Hydro-ecological relations in the Delta Waters

Netherlands
organkation fw
applied scientik
research
TNO f2omdtee
on Hydrdogical
t?eseach
8 hyara ecological relations
in the Delta Waters of the
South-West Netherlands

Hydro-ecological relations
in the Delta Waters of the
South-West Netherlands

TNO Commlitee
on Hydrological
Research
Hydro-ecological relations
in the Delta Waters of the
South-West Netherlands
Proceedings and information No. 41
Verslagen en Mededelingen No. 41
Editors
J.C. Hooghart
C.W.S. Posthumus
Technical Meeting 46
Rotterdam, The Netherlands
8 March 1989
Publ~shed wlth support of
Rijkswaterstaat, Directorate Zeeland
The Hague 1989

COLOPHON
Photographa
- Rijkswaterstaat, DirectorateZeeland:
- Rijkswaterstaat,Tidal Waters Division:
reverseofthetitle
page, l l b, 82b, 1 oOb,
I 52a
- R~jrtswaterstaat,lnstitutefor nlana
Water Management and Waste WaterTreatment
- King Air
- BureauWaardenburg
- W. Oorthuysen
- P.L. Meininger
- E.Sterckel
- L. Peperzak
- M.Veldhuis
- W. Riemens, National Forest
Servicein the Netherlands
Printed by:
DrukkeriJ/Uitgeverij Lakeweld B.V..The Hague
CIP-DATA
HydM-ecoiog~cal relations in the Delta Waters of the
South-West Netherlands: Technrcal Meet~ng 46, Rotterdam,
The Netherlands, 8 March l989 1 eds. J.C. Hooghart,
C.W.S. Posthumus.-The Haaue:TNO Committeeon Hvdrolog'cal
Research. - Illustratio~s. - (Proceed'ngs and inrbrmaton
ITN0 Committee on Hydroloaical Researcn: no 411
With index. ref.
ISBN 90-6743-160-5
SiSO 573.3 UDC 574.5:627.51492.911
Subject heading: hydro-ecoldgy, delta
COWRIGHT Q WTHE NETHERLANDS ORGANlZATlON FOR APPLIED
SCIENTIFIC RESEARCH TNO, W89

2 RIVER WATER AM) THE QPALITH OF TBe DELTA WATERS
L. Biflsnra md J.W.M. Kuipers
Abstract
1 introduction
2 Changes in the Delta
3 The quality of river sedimenta
4 The quality of lake bed and river bed aediments in
the sedimentation areas
5 EFfects on the environment
6 Cmclusions and final renarks
7 summary
References
3 EUTBOPHICATION 02 RIE RXESB WATER8 OF 7I-E DPl W
J.E.W. de Roog md B.P.C. Steepkamp
Abstract
1 Introduction
1 The &lLands DLep/aaringvliet
3 The Volkerak-Zoom Lake spstm
4 Nanagrment nieeswres to prevent or lircit the
eutr~lphicatias of the Volkerak-Earn lake systm
5 Gonelusions
4 ~ P K I OF ~ ESTUARIES I ~ BND BRAGKISH LAGOONS IN
m smn-WEST NETHEBtrnS
P.H. Nieukuis
Abstract
1 Introduction
2 Nutrienz cmcwtrations ad lpadws
3 Effeets a£ eutrmphication
References
5 THE CMANGINO TIDAL LANDSCAPE IN TBE DELTA AXEA THE
SODTH-WEST HETBOaI.p;NbE
J.P.M. Mulder
Abstract
1 Utroduction
E
3
Evolution of rhe tidal landscape durkng histosis time@
Major chenges in the tidal landsaape pvef the last
decades
4 Eonslwaing remark$
Aoknowledgersents
Bef erenoes

6 ECOLOGICAL DEVELOPMENT OF SALT MARSEES AND FOWR TRIAL
FLATS IN THE SOUTH-WEST -S
&.E.& van Baperen
Ahstract
I Introducttw
2 Ecological dpamtcs in various waters
3 Factors influencing ecwystem development
4 Balsnoe of ecosysrem development up to now
5 Future
6 Spatial view
7 Hanagwent
Acknowledgemenrs
RefelrenceS
BY w m m s
P.M. Meire, J. Seys, T. Ysebaert, P.L. Meininger
and H.3.M. Baptist
Abstract
1 Introduction
2 Uescriptian of the area
3 Occlrrrence of birds
4 Distribption of waterbirds welr the different basins
and telatims with the food supply
5 Factors comglicatLng the releeion between distribution
and favd supply
4 Conclusion
Aclmovledpements
References
Appendix
8 HPDRO-ECOLOCtIW &EATIONS IN THE DELTA WATBRS OF
THE SOUTH-WEST EEIETBERLANlXS
C.W. Iedema
Abstract
1 Introduction
2 Sedimentation and aCcumulation
3 Downstream freshwater euerophicatipn
4 Saltwater eutrophicati~n
5 Horphological struare and dpaics
6 Terrestrial nabre1 developments
7 Food ecolog~ connections: bii-ds

B.J.M. Baptist BIJkswaterstaat. Tidal Waters Diviefon,
Middelburg
Wjkswaterstaat, Directorate Zeeland,
Middelkg
Bijkswaterstaat , Directorate Zeelsnd,
Elideelburg
A.M.M. van liaperen -$try of Agriculture and Bisheries.
Beparhunt for nature Eonketvation.
Environmental Pratection and Wildlife
Baxlagement,
6088
J.E.W.
C.W.
J.W.M.
P.L.
?.M.
J.P.B.
P.H.
de Eaog
Iedema
Kuipera
Eeiningsr
&ire
Mulder
Nienhuis
Ftijkmaterstairr, Institute for LnLasd
Water Management and Waste Water Treatment,
Main Division Water Managwnr,
Dor&reaht
Rijkswaterstaat, Directotate &eland.
Middeldurg
Eijkwaterstaat, Directorate Zubd-Holldnd,
Rotterdam
Bijlcswaterstaat, Tidal Uatera Divisicmi
Middelburg
Dlliversity of Gent, tabosatorp of Animal Ecolo$y,
Gent (Belgid
Rifkswaterstaat . Tidal Waters Divisi en,
Middelbur(:
Delta Institute for Hydrobiological Research,
Yerseke
J. Says
University of Gent, Labor&ory of Animal Ecology,
Gent IBelgim)
B.P.C.
Steenlcrrmp
Ilijkswacerscaat, Inssitute for Inland Watex
Management and Waare Water Treatment, Main
Division Wate~ Manegament, Dordrecht
Vazivezaitp of Gent, Laboratory of Animal Ecology,
Gent (B*iuuI)

Before human interference in the Delta of the 1300th-West Netherlands.
the hydro-ecal0gioal relations +n the watexs of this Delta were taken
very much far granted. As a result rzf the man-made changes, thbe
natural relations disappeared. The constmction df the Delta Works
completely altered the face of the Delta. Thie was not the first
occasion on which the relations between the waters of the Delta had
been affected. Take for example the construction of the Sloe Dam and
Kreekrak I)rug in the latter half of the 19th ceatury, which resulted in
the loss of the coneection between the Eastern aad Wsatern Scheldt.
Nevertheless, the Delta ia its present fow ig largely the brainchild
of hydraulic agineerr; of the lattez half of the present centuq.
The origtnal Delta Project treated the Delta as a single entity, but
purely from the point of view of safety and water control. At that
time the integral approach of eool6giaal Interests had yet to be
developadd Although ecological considerations have gained in
impoltanoe over the years, the present compameutalizatlon ef the
Delta is still to s great extent the product of the original Delta
Project.
The completion of the water management infrastmcrure of the Delta
marks, for the tfm being at least, the end of an ere of major
hydradie projects in the Delta. But while these works were being
carried ant, a new era had already begun; one which centred on careful
supervision apd ~esponsib-le managwent of the newly-created systems.

This approach will put to use what has been leaned about the
hbaviour of compartmentallzed systems. If we respand to changes by
making informed use of the scope for control and regulation presented
by the Delta Works, we can develop these new systems to their full
patential.

RrPER WATER
TRE QUALITY OF TEE DELTA WATERS
L. B i j h
and J.W.M.
KuipeIs
Extmsive hydraulic engineering projects have beerr carried out Dver the
last 25 years in the Delta area famd by the rivers Rhine. Meuse arid
Scheldt. This has resulted in parts of the Delta befng divided up
(c~artmentalized) while oche parts have been closed oaf. The time P J ~
which rnaap oi the operations involved
dividing up these areas were
carried out coincided wirh a period during which the pollution in the
three rivers reached its lnaximum level (1970-1975).
34icropollutants (heavy metals and organic compounds) became aztsched to
the Elms in the sadiment carried by the rivers, md this led to the
Delta region also being affected by theae contaminents. However.
because of compartmentalhation certain areas, namely Lake Gtevelingeu
and the tidal basin of the Eastern Scheldt, escaped the wiwe of
pollution. The quantities of micropollutants ia thwe areas are vow
close to natural background levels. The explansion for this is that
the compartmentalization af these areas was completed before the
pollution in the Rhine an2 Wse reached snrb high levels.
In oontrast, the closure of the Earfngvlfet estuary led to the
fornation of new sedimentation areas just prior ta the time at which
the pollwtlon in the Rhine and Beuse reached a peak. Thus layers of
conrantinated material have settled in thase sedimentatim areas. Almost
30% of the total amount of silt transported by the rivers Rhise and
Meuse is deposited in these areas.

Estuaries that have not been influenoed by closure operations can also
un&ergo significant temporary seamentation effects. An example of this
is the land of S~efeinghd in the Western Scbeldt astuary. As a result
of a rfse in the sea level and a deepening 0f the shipping ch-I, the
difference between law end hQher watex bter inrreased which. in C m ,
has resulted in a significautly greater deposition of silt on the
mudflats and salt marshes. Almost 258 of the stlc transported by Eh@.
rlrer Gcheldt settles in these areas.
Although relatively IfttLe is itqown abol~t the effects that contaminated
beds have W% the aquatic ecaapatem, there are clear signs that
ecolorsical camunities which are directly dependent OP the river bed
environment can be disrupted. In additio& the amaunts of pollutants
round in certain oqanisms and plants now exceed the maximum acceptable
level$ for human consumption. Fumre clean-up operations could be
censidm'ed fer the contamfnated $edimentatiou areas when the level of
pollution frorn discharges into the river baeb has been suf£icientl~
reduced. In view af the scale and extent oP the sedimentation layer
concerned, no salutions have yet been fomd to thls problem.
The Delta zegion of the South-West Netherlands
hean the scene of
many extensive hydrauliq engineering projects sver the last few decades
(Fig, 1). These opexatians ware largely a reaorlon to the storm tide
dtsascer a£ 1953, in which large areas of the Delta region were
inundated. The pLam which were &!awn up after this catastrophe and
which have do*1 beon implemented were primarily concerned with
protecting the region againsf Iloodi?~g. In addition. the plans also
addreased dpecific vater management objectives, in particular the need
to control tbe pblwp. of salinization. The basic approach was to
dose off the tidal gullies in the Delta and to create a sertes OF
freshwatet lakes behint3 the dame.

6 )(ollM.Yn DI.0
I L.,. (il .l 1m.n
.-I L&.=- F O,.l*.I.". **.c,sBD,
'I L."* *'S",,,*. (i i..911.1- (8.01 1
" z.-w*-d" .,"U,W,
I htn.a.*,-nswr BTU*. IW~I
Figure 1 The Delta Plan
DvrI4g the ODilStruction phase it was realised that it would be
exrremely undesirable to lose altogether the unFque saltwater tidal
sysrcm that existed fn this region. Consequently, a decision was taken
in the 1970's not to Close off the Eastern Seheldt tidal basin as had
ozigfnally been iataded, but to install a storm surge barrier instead.
This meant that the safety of the region could be @armteed while
still presenting the essential character of a ealturater tidal system.
While CDnsttuctiod work on the Delta Project was in progress, pollution
from the major rivers in the region rose to maximm lavels. This
created a further problem. Closure of the estuaries has disturbed the
morphological balance ancl led to the forreation of new ffedimentatisn
areas. The quality of the lake and river beds in the new sedimentation
areas deteriorated rapidly as the suspended sediment carried by the
wster af the major rivers e~ntained micropollutants quch as heavp
metals and organic wmpeunds.

2 CHIWGES XN TEE DELTA
The Delta are was formed by the interaction 05 the rivers &be. Meuse
and Scheldt and the tidal movements of the Xorth Sea. Tlie average
discharges from the Rhine. Meuse and Scheldt are 2200 m3/sec.
260 mg/sec and 100 m31sec respectively. The original discharge
distribunon over the tidal @lies, prior to thaDelta Project
closures, is given in Figure 2. In the period befere 1965, the
influence of tbe Rhine and Ueuse -tended ta the Eastern Scheldt
estuary. At that time the Delta was Fn a state approaching
morphologioal equilibrium. Until 1970, the amount of sand and silt
brought down by the rivers Rhine and Meuse was ronghly equal to that
Figure 2
Dischatge distribution over the maior rivers 1965 (vduee
in m8/sec)

discharged through the mouths of the estuaries to the North
However, local variations were apparent, with further erosion taking
place in the deeper tidal gullies and corresponding increasas in
sedimentatton occurring in the shallows. Moreover, sea-borne sediment
was deposited in the western part of the Delta. The finer type of
fluvial sediment was only deposited in more sheltered locations, with
the majority of it flowing tkouah to the RarM Sea. Under the
influence of the prevail*
sediment was carried northwerds, where w e
10%-shore current this fine fluvial
of it was deposited ia the
Wadden Sea, an extensive w e s l area ~ in the north of the Netherlands.
Successive closures in the Delta area produced chenges in the flaw
distribution, while flow rates tended to decline as a result of the
complete or partial loss of tidal motion. =is Led to tde formation of
new se-ntation
areas witain the Delta regfoil. By 1965 the central
section of tXe Delta vaS no longer sublect fa the influence of the
Rhine and Ruse. In 1965 and 1969 resptpectively the Grevelingen and
Eastern Scheldt estuaries were effectively &coupled from these two
rivers. The closure of the Harinmliet estuary in the northern section
of the Delta had the effect of reducing tidalmotion over the whale oi?
this area. Xn the sautb, the Western Scheldt estuary remained Largely
unchanged, although the sand bars in the estuarp *ere dug out in the
1970's to improve the shipping route to Antwerp (Belgium). Finally, in
1987, work in rhe eastern past of the Delta led to the fomation of a
separate freshwater lake (Lake Zoom3 in the Eastern Scheldt, irith the
spe~ific aCm of imprwing water supplies for agricultural purposes.
Lake Zom is being flusked out with frwb water from the Eollarids Diep,
The overall eff&t of all these hydraulic engineering projects has been
to change significantly the distribution of river water, which has had
implications for the transport of fluvial sediment. Recent measurements
of the transport of fine fluvial aediments are shown in tern of tons
per year in Figure 3. This fi~uze also gives the yearly irccumulation of
river sediment at: particular locations. Region I represents the
southern lower reaches of the Rhine and the lover reaches of the Meuse.
Following the closure of the Haringvliet estuary, flov rates in the

area decltned cnnside~ably, leadins eo a~rensive sedimentation. This
has cause3 an Inner delta to form at this location, which is gradnag
plovinp in a westerly direction. In the case ef Regien 11, the
lIarinpliet, closure has led ko a thin layer of silt being deposited
over this area.
From 1987 onwards, a new sedimenracion area is developing in Lake Zo&
(Region In). The sedimentarion in this region is essentially similar
to that found in the Haringqliet area and will mainly cunsist of silt.
Pbally, comparisons can be made with the sedhentntion area in the
Western Scheldt (Region IVI. In this region most of the silt carried by
I l
t.l(.rnbsmlsl
W.*l.m
..I"""
Sm.ldl
I
I'
closure dam
J [ waler me.naeernm1 sluice
ir~dlmsnlatlon rsgl~n
X: number ot lhe raglan
200: sedlmenlallorn in
1000 tons I year
Figure 3 Transport rates of fine fluvial sediments 19.87 (values in
l000 tms p.a.)

the river Scheldt is deposited in the mudflats hwn as the l&
Sadtfnghe.
of
The sedimentation areas +-bferted to above will Be discussed fuzther in
the followdug sections.
Prom the turn of the century onwards, pollution in the &ke, Meum aad
Scheldt hssiaa increased rapidly as a rwult of industrial expansion
and fuzther population ~rowth ~5altaoo=,
1981). T'he mount OF pollutien
in these arras antimed to increase until ourxfmute levele were reached
during the period 1978-1915. As a censeqmence Large amaunts of hemetals
and ~rgaaic compamde have becnms attached ca tbe rrver
sediment.
Ja order to IRDnitw ttie sies&tron dxt@t the last Em beea~@s, t&
aSssciated leveLs haha been meas.me8~ at the pdiWs *here t&
t%i&
rivers boss cfre befdees, A BrtaFtd di*%usBi6h of tbe distrfbugion
and cbahpe.8 in We levels of ell rAe sutmtabces that ate etiiteifed is
bey6nd the heepe of tbe :pt@sent papet. ft h- therefore been dec-d to
caiaider the examp&e B£ the bedry eta1 cgdmium a mean of
illurifzatin~ the subje&t. This moral be rqardeli asi a Lacer for
othes pollutrmts. 'Ehe ehq.gS in associated 'e~dmfm level^ measql.e$ In
~nspended sediment is shmm as a Eu=Gion ef tim in Piwxe 4. 8y @aim
data from wre swig takq fw older sedtwnix if h,e. bsen p m l e
te rsganstruet the build-up of poll~tion the %be tthe first part
a£ $AA* centurp. Fellovfng tKe peak y.e%rrs ef the eaz.Llg IPtO's, clem-U$
measweB in the $Wue basttl &YE
led t~ B spectacuiaz sedwctioa in the
afnoudc of cadidurn pa.Uutioh. Coneentratimw &we 40.w he& recture-i te
Skse of

Plan, then it is apparent tbt the separatian of the Grwelingen and
Eastern Scheldt estuaries occurred prior to the peak polluttan years of
the early seventies. The formation of sedimentation regians I and 11, a
result of the closure of the Rarhgvliet, took place hediately before
the major wave of pollution occurred, while sedimentation rsgion 111,
wbich d l be created by the fluehiq operatians of Lake Zoom, will be
affected now that contamLRatiDn levels have been greatly reduced.
Figure 4
Associated cadmium in suspended river sedimeots ar the points
where the rivers Rhine, Meuse and Scheldt cross the border

it (Phalacrocorax carho), a fish eating hird
Volkerak dam and locks

Tiddl flats df the Eastem Seheldt vith "galddiggers"
Flounder (Platiahtkpg f leaus) with ebscessea

4 THE QURLITY OF LAKE BED Bhm BIVER BED SEDIXENTS IN TIE
SEDU4ENTATION AREAS
The series of events, described above, has produced diffesences in the
quality of lake beds and river beds in the sedimentation areas
(Salomons, et al, 1981; Beeft*, et al, 1982). The situation regarding
cadmium levels is illustrated in Figure 5. A regression analysis has
been used in order to compare the meamred c~ncentrations in the
varioue samples.
Roft.rd.m
"'.,.rr.,
U L
R",".
a full rirclc represents
CO m#/kg cadm~um.
Figure 5 Cadmium ooncentrationa in river bed sediments converted KO
an equivalent 50Z-16 v* top lager (1986 levels).
A relationship has been established between the measured concentration
ad the propartioq of fines less than 16 urn in the sediment. It was

tracefore possible to convert the absolute measlured concentrations to
an equivalent concentration that would be expected in a standard river
bed with 550 ~f the grain fraction less than 16 um.
The separation of the Eastmrn Scbeldt and Lake Erevelingen from the
water flowing in the rivers Rhine ad Meuse has resulted in remaining
extremely low on cent rations in these areas. The values found in the
Eastern ScheldC are thought to approximate to natural hackground
levels. The concentration in Lake Orevelingen is known ta be aamedhat
higher. Unlike the Eastern Scheldt, this lake becm stagnant after its
closnre. .As a consequence, there is less interaction between the water
in the lake a d the bed deposits (resuspenaioe) in Lake CreveUngen
than in the Eastern Scheldt. This explains why the concentration of
contaminants in this lake still approaches the pre-1965 values.
The lake beds and river beds in the s,e4imentatlon areas in the northern
part of the Delta are hewily polluted. In the eastern part (Nieuwe
MerwBde/&met) high contamination levels have been detected, affectiqg a
layer at least 2 metres thick. Zhis material was deposited between 1970
and 1975. As an equilibrium has now developed between the hydraulic and
morphalugtcal precesses, it is unlikely tKat this la~rer will be coveted
by new se&ments in the future.
The central section of the Delta (Hollands Diep) hag a thick layer of
less polluted river sediment that was deposited in the years aftek
1975. However, more contaminated material from the 1970-1975 period is
contaised below this layer. Underneath these layers can be fonnd
estuarine sediments from *he period before 1970. l'he different layers
referred to above can be qlearly dlvting=ished fro= core samples taken
from the area (Fig. 6). It is reesgnized that the quality of the tap
layer of sediment will improve as the quality of the surface water
iaproves.

Figure 6 Contwination in rhe Eollands Diep. The recently deposited
contaminated layer can be clearly distinguished by the lead
content.
A relatively thin layer of polluted sedinents (less than 0.1 m) is to
be found in the western section of the Delta (Haringvliet) above older,
mainly marine sedate. As sedimentation in this region takes place
extremely slowly, no significant changes are expected in the near
future (Fig. 7), althmgh in the long term the quality of the top layer
will improve as the main area of sedimentation shifts westward
(Bijkswaterstaat, 1987).

Figsrlre 7
Changes in the cadrnrum contenr 08 the Bbine vater W) and fn
river bed sedineat from the Aari~wlieE (K) over the period
1971-1W.9
As flushing +pexasrions ere vnly sebeduled hc cBmaenc@ Sn 1981, the
quuli@ oZ the! eediwn2 in take &m is *=ill the as that faund in
o~het part$ of the Eastern 5cheldt. %wener, theue ia a small area
Wtnd trhe Volketaklocke wMch has been afteored by the inflw Q£
water From tke Hollaxrds Dig. For water management purposes it h@% been
decl&d tu re~tticthe fldinp; qerationw in Lake Zom to a miaimum
(the salinity level in eke lake *ill not eiceed 4170 &l),
the iuput of ppollutants.
to reduce

It is possihle that in the future a more liberal flushing policy might
be adopted $n connection with the improved water ~ualitg in the rivers
Rhine and Lleuse. Hawever, it should be borne in mind that not all the
pollution parmeters have been reduced to the same ertent as cadmium.
In the Western Scheldt the situation is different from that described
abwe since this rwion haa not been faflueneed by the closure
operations. Boww@r, significant amaunts of sed-t have been
deposited over the last deaades in the Land of Saeftinghe area, Which
conaists of extensive salt marshes and mudflats. This has been caused
by the deepening of the bars in the eaetern part of the estuarp for
shipping purposes, which have increased the t%dal difference and also
the turbidity. Conruequeutly. a layer between 0.2 and 0.4 m has recentbeen
deposited in thLe area. Polluted silt from the river Scheldt is
sect* here together with uncontaminated sea-bane sediment, in the
ratio 608 liver silt to 44% sea silt.
& a result of the comfruction of the Delta Works, as described in the
previous chaptezs, the central seetiou of the Delta has been cut off
froa tbe ingluence of the major rivers to some extant. Hewever, this
does not necessarily guarantee the quality of the estllary and lake
beds, which 3s affected bot& by past influences as by local 8ources of
pollution. It is inpurrant to take this inea account view of the
radical changes which tbe hgdranlic and aiorphblogical confi@uraeian of
the regLon had mdcrgone. For instance, rhe cmpartentslization of the
takes Grevelingen. Beere and Vblkerak-Zoom bad cbnsidetahly ibcrea-sed
the residence tima of water. The construct'ion of the storm-surge
barrier in the Eastern Soheldt haa also led to a oonsiderable reduction
in the hoxizontal tide, as a result of which residence the had
ineressed and the Eastern Sch%lde has becorn a pure sedimentation area.
In shsrt, the centrl section of the Delta has becoma much more
vulnerable to local pollution.
Local pollution mafnly emmates from diffuae sources; the main ones are
acrivrties in the smaU hsrbonfs in this region and the discharge of
water frm the polders. It has been obsemred that the beds of all of
these harbours are moderately to aeverelp polluted. Clean-up and

prevention measures are desirable here to prevent the diffusion of
pollution to adjacent waters.
5 EFFECTS ON THE ENVIRONMENT
Relatively little is knm about rhe impact polluted beds have on
aquatic ecosystems, partly because these areas are not directly
obsesvable. Future research strateges will therefore be mainly
concerned vith addressing these shorreomings. There are, however,
various indications that pollution m y be a hazard to public health md
may influence the functioning of aquatic ecosystems. The presence of
heavy metals ad organic micropollutants in benthic organisms is
clearlybigher in polluted areas rhan is less wntaminated areas. This
has been verified for various organisms such as Dreissena palymol-pha.
Tubifex spec., miromid larvae and also zooplankton (Urk, 1987). There
are, of course, specific differences between individual species with
regard to the type and mwunt of heavy metals found. In molluscs, for
example, contamination levels are Ngher in species that live ia the
top layer of sediment than surface dwellers.
Kelatively low levels of p@llution are found in the eastern part of the
Xarin$vliet basin (Hollands Diep), whereas higher levels can be
detected in the western part of the Baringvliet. This is a coneequence
of the sedimentation process, which has influenced the quality ef river
sediments in the areas, as outlined in the previous section. These
gradients are also reflected in the concentration of contaminants found
in specific organisms. Xigure 8 shows how the cadmium content 3s the
gills of Anadonta anatina varies at different locations. The levels in
the western part of the basin (Haringvliet) are two to three t h e
higher than those found in the eastern part (ILollands Diep) . Simular
problems have been encountered with orgsnie micropollutante. For
instance, from 1977 to 1985, PCB levels in eels from the different
surface waters in the Wetherlands were monitored bg the State Institute
for Fishery Research CRIVO). At the end of the seventies PCB levels in
eels from the BaringvlieL basin were extremely bigh, prohahle as a
result of local discharges, some of them illegal. Since tbis time, the

Figure 8 Cadmium content in the gills of Anadonta anatina found at
several places in the Raringvliet (a) and the Rallands
Diep (b)
level had decreased to about the same value as that nomally found in
the upstream areas of the Rhine. Nevertheless, the level in eels from
the Haringvliet basin is still above the recommended Limit specified
for human consumption. To give another example, table 1 shaws BGB
levels measured from the Haringvliet basin and some cZean referenee
areas. The accumulation is clearly discernihle,
The amount of contaminants in plant and animal tissues does not throw
very much light on the functioning of the ecosystem. Pollution map be
regarded as a form of strees imposed upon the environment. Its effects
vary from species to species and may manifest themselves in stunted
growth, a fall in the reproduction rate and an increased morrality
rate. Given tbat certain animal species are more vulqerahle to
pollution than others, it may lead ro shifts in the competitive

elationships between different species. The more vulnerable species
will decline and finally disappear, while the less vulnerable species
will becm established or increase in particular areas. In general,
the total number of species wtll decline, thus impoverishing the
ecosystem, poasihly to a severe degree.
In the Earingvliet basin the levels are so high that specific
environmental effects are to be expected. Inventories carried out in
the area have sbwn that the ecosystem in the Eollands Diep is
functiot&~g much better than that of the Earingvliet. For instance, the
biomass of fauna found on the river bed has been shown 50 be higher in
the Hollands Diep. This also applies to the number of fish and to the
numher of birds that feed on fish and fauna, living on the river bed,
such as cormorants, grebes and tufted ducks (BoudetPijn, et .l, 1986).
The differences observed between the Eollands Diep and the Earingvliet
may be explained partly in terms of the difference in quality of Ehe
beds. But even, the ecosystem qf the Hollan& Diep is poorly developed
by compaison with ether large freshwater systems in the Netherlands.
Ror exantple, large water plants are scarce in the whole area, even
though slight increases have been detected aver the last few years. As
far as iudividual species are concerned, sme of the effects are
probably connected with the contamination of the river bed. A liak has
been est&lished between deviations in the chitin structure of the head
of chironomid larvae and the degree of pollution found in the sediment.
Lahoratory research hae indicated that the reproduction rate of tufted
ducks fed on freshwater mussels from the Haringvliet is considerable
lower than that of tufted ducks fed on mussels from less pollated areas
(Marquenie, et al. 1987). Moreover, the eggs of tufted ducks and grehes
from the Earingvliet area appear to have mueh higher levels of organia
micropollutants than those of birds from less contaminated reference
areas (Table 1).

Table 1 PCB levels in ees of grebes from the Haringvliet basin and
sore clean reference amas (values in ~g/kg)
basin
areas
PCB 138 4 900 - 18 500 120 - 1 450
PCB 153 9 MO - 16 500 205 - 2 $50
PCB 186 1 850 - 3 450 10 - 195
Recent research also show8 that cormrants, a fish-eat*g
spaeies at
the end of the fopd chain, are reproducing far leas snecessfully iU the
northern part of the Delta region (Biesbasch) than in other areas
(Boudewijn, et al. 1989). In a number of colonies along the major
rivers (IJssel. Rhine and Waal) the reproduction rate is two to three
rirpes higher than in the Biesbaach, and it is four tines highex in
areas witb a relatively low level nf palltLTian. (See Table 2).
Table 2 Number of puns leaving the nes per clutch, in a d e r
connorant colonties in 1988.
of
&ss with a MBj or river areas Biesbosch
reiatfrely low
level of gollrttion
OV Oude Venen (Friesland)
BW B~ede Water CVoorne)
W Wijhe (Gelderse IJseel3
0 OZst (Gelderso TJssel)
P Pannerden (Upper Rhine)
W Baaften (Waal)
bB Dordrse BSeabosch

Bio-essay Weriments with polluted sediment from Due of the smaller
harbours in the Delta region (&resicens) revealed effects on oyster
larvae (Crassostrea Awulatal and a ernstacean Lhthypreia).
Even when B large proportion of Clew sediment was added to the
polluted harbour sludge, mortality rates were demonstrably affected by
pollution (Hnrk, 19883.
The effects of increasing pollution in water md riverbeds are al-so
noticeable in the Western Srheldt estuary. The popuLation of hatbaur
seals (Phoca vitulina) has now disappeared. Food chains in rhe eastern
part of the estuary have been dfsthed and the number of benthic
organisms significaatly raduced. It is acknowledged that the amounts of
cabmiwu now exceed acceptable leveLs for hum consumption.
particularly in flounder (Platickthys flesus), in shellfish such as
mussels (Mytilus edule) aad in sea plants such as Aster tripolium and
Salicornfa spec., foutld in the Lad of Saeftinghe area.
6 CONCLXISIONS AI43 FINAL ReMaaKS
Although water quality and the condition of the beds we*e not taken
intn consideration at the time of the origtnal decisions concerning the
phasing and implementation 05 the Delta Plan, it is now recognized that
the separation of parts of the Delta from the waters of the Rhine and
Eeuse, as part of the compartmentalization programme for the regiwn,
has played a significant role in wtntainSs& a satisfactory level of
quality in the majority of the beds in the Delta. As a result, the
quality of the sediment in tlle central secrion of the estuary is clese
to natural backgrrrwd levelS. However, the closure of an estuary such
as the Rarin8vliet ean lead to large amounts of polluted river ~ilt
settling in new sedimentation areas. An other ntethod af controlling
the distribution of contaminated rmaterial is to limit the tkrou~hpm of
water if sluices for water management are available in the
colnpartment~llzation-dams. This idea has, for instance, been
incorporated into the plan for managing the quanrity of water of Lake
Zaoin.

In the sedimentation areas, there are clear signs that the ecological
commuaities that are directly dependent on the quality of the bed have
been disrupted. The long-term pollcy aims of the Wetherlands government
are to improve the quality of riper beds and lake beds to such a degree
that the aquatic ecosystems can fuaction satisfaccarily, quaranteeing
optimum use of the water systems. Short-term policy objectives are to
obtain further information about the present quality of the river beds,
the causes and effects of pollution and possible means of prevention.
Furthermore, uniform standards for the aseessment of all types of soils
and clean-up methods for the most polluted locations are being
developed. In the last fifteen years almost 80 million m3 of pollnted
river sediment have been deposited in the lower reaches of the Rhine,
&use and Saheldt. The south-western reglon of the Netherlands can
therefore justly he referred to as the sedimentation pit of these
rivers. It is essential that clean-up operations in the river basins be
iqlemented quickly. However. the eleaning-up af the river beds can
on*
really be considered when the contamination in the three rivers
has been reduced t~ an acceptable level. As a result of consultation at
international level, the Rhine riparian states have agreed measures to
halve the discharge of a number of major pollutants in the Rhine basin
between 1985 and 1495. If necessary, additional measures will be taken
after 1995, in the event of failure to achieve the aims lald down in
the agreements (such as the elininacion of d~positions of pollutant5 in
sediment). The states bordering on the North Sea have arrived at
similar agreements with respect to the t-ivers and these will have most
effect on the Delta Fn this regard. once the transport of polluted
sediment has been halted, a decision may be takw on the cleaning up of
the estuary and lake beds.
Those areas. where the sediment is of an unacceptable quality in view
of the uses te which the wafer is put, are first in line for a clean-up
operation. Other important criteria are whether or not: the polluted
sediment is gradually heing covered by cleaner sediment as a result of
natural processes, the effect which the puSlufed area has upon the
surromding environment and the practicability of a clean-up operatioe
from the technical and administrative points of view. As such
operations are invariably costly, it will be necessary to give

considerable thought to drawing up the list of priorities.
Clean-up operatiens are probably uenecessary in areas where a cleaner
top layer is being deposited as a result of continuing sedimentation,
as is happening in the Hollands Mep and the Land of Saeftinghe.
Hwever, in cases where pollutants remain in the top layer and may be
released and spread further afield, such measures must be emsidered.
The degree of priority to be accorded to cleaning up the area is
deteaed by the gravity of the effect of taking no action and by
whether the source of palLution has been removed or continues te exist.
The polluted harbours in the Eastern Scheldt region are to be cleaned
up in the course of 1988. It is certainly worth considering cleasing up
the pul$uted sediment in the br, the Nieuwe Menfede and the
Biesbosch, as virtually no further deposits of clemer sediment can be
expected in these aeeas in future. The need is all the more pressing
because it has been prown thAt pollution seriously detracts from the
funetion of this azea as a Wure reserve.
since simiIar problems with polluted river sed*nts are Ekely to
accompany the huildag of dams in other parts of the world with
indnstrialixed catchmene areas, the experience gained in the Dutch
Delta Project should prove to be generally applicable.
Zxtensive hydraulic engineering projects have been csrried out over the
last 25 years in the Delta area formed by the rivers Rhine, Meuse and
Scheldt. This has resulted in parts of the Delta being divided up
(compartmenta15zed) while other parts have been closed off. The time at
which many of the opmations involved in dieding up these areas were
carried out coincided with a pezioa during which the pollution in the
three rivers reached its maximm level (1970-1975).

Bicropollutants (heavy metals and mganic compounds) became attaohed to
the fines in the sediment carried by the rivers, and this lea to the
Delta region also being affected by these contaminants. Ifowever,
because of compartmsntalization. eertain areas, namely Lake Grevelingen
and the tidal basin of the Eastern Scheldt, escaped the wave of
pollution. The qnautities of micropallutants in these areas are very
€lose to natural backgmund levels. The explanatian for this is that
the compartmentalization of these areas waa cmpleted before the
pollucibn in the Rhine and Meuse reached such high levels.
m contrast, the closure of the Itaringvliet esfuary led to the
formation of new sedimentation areas just prior to the time at which
ehe pollution in the mine and Wuee reached a peak. Thus layers of
contaminated material have settled in these sedimentation ar-S.
30% of the tutal am~unt of silt transported by the rivers Bhine and
Meuse is depasited in these areas.
Almost
Estuaries that hare mt been influenced by closure operations can also
undergo significant tempmrary sedimentation effeots. An uample of this
is the Land of Saeftinghe in the Western Scheldt estuary. h a result
of a rise in the sea level and a deepehg of the shipping channel, the
difference between low and higher water hqs increased which. in turn,
had resulted in a sigaiftcz~tLy greater deposition of silt on the
mudflats and salt marches. Blmosr 254: of the silt crsnspdrted by the
river Sehaldt settles in these areas. Although relatively little is
born about the effects that conraminated beds haae on the aquatic
ecosystem, there are clear signs that eecelogical contmu~iities whPch &re
directly dependent on the river bed environment can be disrupted. In
addition, the aouots of pollutants found in Certain organisms and
plants now exceed the maximum acoeptable levels for human cm%umption.
Future clean-up operations could be considered £m tbe conraminated
sedimentation areas when the level of pollutian from discharges into
the river baeiw has been sufficiently reduced. In view of th8 scale
and eeent of the sedimentatim layer concsrned, no solutions have yet
been fuund to this pmhlm.

ShLOUONS, W. and BYSINK, W.D., 1981. Pathways of mud and particulate
trace metds from rivers to the southern Borth Sea. In: Nio SD,
Schiittenhem RTE, Weering TCE of (eds) Holocene marine
sedimentation 19 the florth Sea basin. Spec. Publ. Intern Aesoc.
Sedimental 5: 429-&50.
BEEFTINK, W.G.. NI~UIZE, J., STOEPPLER. M. and WHL, C., 1982.
Hewy metal accumulation in salt marshes from the Western and
Eastern Scheldt. Science Total Environmental 25: 199-223.
BOUDEWIJN, T.J. and MES. R.G., 1986. De ontwikkeling van de vogelstand
in het Bollands Diep/Baringvlietgebied in de periode 1972-1986 en
de invloed van het waterpeil op watervogels. Ecoland. Leewarden.
RIJKSWATERSTBAT, directie Benedenrivieren 1987. De waterbodem van het
noordelilk deltabekken. Dordrecht.
BOODEWIJN, T.J., DIB$SEN. S.. MES, R.@. and SLBGW, L.K., 1989. ne
aalscholver: indicatiesoort voor de kwaliteit van de NederLandse
wateren? (in prep.)
M&RQUENIE, J.M.,, ROELE, P. and HOOWSHA, G., 1486. Onderzoek naar
effecten van contanrimten op duikeenden. TWO-M!I Den Helder.
Rapport nr. 861066.
BURK VAN DEN, P., 1988. Voortgansrapportage indicat-Biamon 2.

EUTROE%ICATION OF Tfts
WATERS OF TEE DELTA
J.E.W. de Hoog and B.P.C. Steenkatnp
ABSTRACT
A number of stagnant and serai-stagnant water systems came into being a6
a result of the recently completed hydraulic ehgiueeritlg works in the
Delta area of the Aerhe~litnds.
Eutrophication can occur in these waters as a result of the input of
nutrient-rich river water. l%ia paper describes the eutrophication
situation of bwo water systems in the Delta area2 the Hollands Diep/
Heringvliet and the Volkerak-Zoom lake system.
In the E~llaods ~ie~iliaringvliet the residence time of the water
depends on the discharge of the sivex Rhine. In years of low discharge
the restdance time is relatively long and, especially in the western
part of the Haringvlier, a large biomass o$ algae can appear as a
result of eutrophication. In pars of moderate OT higher discharpes sf
the riv$r mine this water system behave8 like a slowly flovlng river
with relatively short residehce times ad a s41all hiamaes of algae,
The Volkerak-Zoom l& system came into being in 1981 and after a
period of desalinization changed from a saline tidal area into a freshwater
stagnant reservoir. The most important input of rrater is from the
XolLands Diep and the river Dintel. Computer modelling indicates that
the lake will develop into an eutrophlc lake anleas adequate measures

are taken. The first year after the desalinikation proved mmre
favourable thm predicted (the himas8 of algae was relatively small
and transparency was ghod), but the input of nutrients meds to he
limited still further in order to arrive at an enduring Iavouzable
water quality, In addition, "active biological management" should be
considered, i,e. the applicatimn of biological management raeasnres to
obtain a healthy water system wfth clear water, an abundance of aquatic
plant@ and a stock of predatory fishes that wrll keep the whitefish
within bounds and hence allow sufficiefit eooplankton to remain to graze
on the algae.
Eutrophication is the enriehmerrc of a water system r*ith plant nutrients
(also simply called nutrients), of which nitrogen and phosphate are the
mast important. Eutrophication is partly a natural pzoeass: erosiofi and
leaching wash nutrients inta zivers, wkich transport them to the lower
reaches. Therefore, delta areas are natur.%lly wtrophic, and the Dutch
Delta area is a good example.
In the last 100 years humanity has greatly exacerbated the natural
process of eutrophication. The insrallatien of drains ad sewers, the
use of detergents and fertilizers and the increasing intensfficstion of
livestock fanning have led t0 the amounta of nutrients in the rivers
being increased many times over.
Tn sttagnant water the enrichraent with planr nutrients can lead to m
increase in the growth of algae, which can reault in high 4ensities of
algae. Large numbezs of algae 14 the water results in a lev
transparency; when the algae dh aeaerobim can occur and thie In turn
can lead to fish mortality and a bad emell. Furthermore, toxic species
of algae can occur (blue-green algae). Some species £am floating mts.
Sa~tions 2 aad 9 of this paper describe the eutrophication situation of
two Serni-stagriant water systems in the Delta area: the nollands Dlepj
Haringvliet ad the Vohrak-Zwm lake system. The wtrophtcation state

of the latter is compared with that of Lake Brielle, which is
comparable in terms of morphology and residence time. Figure l shows
the location of these water systems.
Section 4 discusses the possible management measures with which the
eutrophicatian problem of the Volkerak-Zoom lake system can be
prevented or limited.
NOORDZEE
Figure 1 Location of Hollands DieplHaringvliet and Volkerak-Zoom lake
system

The Hollands DiepJnaringpllet is part of the northern Delta basins,
also called the lower area of the peat riiuers. This is the area where
two large Western European rivers, the Ehine and the Mewe, discharge
into the sea. The river vater enters the area from the east along thtee
river channels and can. in principle, reach the sea via two routes.
When the Rhine discharge is less than 1700 m3/s
tbe Haringvliet sluices
ip the west rermtin closed and all the river water enters the sea via the
New Waterway. At Rhine discharges abwe 1700 ms/s the Hazingvliet
sluices are also used to discharge the riaer water. These sluices are,
haqeaer, ~nly opened at low tide. This management of the hringvliet
sluices obviously has repercussions on the water movement in the
Hollands DieplHaringvliet and henee also on the eutrgphicatios
sitnatLon.
The eutrophlCation situation of the basins ~im he characterized as
falLows. Large amounts df nitrogen and phosphate are brought in by the
rivers, thns the concentrations of nutrienks are high. Howevet, it is
striking that in these waters a low biamass of algae U usually formd.
Rlsa. tlie bbmass of algae usually decreases fxom east torest; as a
~le, the £eves* algae are found near the Wingvliet ailuices.
Generally. fev blue-green algae oceur in these waters and the
trausparency is reasonable on the whole; near the Karisgpliet sluices a
twsperencg of 1 to 1.5 m is regularly found.
The relatively low biarsasses of algae are often attributed to the
influence of toxic substances. It will be shown below that the watez
movemat provides a plausible exalanation. Figure 2 shows the chasges
in mean snmaer Level of chlorophyl along che traject from Lohith to
the Earingttliet aluices for the pear 1982. Chloropbyl is an indicator
of the amaunt of algae in the water.
From Figure 2 it can be seen that there is a clear decrease in the
biomfass of algae along the traJect. However, the picture changes if we
look at figures from other years. Figure 3 shows the mean smmer levels
of chfwrdphyl between Lohith and ehe Earinpliet slaices in 1976, L982
and 1987.

1
Loblth Vuran NMlS H7 H8 H9 H10 H12
HOLL. DlEP
HARINGVLET
Figure 2 Mean s m r
level of chlorophyl in 1982 along che Lohith-
Baringvliet sluices craject
HOLL. DlEP
HARlNGVLlET
Figure 3 &an smer chlarophyl levels in 1976, 1982 and 1987 along
the Labith-Haringvliet sluices trajeet

In 1976 the determination of chlozopkyl levels was still in its
infancy, put the few daca available indlcate that in that exceptionally
dry year there vas an increase in the biomass of algae towards the
Baringvliet slnices. The picture ww very diffezent in 1987: in that
year the was very little dffference in the amounts of algae along the
weire trajert from Lobith to the Haringvliet sluices.
This disparity can be oXplained by studying tXe river discharges of the
various years,
In L976 the mean smer Rhke disaharge was well below 1700 mars; in
that year the Haringvliet sluices were slrarcely used the whole emer.
In 198% the mean s u s Rhine discharge was approximately 2200 m','$;
therefete in that year the sluices were regularly in use. In the smer
of 1987 the Ehine discharge was well above the mean discharge. and
therefore the sluices were used almost daily to discharge the E-l~er
water into the sea (Table 1).
Table 1 Mean smer chlorophyl level and Rhine discharpe in 1976,
1989 wd 1987.
chlorephpl
(ms/m3)
Ehine discharge
(g/.)
Computer Sirmtl&EionE dbne at Delft Hydraulics have also shm that the
behaviour of algae in these watetri greatly depends on the vater
movement is this area. The eutrophicatian in 1976 and 1982 was
sfolulated with the DBLWbQ-BLOUX model. &B input the made1 requires data
@n the water mowameat in Ehe Various river channels, the concentrations
df uzttrients and algae in CIre incoming river water, climatological
factors rlna properties of algae. As output the model supplies the algkl
hiomass far each location in the area am4 for each day of the year.

chlorophyl [u~/I) l 8.14- 8.12 IUU~U~ a.ffi- 8.83
m 0.lZ 8.11 8.83 B.82
DAY p 8.11 - 8.89 1 8.82 - 8.88
IZ1
8.U - 8.68
%igure 4 Simnlation produced by DGLWAQ-BLOOM model for day 121, 1976.
Figure 4 &wa the result produced by the model for day l21 (the end of
April) in 1976. It indicates tbat there are appreciably fewer algae in
the EIollmds Diep at the Volkerak sluice6 than in the river channels
upstream, and thac very high levels of chlorophyl wcur near che
Earingvliet slgices .
The picture is different: fer 1982 (see Fig. 5). In the Bollasds Rdep
the chlotuphpl levels are lower than is the river ehwnela, but
towards the Hafiupliet sluices they decresse even furLher. mia is
fully in agreement With the picture of measured chloropliyl levels in
that year.

, 0.66 m 0,s - 0.02
1989
I 0.86 - 0.85 @ 0.82 - 0.02
chlorophyl (~g/l] m 0.85 - 8.6 11111111 0.82 - 0.01
DAY U1
88 s.es - a.ai I @,at - s,e~
B3 0.04- 0.84 6.01- #.M
Figure 5 $imlatlan produced by DELWAQ-BLWMmodel far daF 121. 1982
Table 2 Summarized results @Of research on eutrophication in Eellands
DioplHaxingvliet
long short v. short
residence time agpron. apprw.
69 days 6 days l - 2 days
system lake transition river
lake-river
behaviour of aka*
Hollands Diep settling out settling out transperted
Earingvliet growing settling out transported

Table 2 suonna*es the mast importmt results of the research on
eufrophicath in the Hollanda DiepIIlaringvliet. From thia table it can
be seen that in 1976 rhe residence time in the Ballads Diep/
Haringvliet was Long; 60 days on average. The smtem behaved like a
lake. The algae in the IEollands Diep settled out, but in the
Haringvliet they were able to grow well.
The residence time was much shorter in 1982: apgroximately 6 days on
average. The system behaved Like a transition between a river alld a
lake system. Algae settled wt both fxi the HolLands Diep and in the
Earingxliet.
In 1987 the residence dm. was vely sbrt; the system behaved like a
slow-flowing river. The algae transparted with the pates -re egrried
along rwards the sea.
l'he water movement thus offers a very acceptable explanation for the
behaviour of algae in this system. kence. other reasons, such as
t~xological effects, seem less probable.
The VoUcaak-Zoem lake system eame inta being in April 1987 after the
closure of the Philips dam. Ptsm a saltwater tidal area this area has
gradually chaaged, first ints a bSacld8h lake. During 1987 fhe
desalinieation was acaelerated bg allow* an ex+ra intake of fresh
watez via t&e Volkerak slaices. The Dintd, a small river that draing
much of Wear Brabant end z& smell part of Belgivm, is annthiir important
smrce of frbsh water for this lake.
Before the l&e was created predirtions were made about the
eutrophication that could be expected. Table 3 gives an impression of
this and also shows the eutrophicatima observed in 1987 and 1988.

Table 3
Eutrophication of the Volkerak-Zoom lake system and
Lake Brielle
Lake Brielle
Volkerak-Zm lake system
predicted 1987 L988
total P
(mgfl)
transparencp approx. 1 approx. 1 1 2.2
(m)
!t%e general tendency of the forecasts was that the Volkerak-Zoom lake
system, with 8 residence tinle of 2-3 months, would develop into a
eutrophic water hody with a chlorophyl level of 60-100 mg/ms, a total
P-level of 0.20-0.35 mgll and a transparency not exceeding l m. It is
not possible to compare the actual situation in 1987 with the
predictions, because iri that year there was a sharp fall in the
chloride level during the growth season and hewe there was no
equilibrium situation. Rowever, the values measured in 1987 fall within
the range of forecast values.
The situation ia 1988 was clearly different. That was the first year in
whi& the lake was largely Eresh water: the measured values were far
more fwontahle than predicted. The mean sunmeer chlorophyl level was
approzimately 20 %/m3, tbe tQtal P-level was 0.17 mg/l and the mean
transpan&ncy was 2.2 m.
For cqarison, the values measured in Lake Brielle have been included
in Table 3. This lake is reasonably comparable with the Volkerak-Zoom
l& system in terms of morphology and residence time. The meam
chlorophyl Xevels in Lake Brielle are relatively low, but the

occwrence of floating mats of blue-green algae often causes a nuisance
in tMs lake. The mean transparency does not exceed 1 m.
Figure 6 shows the changes in fhe chlorophyl level in bofh parts of
the lgke system in 1988.
' Laks
VoLsrak
---- Lake zom
-
0 6 TO 9 6 20 26 30 85
week no.
Figure 6 Chlorophyl level id take Volkerak and lake Zoom in 1988
From this Figure it can be seen that peaks of up tn 100 mg/m9 o a r tn
the s~ring. There fs an abrupt downturn at Che end of &lax/he~inning of
June, and fn the reat of the summer the chlorophyl levels remain very
low - especially in Lake Volkerak.
The next two Sigures (Fig. 7 and 8) show the changes in the total
P-level and the transparency.
There was a clear decrease is the level of total pho~gharrrs in the
spring. In the perirvd of the sharp deeIine in the ehlorophyl level
towaras the end of May there is a clear increase in traneparency, first
in Lake Polkerak and later Ln Me Zoom too. In hrvth lakes a
transparency of alnurst 4 m was measured in August - a unique situation
fos fresh water in the Netfierlds.

Laks
Volkarek
---- Lake Zoom
0.40
0 5 10 16 20 25 30 35
week no.
Figure 7 Total P-level in 1988
Lake
Volkerak
---- Laka Zoom
O L ~ " ' ~ ~ ' ~ ' ~ " ~ " " " ' " ~ " " ~ " ' ~ ~
0 5 10 16 20 26 30 S6 40
weak no.
Figure 8 TrBmpareucy in 1988

Figure 5 show the numbess of Cladocera (an o~der of zooplankton which
includes the water fleas) in Lake Volkerak and Lake Zoom in 1988.
The unusual rransparency can be atfributed to the intensive consumption
of algae bp zooplankton.
- Lake ---- Lake zoom
Votkarak
260
zoo C
1
0 5 l0 16 PO 25 3 0 36
week no.
Figure 9 Numbers of Cladacera in Lake Volkerak ad Lake Zaom in 1988
The fact that ths group of aooplanktm peaks in W/June indicates
that the clear deel%ne in the Biomass of algae may be caused by intensim
grazing by the zooplankton. Although not sham in Biguxe 9, during t b
reat of *he stumer it was striking that e relative17 high n&r vf
large water fleas were present. 3usr these water fleas can create an
effective grazing pre$sure on algae.
In turn. the large nuwbers of water fleas e m be explained by the still
limited numbem of plankton-eating fish.
Floating mats of blue-grem algae accured very locally L& Volkexak
and Lake Zoem in autumn 1988.

4 WAGBEET MEdSIIRaS TO PEEVBT OR LTXIT TAE EUTROPEICBTION OF THB
VOLKERAK-ZDOM LAKE S%STEiT
A large nurober of goals have been formulated Tor the management of the
Volkerak-Zoom lake system. They include the prevention or reduction of
eutrophicatian. Two types of IMma@ement measures could be used to
achieve this goal:
- reducing the nutrient load;
- acteve Biological management.
It mnst he stressed Ulat these are nat afternatlves. Active biological
management eaanplemencs the reduction of nutrient load.
4.1 Dealing tdfh nutrients
When reducins the nutrient load the accent will lia on phasphate.
becatwe in the fresh waters in the Netherlands this Es the natural
limitfug faetor to the grovth of awe. For lakes like Lake B~rieLle and
the Volkeritk--Zoom lake system che pfissphate is only limiring Bar algal
growh if the phosphate load is below 2-4 $Em2 per year.
Frm Table 4 it cen be seen fb% the phosphate load of Lake Bridle has
been drasticallp reduced since 1972. Uovever, chia has not yet led to a
reductian of the eutrophieation preblem. The fact that the bed of this
lake has meanwhile been accumulatFog phospliatea that, in prineigle,
could again cause an iaterna-l lead, is ullquestionably impartat hers.
Zn this lake, largo-seale occurrences of drifting mats can be limited
ts some extent by 5nterim eera flushing in the sumer period.
In the Volkerak-Zoom l&e system the phosphate load has turned out
10- than the foreoast, but in l988 it was still far too high eto he
limiting £er the growth of algae. The option of extra flushing to
cwbat drifting mats is not considered hero. because tt conflicts with
anothar goal of water management, i.e. trying to prevent the water
systw~ from being loaded with toxic Substancee from the Rollands Diep.

Furthermore, it is questionable whether such flushing would be
effective, given the shape awd size of the lake system.
Table 4 Phosphate load in Lake BrieUe and the Volkerak-low lake
sye tem
Lake Brielle
1972-1976
1980-1982
1983
1987-1988
Volkerak-Zoom
Lake system
preaicted
1987
1988
guide value
P-linitation
Table 5
Phosphate load in the Volkerak-Zoom lake system in 1987 and
1988
m3/s ton P ms/s ton P
Volkerak sluices 28 250 16 140
Brabant rivers 15 460 12 240
Other 5 60 5 60
Total 48 770 33 440

Table 5 gives estimtes of the phosphate load originating from the most
important sources in 1987 and 1988.
The difference between the two years for the Volkerak sluices is caused
by the extra amount of water needed to desalipFEe the lake in 1987.
MW& less phosphate was brought in by the Brabat rivers in 1988 because
in May 1988 the effluent from the sewage Ereatwent plant at Bred&
Nieuweer, was diverted from the Dintel to the Holland6 Diep. When, in
the future, water is extracted from tbe lake for agriculture, the
phasphate load will be higher hficause more water viz1 have to be
sdmirted via the Volkerak sluiees.
The phosphate load in the Volkerak-Zoom lake system could be reduced
by:
- diverting water from the Nieuwveer sewage treatment plant from the
Dintel to the nollands Diep (this wes done in Bay 1988);
- the Rhine Action Plan (RAP);
- the North Sea Action Plan (NAP);
- optiolizing the intake from:
t water losees from locks;
* agriculture;
- treating intake water;
- rsducing agricultural emission*;
- reaching agreement with Belgium on the reduction of the load in the
Belgian part of the Dintel catchment;
- drastic dephosphatiuation sewage treatment plants.
It is expected that the implementation of the Rhine Action Plan and the
North Sea Action Pla will result in a halving of the phosphate load in
the Volkerak-Zoom lake system too. However, this will nor happen before
1993.
If the almve-mentioned measures are implemented vigorously. it seems
possible that phosphate limitation for algal growth could be achieved
in the medium term; in the short term thb is not expected.

4.2 Active biological management
If it is considered desirable to have clear water in the next ten years
too. then "active biological management" could be emplop3.
Furthermore, experience elsewhere (e.g. in the Veluwe lakes) and
computer modemg btdleate tbat Sr is difficult far a lake that is
already in "turbid equilibrium" to be made clear again.
/
transparency
algae
+++i-t+++t+U
/--L +++*M
+
asuatiq plants
++-+tit+
\/
preatory fish
+++CH*H+++++
nUttianfs
'2w00000~
l *&a,
wblteeirih
-----
MOOOWOO~O
aemal state si Vol%asak-%oon lake aystem
autrophic rster
o m mixed agsten
+++c healtay water syseem
- path of influence er flaw of mather
HignEe 10 Schematic representation of ecosystem relatianahips in a
autrophic water body
Bigure 10 shows the most important relationships between the various
components of a lake eeosysfem. Nutrients enhance the growth of algae;
algae are eaten by zouplaakton; zooplankton are eatea. bp whitefish;
+#kitefish are eaten by predatory fish; pre&cory flsh, particularly
pike, requite aquatic plants fot shelter and for breeding, atld the
occurrence of aqnatic plants depends on the transparencg of the water
axid thus also an the absence of algae.

In principle, two states of equilibrium are possible in such a system.
One is an eutrophic, relatively turbid system dominated by marry
nutrients, many e a e and m y whitefish; this is indicated by - in
Pisure 10. The other possibility is a healthy functtoning system with
transparent water, many aquatic plants, a predatory fish populaticn
that keeps the whitefish population in check, and sufficient
aooplanktan to be able to grase the algae effectively. In Figure L0
this is indicated by +Wt.
In 1988, contrary to all ptedietions, the Volkerak-Zmom laka system
contained elemnts from both passible equilibrium situatians - many
nuttients, transparent water and m y zooplankton: this is indicated by
ooa0 in Figure 10.
In 1988 aquatic plants started to develop, but not sufficiently ta
offer shelter and a breeding enviranmeot for predatory fish. The few
data available on stocks of fish suggest that the bream, a whitefish
that often causes problems in other eutxaphic waters, is not present
here in large mumhers. Predatory fish, especklly pike-perch. are
present in the system.
The most obvitlus way of acaidy intervening in the biology 05 the
system is to stiPtulate the develspmnf of aquatic plants as much as
possible. Few propagules of freshwater aquatic plants vtll be present
in ttte system, and therefore it is advisable to introduce material from
the desired species into the system on a large scale. It is also
important to ensure the presence of sheltered shallows, because in
princfple here it is that aquatic plants can develop the hest on q
large seal&.
Proper management of fish stooks requires agreement between the water
authority and the persona with fishing rights. Quantitative wnitori~g
is necessary to be able to properly anticipate developments towards an
excess of bream.

Floating mats of blue-green algae (Microcystis aerigunosa)
Benedensas, before the enclosure of Lake Zoom

Zooplankton (Alma affinis)
Pike (Esox lucius) in i ts natural habitat

Breeding areas for pike can be provided by the large-scale installation
of defences in front of the banks and by -g
areeh suitable.
Xf d tor$ng shows that the numbers of bream threaten to become
excessive, then predatory fish could be introduced, or whitefish could
be qetted and removed.
Stimulatiu~ the growth of freshwater mussels is a third type of measure
appropriate to active biological management. These mussels are net
shown ia Rigure 10, but the^ fulfil more or less the same role as the
zooplankton: they fflEer suspended matter from the water and thereby
also contribute to the grazing pressure on the algae. Mussels require a
hard substrafe. This could be provided by dumping large amounts of
shells in suitable plaoes.
5 CONCLUSIONS
In general, the dwming off and comparcmentalizatim of the Delta
wters leads to stagnant or semi-stagnant waters whose very location
m&w them ausceptihle to eutrophication prohlw.
In she Bollwds Diep/Bazingvliet eutrophic+tion problem can be kept
within limits by feeustug management on preventing long residence times
fn the saer. The critical level seqms to be arousd three weeks.
Ln tbe Volkerak-noom lgke system the startiag pobt is favourable: the
water is transparent. At present the phosphate level is too higb to
lMt eutrophication, hut active biological magement offers good
prospects for keeping the water transparent. The phosphate load needs
to be reduced fnrther. It is likely that in the next few years bluegreen
algae will occur regularly in the autumn. It will probably be
possible to tackle the symptoms of thie in areas tlar suffer grerrtly
from the problem.

EUTROPHICATION OF ESTUAAIES AND BRACKISH LAGOONS IN TW SOUTII-WEST
NETHERLANBS
P.R.
Nienhuis
Eutrophication of the saline waters in inhe South-West Netherlmds is
mainly nlrc caused by water fromthe rivers Rhine and Meuse, contrary to
the nutrient enrichment of freshwater bodies (Nfeuwe Waterweg, Hollands
Diep. Aaringvliet) and the western Wadden Sea. Eutrophieatim of the
Westerschelde, Oosterschelde, Grevelingenmeer Md Veerse Mar is
connected to the specific hydrogra~hy and geomrphalo~ of these
isolated waters; eacept for the 'GteSterschelde, nutrient loading mainly
occurs thraugh agricultural run-off and drainage channels. The
lXesterschelde, a heavily euthrophicated estuary, he5 only an
insigz~ificant impact on Dutch coastal waters, owing to the very limited
discharge (4X of W 4 discharge) and long residenae time of the
warensass. The resilience of saline waters £er eutxophication by
nitrogen is presumably c~nuected to the psocess of denitrification.
removing N to the atmosphere, and the filter fee&g
actidcies of
bivaLves, depositing N on the botm sedimpnt, and enhgneing tke
regeneration of nutrfents. Low N-load map be sompasated bp
denitrification. We vulnerability of stapme, saline waters with a
long residence time is expressed by the fact that a l~ad of
approximately 10 g N m-$-'
or more presumably exceeds the resilieace
capacity of the ecosystem. The contrast between the ereveliagenmeer
-2 -1 -2 -1
(load 4 N m y ) and the Peerse Heer (load 34 g N m y f is
significant; next to highly productive phytoplankton, macro-Algae play
a dominant and unpredictable role in the carbon budget of the Veerse
Meer lazeon.

Eutrophication of marine coastal waters and estuaries is increasingly
regarded as a nuisance for the near future (Bosenberg, 1985).
Eutrophication is defined here as the pzocess of increasing
concentratian and load of inorganic nutrients, inducing changes in the
aquatic communities. In general terms the large European rivers provide
the main stLnuLus for nutrient enrichments in coastal waters. The main
rivers loading the surface waters af the South-West Netherlands are
Bhfne, Mense and Scheldt (Fig. l].
Figure 1 The Sduth-West Netherlands with the estuaries Oosterschelde
and Westerschelde and the brackish lagoons G~evelingenmeer
and Veerse Meer. The Bieshesch. Eollands Diep, Karingvliet
and Nieuwe Watemeg form the lower reaches of the Rhine and
M~use. The Westerschelde estuary is connected to the river
Scheldt. The Delta Works started in1960 ancl were finished 19
1987. The Volkerakdam was closed in1969 between Eollands
Diep-Haringvliet and Zoomeer. and blocked the main southern
exit of Rhine water.

The average discharge of Xhine and Xeuse together is 2538 m3s-I
Ruyter, et al. 1987) and the nitrogen (ammonium and nitrate) load of
these rivers has increased over the peried 1950-1980 with a factor 2-4,
whereas rtre phospkorus load has increased with a factor 5-7 (Van der
Veer, et al. 1988). Contrary tn the rtitrogen load, the phosphorus load
decreased slowly after 1980. The concentrations of plant nutrients
tncreased even nore markedly than the load: a fivefold fncrease for N
and a tenfoLd 3icrease for P, with recently a slight decrease (Pig. 2).
The increased concentration6 of nutriant.8 in the Bhine-Meuse resulted
in a 3- to 5-foLd rafse of N-
(De
and P-concentrations in the Dutch coasta3
waters. The increase of nutrient concentrations an3 -loads in the
Wwterschelde ebtUary showed the same dramtic trend over the past 30
to 40 Bears (Van Buuren. 1988).
Figure 2 Nitrate- and orthopho9phate concsntrations of Ehinewater
near the Dutch-German border, durins the period 1966-1982
(lUWA Bnnual Report 1982. in Slooff, 1983).
Approxisrately 15% of the mine run-off flows Suto nartherly dinecticns,
aYa IJssel and 'IJsselmeea, ad fiirinlb enters the wwtern Wxdden Sea,
causidg oonaiderable wthrophi~ation to chat estuarine area (Van der
Veer, et al, 1988). More than 50a of the EWnewarer mns via tbe Niewe
Uatemeg (Fig. 1) direct13 into €he North Sea. &bout 3QZ flows into the

Baringvliet and finally also enters the North Sea, depending on the
management regime of the Wingvlietsluioes (Pi$. 1). Owing to the
residual currents alung the Dutch coast the Rhlne wafer is mainly
Kra9spo~ted in a 51) to 70 kilonreters wide zone iiito mrtherly
directions (Fig. 1). The imerage nutrient loadings of the Dutch caastal
Figure 3
Flop of Rhine-Meus* water along the coasts at the
Netheslands. (iermaay awd Denmark; SW Mnd 4.5 m S-', aarer
mass fractions in %, age distribution indieared as solid
lines in dars (derived fzool De Ruyter, et al. 1987). At NW
winds a maximum fractian B£ 10% of Rhtne-muse water eaters
the Ossterschelde estuary.
watera reflect the discharges of marine and riverine water: the Rhine
wd Meusa (including the Noordaeekanaalf contribute raaghly &OX of the
total N- and P-load. the residual ctlrrent of Chann@l vacer adds 90Z

and only l-2% cows frm the aive Scheldt (Pig. 4). The Scheldt has an
averege discharge of only 112 m's-' CVe Ruyter,, et al, 1987) which is
only 4% of rho Rhiae-Meuse discharge.
Total nitrogen
Totai phosphorus
ennel
37 %
Rhine / Meme 57 % Rhine Meuse 16 %
Figure 4
The ahares of Rhine-Ueuse load, Chamel load aqd Scheldt load
in the rota1 eanceqtratiop aP N and F Dutch cpastal waters
(Van Buuren, 1988).
Only a veky small pereentzse of the original We-Mewe water reaches
directly (or indirectly via the North Sea) the estuariee and brackish
Lagoons in the South-West Netherlands. Especially the construction of
the Volkerekdam in 1969 (Fig. 1) deprived the saline waters of the
direct influx af fresh river-w-ater. me Oosterse.helde, Grevtrlingenmeer
and Veezse Meet are mainly loaded with nutrients from dSSfuse Bourees
such W apicrienral run-off, treated vasce water and drainwe canals.
The saline water bodks in the South-KeSt Netherlands have been
separated spatiallq fcompartmentalization) ovlng to the Delta Works.
Cmseguentlg each of these waters has its oun eutrophicatim history
and its orm sperffic ehareteristics, excluding, an integrated epproach
to manage eh= nutrisnt 10.oading~ B$ these syatema.
Table 1 sumeariaes a hlmmer of ~paLem peameters. The residence tines
of the uater masses Zu tke stagnant, nen tidal OreVdingewer and
"ieerse Beer are long, cwmpared to ch8 saate cha+ac+etistic in the tihl

estuaries. The net freshwater load directly from the rivers Rhine and
Mwse is extremelp small (1% of the discharge). Tba Veerse Meer lagoon
eaperiences almost permanent stratiEicaeion, whereas the
Oreveliugenmeer has only a few deep channels, stratified during summer.
The Westerschelde and Oostersehelde estuaries are completely &ed
tidal systems.
Table 1
System parameters of the saline waters in the Swth-West
Netherlands, derived from Wollast, 1988, and Bokhorst, 1988,
for the Westerschelde; Projectgroep Balans, 191d8, and
Wetsteyn and Peperzak, 1988, for the Oosferschelde; Nienhuis,
1985, and De Vries, et al, 1988, for the Grevelingen: Daemen,
1985. and Stronkhorst. et al, 1985. for the Veerse Meer. Load
= direet water load from Rhine-Meuse.
reddenGB time (4)
bad h%-1,
tides
stFati£icatlan
extinct-.
(m-')
The Westerschelde is extremely turbid (extinction coefficient 0.5-7)
and the Grevelfngenmeor contains very clear water (extinction
coefficient 0.2- 0.5). with the Oosterschelde and Veerse Meer in
between.
The range in nutrient concentrations in the saline Delta waters differs
greatly (lfig. 5). The Oostersehelde and Crevelfngen reach only seldom
for N. P and Si; nutrient values frequently
values above 1 mg I-'
approach zero concantratione during heavy blooms of phytoplankton.
The Veersa Meet has higher maximum values for N and Si, but depletion
occurs during the growing season. The Westerschelde has far out the

highest trophic potential: nutrient concentration values never approach
zero during spring a~d sumer and reach hi&h levels Buring wtnter
(8 mg 1-l N; 4 m$ l-l P and 9 nrg 1-l Si). Compmd t9 the nutrient
coneentratious in the river Rbine (Fig. 2). F-concentrations are wo
tiares lover and N-emcenttations are ten times 1-
water badies (Fig. 5).
exeept far the Weaterschelde,
id the saline
Veerse Meer -N
Figure 5
Range in nutrient concentrations in esfuaries and brackish
lagoons in the South-West Netherlands.
See legend Table 1 for souzces.
The question arises which factox acts as the limiting constituent for
the productivitr of algae in the saline Delta Waters. In the turbid
Westerschelde it seems obvious that the availability of light is
limiting phytoplankton dynamics. In brackish coastal waters phospate
may he a potentially limiting factor (Veldhnis, 1987). It is generally
assumed that nitrogen is the limiting factor for phytoplankton
development in the Oosterschelde and Grevelingen and occasionally also
in the Veerse Meer. Knowledge about phytaplankt~n and wcrophyte
kinetics in the South-West Netherlands, however, is scanty. Especially
in polluted estuariee like the Westerschelde other factors than
nutrients and lighc availability may he responsible fox the limitation
of algal photosynthesis. It is now becoming increasingly obvious that
antagonistic effects beween metals on phytoplankton exist, whereby an
organism may tolerate high concentrations of a given metal in the

presence of sufficient quantities of another meral hut not in the
absence or at low concentrations of the second metal iRaven and
Richardson. 1986).
Table 2
Etitrogen and cholorcphyl in Dutch coastal waters, eetuies
and bckieh lagoons (data deni~ed fremDe Vries et al, 1988.
and Smoes - Dos+se?rschelde model cdculatloos; E. Schalrea,
personal communication).
load &eez cone. rhlorophyl cm@.
Nor& Seb (coast.)
Wadden Sea
Westersrhelde
0ostersche3.de
Gravelbgenneer
Veerse Seer
Table 2 shovs a large variatiw in tohd nitwgen loads (4 - 235 g N
-2 -1
m y ) in Dutch coastd waters, estuaries ad brackish lagwns,
whereas N-edncencratims only &a4 a rwe of 0.5-4.6
Obtrioustg, a nitrogen load of 40-200 g N m-'T-'
mg 1-l.
does not giae rise to
extremely high chlurophgl conkentrations during summer, usifher in
North Sea coastal water, nor in the Wadden Sea and the WssCerscheLde
e s t q . It is assumed that l&ght availability is the limiting factor
in these turbid c-tax
and estuarine waters, end not nitrogen,
explaining the relatively low phptoplankton tchlarophyl) biomass in
smer. Contrary, in the clear water of the Veerae Weer lagoan a N-lead
-2 -1
of 34 g m p results in high ~hlorophyl concentrations (l00 mg CB1
m-' or even higher; Table 2). This large productien of phytgplanktm
bioaass ip the Veerse Meer lagoon, and soasequent de~sition of
particulate or!&anic cnrbon on the bottom sedimenfs, resale& during the

period 1980-19$3 in an increase of the anaerobic sed-t suzface area
of 4 ta 258 of she entire bottom awfkce (Stronlrhorst. et al. 1985).
Qbvioualy, the Veerse Meer is vulnerable to eutreghicatian, owing to
the long resid&&e the of water msses, the low extinetian coefficient
and the ;Umost permanent stratification.
The Oastesschelde estuary has a very low N-lead re6nlting in low
chlorophyl concentratisne (Table 2).
Wing to the execwtina af the
Deltaplan (Pig. 1) the Oostarnehelde estuary has mainly been depriVed
recently from its Bhinewater loading (Table 1). Model calculations
revealed that a reduction of 50% of the nitrosen bed of the river
W e
would only lead to a rsduetian of less than 6% for nitrogen
eoneentratlons in the 0oster.jkbelde estuary (Smobs-swdel; X. Scholten,
peraerial co~u~ication). The Uoster~ohelde estuary is rnainly ir;fluenced
by tearhe foastalwffter, containing low conewtrations of total
nrtrogen (less than 0.5 slg N 1-l; Srockolano, et al. 1988).
Contkaq to the Veerse Meer lag~an. the Creveliagen lagoon has an
-2 -1
e%treaely Low N-load (4 g N m y ; Table 2). The CaeWA~mdel (De
Vries, et al. 19BEI) revealed that Khe prodtlrcim of phytop.lanktan La
the Gzevelingen is liodted by nitrogerl availability and not by light
availability. The hl
: P ratio of tae dlsselved nufkiems in cke
watercolm of the GrwsMngenmeer durins winter is 2-4,
poirtcing into
the direction of a large surplus af phosphate, notwithstanding Che
dominance of nitrogen in the discharge utatet, loading the lagoon
Eff : P = 22 : I f. Model calculations showed s Mgh tunrover of ni~rog.~u,
in the watercolumn. The chain of processes: nutrient uptake by
phpoplanktaa - forslation d organic matter in algal cells -
mherallzation of dead algae - regeneratbn of nutrients etc.. occurs 8
tWs a year. &out
90X of the anme1 prisary pr&uction of
phytoplanktoa occurs on the baeis of resewrated nutrients, especialLy
NB4 CDe Vtim, et al. lS88).

In the GZ3WQ-model ~alculatians CDe VrieB, et al, 19881 it is
hypotkerriaed that in the Grevelingen lagoon the process of
eutra@ication
is prevented by the rate of the denitiificatian process.
Dnnit-rifioation is undoubtedly a significant process in marine and
estuarine ecaqstems. Rates of gaseous losses af nitr-en in the range
-2 -1
of 5-70 mg N m d were determined in Bebgi-an, Dutch and Danish
caastsl sedimnts, representing 8-23% of tha amount of nirrwen
dnerdtliration in the benthic subaystein Caillen and Lancelot, 1987).
Notwithstanding the Eaet that no actual measurements exist on
denirrification rates in the Grevelingemeer, it is assumed in the
OReWQ-model that deeitrificatiw compensates the load of nitrogen,
especially in the benthic bonndary layer in the sedismnr, where aerobic
and anaerobic 'patches' o;f sediment join each other.
In mahp sballw estuarine waters the nain Foed chain ia dpminated by
phyto~laakron and bmthir Eilterf eeders (mussels. cockles) , Just as ie
the case in the Greoelingenmeer, Oosterschalde and Wadden Sea. T3e
rumover rate sf nutrients in these ecosystems is deteraELned bp the
filtering capacity of bsnthic filterfeeders. Theoretieally, every 5 to
10 days the entire valme of water of the estuaries circulates through
the filtering apparatus of suspension feeders. Filterfeedem act as
natuzal cantrollers of the eutrophicatian psocess (Office, et al.
1982): they deposit organic material from the watercolmn Dntn the
bottom mdiments. Mnreover, they accelerate the regeneration of
nutrients from the deposited perticulate o~anicarbon, thereby
enhancing the primary produetion of pbytopLimktun, as was assuxned for
the @xevali~enmeer (De Vries, et al. l$&%)
'be chain of pzoeesses - parcly measered. partly cheureticel - from
biode~osition to reaenesation of nutrients and the coupling between
nitrification and denicr%£icatie$n is limited by the load of szgani~
nace~ial ro t&e
sediment. men the increaeing load of ozgulic material
runs faster t4w Ehe grocess of heaerotrophic lotneralization. anaerobic
cQndFeions wlll prevail in the sediment leading to death of bottom
Fauna and a disconnection BE nitrification and dea5trification. GREWAQ-
.

model calculations reveal that a nitrogen loading of approbimately
-2 -1
10 g W m y or more unwuples the eutr@hiulti@n controlling
processes (Qe Vries, et al, 1988).
-2 -1
In the Veerse Meer lagoon, experiencing a N-load of 34 g m y and
haaing a long residence time of the water, permanent stratifloation
mainly in the eastern section and dear water during considerable parts
of the year, obvionsly, the chlorophyl concentration cmot be
controlled by the bottom fauna, although a fluctuating but substantfal
benthic biomass is available.
In the recent literature (Benkema and Cadee, 1986; Cadee. 1956a. h;
Cadea and Hegeman, 1966; Nelisseg and Steffels, 1988) a number of
biologi~al effects of increasing eutrophioatbn have been mentioned for
the Dutch ccoastal waters and the western Wadden Sea: (1) increasing
primary prodwtivn; (2) dominance of flagellates cwer diaroms; (3)
incrwing biomass of benthic macro-fauna; (4) increasing catches
tbimass 7) of demersal fish and shr-S. It ia beymd the diwcussibn
of this paper whether or ~ othese t effects of euthropicatlon are proven
~ausnal facts or jusr assumed tendencies. The question arises whether
the correlations menrfoned for Dutch coaxtal waters can dso be
revealed for the estnaries and lagoons in the Sauth-West Netharlaads.
The answer is negative, for several reasons: (1) long terms series.
6uch as published for the Wadden Sea by Beokema and Cadee. 1987, are
not available for the Oosterscfielde and Weaterschelde esruarieti; (2)
any possible trend in eutrophication of the Delta estuaries has been
obscured (except for the Westerscbelde) by the ==cation of the Delta
Wsrks, changtng the hgdrography of the area drastically. The Veerse
Neer and Grevelingen have their om euthraphicatipn story, starting
respectively in 1961 and 1971 when the basins were cloeed off from, the
sea (cf. Pig. l).
In Figure 6 a restricted dataset on the occurrence of the flagellate
Phaeucgstls poucharii in the weecerh part of the Oasterschelde eahtary
is compared with data fro= the western Wadden Sea. The Wadden Sea data
reveal. a telacian between increasing eutropbicetion and increasing
incidence of Phaeocystis blooms. The Oosterscbelde data show a

Phaeocystis pouchetii
WADDENZEE
OOSTERSCHELDE
Figure 6
Duration of the spring$lonm of Phaeocystis pouchecii
expressed a5 number of days wit3 cenc%ntratbns of cells
above 1000 ml-l, and average concentration of cells,
eqressed as nrpnbez of cells ml-l,
for the western Waddenaee
(tadee and Aegeman. 1986) and the western Oosterschelde (data
rrom C. Bakker. DIHO). For the Oosterschelde: rider af deys:
y = 819-9.178~~ = 0.94399, P < 0.01; cells ml-l : P > 0.05.
significlmr decrease in the lengCh of the petied of the Phaetlcystis
bloom, but aa increase in bloom intensity. Thase partly contzadictory
results question the asition of Phaeoegatls pouchesli as an indicator
for increasing eutrophic*timn of Dutch coastal waters in general. Older
records from before the second World War also reveal incidental heavy
Phseocysris blooms in the southern North Sea (personal comnlcation
S.H.R. hip) which make the position of the flagellate as an indicator
oven weaker.

Table 3 PreUmInary carbon budget of priwry producers in estuaries
and brackish lagoons in the South-West Netherlands (expressed
-2 -1
ihgGm y ).
Westerschelde: phtoplsnkton data detived from Billen, et al.
1988, and Beip, 1988; microphytobenthos data rougbly, based
on blows estimates, DGW, Middelburg; data on above-ground
production of saltmarsh plants - both for the Westerschelde
and Oosterschelde - derived from Gromendijk, 1SI86, an8
Huiskes, 1988. Oosterschelde: phytoplanktan data based on
Weteteyn and Peperzak, 1988, and Vegter and De Visscher,
1987; microphytobenthos data from Nienhuis and Daamcn, 1985;
data on seapassea and macro-algae on soft and hard
substrates - bath for the Oosterschelde and Uesterschelde -
derived frm Nienhuis, 1980, and Nienhuis and Daemen. 1985.
Drevelingen: data from Hiedluis, 1985, and De Vries, et al,
1988. Peerse Meer: data £er phytoplankton Eram Daemen, I985
and Stroxkho~st, ec al, 1985, fer ntlcrophytobenthw based on
best professicnal judgement; daca on macrophpt~s of soft
sukstrates derived fra Ha~ewijk, 1988, and Sybema, 1980,
and mpublished DIE0 re%ords; data on seagrass production
deriuad frem personal codmutication F. Vat, Lent, DIIIO.
-tea babitat sgatw Wfszt sgsW habitat Sysren -C
Total fa. 7.75 ea. Z&O ea. 320 ca. 450

In Tahle 3 a tentative carbon budget of the 4 saline Delta Waters Is
given. as an expression of the transfotmation of inorgank nutrients
into organic carbon of primary pxohcers. Table 3 does not girre
infetmatioa about the turnover of nutrients between the successive
generations of plants. Notwithstazdlng the large dlfkrences in
nutrient loadjags of the separate waters, primasy production of
phytoplankton only shows a difference of a factor 2 between the light
-2 -1
limited, tUrbid Westerschelde (l25 g C m y ) and the clear,
-2 -1
presumably not nutrient limited Veerse Meer (249 g C m y ). The
Oosterseblde and Orevelingemseer hold int-diate positions.
Production of microphytobent!hos (benthic diatoms, gzeen algae etc.) is
roughly one third of the production of phytoplankton. In the
Westerschelde estuary, however, the relative skre of benthic
microphytes is considerahLy higher, based on P/B ratio's derived from
high biomass data, taken on intertidal flats (personal eonrmnaicatFoa
D.J.
de Jong, DW).
OwFng to the relatrvely large surface area of supratidal wetlands, the
contributivn of ahoae-ground peimarp production to the carbon budget in
-9 -1
Westersehelde estusry (60 g C m y is about 20%. The larger part of
the saltmarsh plants is produced in the eastern secrion of the
Westerschelde estuary (Verdrmken Land van Saeftinghe) and is not
transpwrted over the entire estuary, In contrast with the
Westerschelde, the Oosterachelde salt marshes pay only an insignificant
contrLhution (lees thau 5Z) to the system metabolism of the entire
estuary.
Maarophytes growing on bard substretes (i.e. seaweeds attached to
sewalls and stone-clad dilres) have only a minor contriburian to the
carbon budgets of the entire water hobtes, notwithstandiag their high
production per square metre habitat.
Eigh turbidity and exposure to waves and tides prevents the potential
sediment habitats in tke Weatersehelde egtuary from being invaded by
macrephytes, The Oosterschelde estuary has only local growth of
laactophytes an sediment substrams in sbeltered regions (Zandkteek,
Ktabbenkreek, eastern part). In the sheltered, stagnant lagoons

Water life of Lake Grevelingen

Foam of algae as a result of eutrophication
Geld Ulats df the Bas+era BeheLdtl

macrophytes livkg on or rooting in sediment, respectively macro-algae
(mainly green algae) and aeagrasaes, offer a significant share to the
carbon budget. In the Orwelingenmeer the seagrass Zostera marina
dominates, coverkg 30% of the surface area of the lagacm, but
contributing only 14% to the annual carbon bu&et, owing to the low P/B
ratio of 3.
Coverage 1%)
80 GREVELINGEN
seagrass
Losfera marina
GoveraQe 1% 1
90- VEERSE MEER
Sigure 7
Percentage coverage in vertical projection on the sediment of
Zostera marina and mcro-algae in permanent sample plocs in
the Grevelingen and Veerse Meer. She of sample plot is 15 x
15 ma, waterdepth 14 both localitaties is 1.25 m (data
bsrrowed from F. Van Lent, DIEO).
The Veerse Meer &Ears a &ifFerent picture. Seagrassee csver only 3% of
the surface area of the lagoon (IhnewFjk, 1988) and have to compete
with epp~rtunistic green algae (mainlp E) for space and light. The
lagoon is dominated by plva during the smer season, showFng a roughly

estimated primary production of 500 g C mdZy-l in shallow are-. The
contribution to the annual carbon budget of the Veerse Meer is roughly
-E -1
120 g C m y , which is 27X of the lagow' budget. The higb nitrogen
load not only leads to a relatively high production of phytoplanktou,
but also to intensive and undesirable mass growth of =in
areas.
shallow
Figure 7 reveals the inareae and decrease in biomass, eapressed as
perrentage wvezage, in a permaaent saaple plot In the Grevelingeu and
in the Vwse Fleet!. In the Grevelingemeer the annual cycle of cbaagerr
in biomass of seagrass is not disturbed by macro-algae. which have only
a low presence. In the Veezse Meer the seagraes plot becomes cmpletely
damindted by quickly growing-, suppressing the growth of zostera.
Table 3 shows that the eutrophicated Veerse Meer has the highest tmtal
-2 -1
primary production (roughly 4-50 g C m y ) and that the nutrient
limited Oosterschelde estuary has the lowest values (rdu@ly
-P -1
240gCrn y 1.
For Ereshwater ecosystems a tentative model has beau developed,
descrihlng the relation berween the relative domtuanca of primary
producers wmected to the availability of nutrients. and the
successive phase$ in the process of %acreasin% eutrophicatton
(Phillips. et al. 1978; Van Vierssen, et al. 1985; De Nfe. 1987: Fig.
8, upper panel). The model for esruarine and lagoonal situations Rig.
8, loaez panel) is adapted, based on data from the Qrevelinpn and
Veerse &er. The model is odly applicable to stagnant brackish lagoons
and extremely sheltered parts of tidal estuaries. In "healthy" saline
waters, waterplants - such as seagrasses - dominate. Nitrogen load and
concentrations are lm and the relative importance of phytoplanktm in
the shallow seagrassbeds is iusigniEicant; the Grevelisgeumeer is an
example of phase I. Zn brack3sh waters where eutropbiearim increases,
revealed by higher nitrogen loads and concentrations and lower,
instable salinities, waterpLants am outcompeted by macto-algae.
Epiphyte gowth gn seagrass-
increa~es considerably together with the
relative dsmtnanee of pbytoplankfou. The resilience of the aquatic
cmumnitied decreases, which makes the system less constant in time end

less predictable for water managers; the Veerse Meex is an eraple of
phase 11.
Relative dorninonce
l SALT WATER I /
I
n
I II m
EutPophlccltion phase
Figure 8 Tentative model for the relation between primary producers
and nutriats and the snccessive stages in the eutmphication
proce6s. Explanation see text.
In hypereutrophicated systems (phase 111) heavy uncontrolled
phytoplilnkton blooms alternate with mass growth of macro-algae.
Nutrient concentrations are continuously high. Rooting waterplants hwe
completely disappeared. Bottom sediments suffer from permanent anoxia.
The lagoon of Venice is an example of phase 111, where E bimass
-2
reaches values of 1.5 kg dry weight m (Sfriso, et al. 1987; Sfriso,
et al. 1988) which is 6 times higher as in the Veerse Meer.

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genoeg of a1 te -ell Vakbl. Biol.. 67 (9): 153-157.
BILLEM, G. and LAhTOELOT, C., 1987. Modelling bemhic microbial
processes and rheir role in nitrogen cycling of taperate coastal
ecosystems. In: E.T. Blackburn and 3. Sorensen (editors) -
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pp. 341-378.
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1988. Modelling
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XBE CHANGING TmAL LAMXiCAPE I N TEE DELTA ARM OF THE SOUlX-W'ST
N-DS
J.P.M.
Mulder
Implementation of the Delta Prpject has initiated a procesa of major
changw in the tidal landscape of the South-West Netherlands.
The win geomorphological developments are described in comparison &h
the historic evaluation of the Delta area and in eelation to changes of
the mjm natural driving force*.
Channels. sandy shoals, mudflats and salt marshe3 make Up the
characteristic elements of the landscape in ~e {formerly) tidal waters
of tbe South-West Netherlands. Over the last decades thls lmddcape hhs
became subject to dramatic chases. Main causes: implementation of the
Delta Project, aimed at securing the South-West Netherlan&s against
flooding, and in the Westerschelde, large scale dredgfng for the sake
of free shipping t9 the harbours of Bntwerp. Consequences: the coherent
geomorph~logicnl system of ebb tidal deltas, tidal. inlets and
estuaries, has beep divided into several were or less hiependent
subsystems. The tendencies towads a dywmic equilibrium between tbe
tiW landscape aad natural driving forces - tides, waves and fluvial
currents - have been dibtnKM.

In this paper (Section 2) first the historic evolution of the tidal
landscape is analysed, resultbg in some 'hypotheses' M general
response mecbnism to chaqges in the natural driving forces. !hen
(Section S) the major changes of the tidal landscape in the respective
basins of the Dutch Delta area over the last decadee are broadly
reviewed. The observed phenomena are confronted with the general
'hypotheses' of Section 2 and discussed in relation to the specific
shift in natural driviog forces. Flnatly a short outlook on future
developments is given.
2 EVOLUTION OF THE TIDAL LANDSGAEE DURmG RISTORiC TINES
2.1 Kelinium and Oosterschalde
During Roman times the coastline of the South-Vest Netherlands only
showed two main tidal inlets (Fig. 1).
At that time the northernmost inlet, known in Roman writings as
Aelinium, was the main distributary of the rivers Rhine and Ueuse, and
partly of the river Scheldt (Zagwljn. 1987). During the next centuries
the importance & this estuary continually diminished, in favout of the
development cf the sarly Haringvliet, GreveBgen and Onsterschelde
estuaries.
Figure 1 The South-West Netherlasds 2000 B.P.
circa 1550
(Zagwiju. 1987) and

The storm surke of L&Z1 A.D., knorm as St. ELisabeth's flood, had a
big impact in this respect. It caused a drastic change in the courses
of the =in rivers and conaiderable losses of laud (Rig. 1). Due to
these land losses - the extensive tidal marsh lands of the Biesllosch
originate from this flood - the tidal prisms of the early Haringvliet.
erevelingeu and Oosrerschelde increased considerably, resulting in a
further deepeniag and widening of these basins. The former Eeliniua on
the contrary showed a decrease of tidal prism and as a result silted
up. This pzocesa of siltation still was continuing during the 19th
century, lead* to the need of coustmcting the Botterdam Waremay
(1868) fm order to ensure aecess to the Botterdam harbours (Van den
Berg. 1987).
The Qosterschelde inLet hardly did not change position since Roman
times. Bowever, the shape oE tbe estuary ewersince has changed
considPrably. Originally the Oosterschelde was the main distributary of;
the river Skheldt. From about 700 &.D. the mere southerly Westerschelde
estuary developed from a small tidal inlet, during Rman times located
approximately at the present Dutch-Belgian border (Rig. 1; benders.
19861. The Westet~chelde gradually took over the discharge of the river
Gcheldt. At the end of the 16th century the Oosterschelde practically
had lost all importance as a distributary of Scheldt waters.
Revertheless, in contrast to the silting up of the Heliniua after
diparsim of the mHin courses of the rivers Rhine and Pleuse, the
Oostersmhelde showed intensified erosion. An explanation for this
renarkable different behaviour Q the dramatie increase of the
Oostersehelde's tidal prism due to extensive lwd losses. Apart from
the effeers of the earlier mentioned St. Wsabeth's flood. it 1s
estimated that as a result of another notorious storm surge
(St. Pelix's flood of 1530). the Ooseerschelde basin was enlarsd to
such extend that it's tidal prism must have increased by at least 50%
(Van den Berg, 1986). The erosive respouse to this inczease of tidal
prism still is visible during the 19th century CFig. 2).

Figure B
GnDSs :sectiun~2 df the '(108tersche1Be falet h in&erent
p'er.Faak, i1l~afzat.i~ eh& &aep.egiq@ of &amrIs md ,expaneaon
M
shoals (.Van BeetpX, 1877; Bm., 19XI
ltwex, tha ezqsboa preews,. dus4ng. cWs piad Camat he eqlaiaeB &S
!a s& +W$?tion :CO tke meB*aL Saad Z@ssps. DwtW the lStIi aM Wi3i
cwt- 6he tidal .prig@ of the .WterSchaLde Xilm-remed -Bet due td
.a penias ,eiuil ,.cag.%wez&g &S. ewh fwti@iae a m1.e U&me
pmpapaZb af tib CZs8le 1;. p9rs .den Ba*, 1987; ,%a Vq&cSel, 1977).
Table 1
Ciail eu@kaering marks fn the l.a&wexd pwfs vf the
@eseerach&Cie IW34370 M. 1978)
1851-1885 construction of the Nieuwe Mewede
1888-1907 construction of the Bergse Naas; normalisation of the Amer
1931 construction of the Bellegatdam
l965 construction of the Grwelingen Dam
1969 construction of the Volkerak Dapl
1850- dreQing and saadnhing

osi*
rn@8IOIP
m .SL
4 Has
F~~ULB 3 Dpp~sitie @~pee3s Xn Ooster~whelii& has%n m& etjb eim delta
11;@?4-lbtS),, and xyt sadit+aqtat~&w wf gboals fmone betwem
level MS& 4 mm ht&h vacer slack in an
8werall cz&,&g B~sin Crib@, K?TS.j V.- Ocehgel, iSTT1
2.E Eypothe$es and preliudnaq ConcIuaions
Clraoges in tidal valume play a key roIe in the geamorphoLogica1 history
of the South-West Netherlands. The well documented changes of the
Oosterschelde ovet the laet 100 reaxe cleaely illustrate the general
r=ZSgOBSe of mdy estuaries to an increase of tidal volume:
- the basin as a whole ahovs a tehdency of arcsion: at the s a time
the connected ebb tide1 delta is growing (Fig. 3);
- morphological differences within the basin are accentuated; channels
deepen, sandy shoals expand (Fig. 2 and 3). In general this response
might be visualized as given in Figure 48.
The history of the Eelinium shows the general response to an opposite
trend in tidal volume: the basin starts silting up. Unfortupately, in
contrast to the Oasterschelde, detailed infomatlon 04 the historic
development of iieliniwn's ebb tidal delta or of single channels and
sandy shoals is lacking. Nevertheless, in negative aualogg to the
Oosterschelde case, rhe effects of a dscreaae of tlldal wlww nilght be
stated as:
- the basin as a whole shows s tendency of sedbentatia4; the
connmted ebb tidal delta is erodlllg;

- wrgbolegical differmes wititin tk hasin axe smothed; channels
silt up. sandy shoals degrade (Fig. 4%).
A.
tidol volume INCREAf;E
8
tidol volume DEGREASE
Figure P
eeneral respame mechanisms of the tial landsrape in cmcel
and estuarine envirgnnenrs to chmsas Ln the tidal volnme
l'hesn wtatemencs on general response oecbariisms pumide weful
'llypoFpoMeses' eo be tested on observations 03 the major chaees fn the
rMaI landscape over tBe last 30 rears (Section 3).
The respanae mechmism no change* of tide1 volume trends to create a
new dgnsJaic equillbbl-ium hemeen tlie tidal volume end the shape and
dimeasions of the tidal landscape. The comtinuing sSltatiun of the
hLelL&tlht and erosion of the 0ostere;ehelde during the 19th centruy, Cn
respanse to dsastic charrges of their tidalvolme &ring the 15th and
16th ceutlvry (see seation 2.1). indieaces abe time scale sf
geomotphologfcal remetlens: a net? dwmic equilibrium takes centuries.
The tnqract of hum iqterfm-e
hecornea Fncueasingly appirrent tiuough the ages.
on deu@lop@ents of the tidal landscape
Duriry: the Late Miadle Ages man was a mjor cause of the land losses
alwg the Ealiaim and Oasternobelde es';narl.es.
The constlvction of
palGers (sinee &ant lIOD RD.), the artificial Lmprovel~e~~t of dr&age
and extensive salt srining initwed large scale lowerinrg of the limd
level. Consequently the land beeam a more
prey to storm ~uzges
(Van den Bezg. 19863. Man indirectly initiated major changes in the
tLd& voluures P£ the estuaries.

During the 19th century man starts interfering directly by engineertns
works of ever increaslag dimensions. me last decades are marked
h W
fnfluence of mprecedented scale and Fntensfty.
a
3 MAJOR CHANGES IS TEE TJDAL LANILSCAPE OVER WE LAST nEGaDES
3.1 Different categories
Pfter empletimn of the Delta Project in 1987, the waters of the South-
West Netherlands may be divided into faw categories (Pig. 5):
I
L
1
Figure S
The South-Ueet %echerIands after cwple€ian e? the Delta
Prnject
- category BaringvlTet: basins wit% wly a closure Bern at eke sward
side, almDst totally discomscced frmn tidd iafheaces but nick a
substanrial direct fluvial inflow (Earfngvliet - HolMsch Diep,
Brlelse mer) :

- category Grevelingen: basins with closure dams at hn, sides,
disconnected from tidal influences and without any [Qrevelingenmeer.
Veerse Meer) ar with very limited fluvial influences (Volkerak -
Zoommeer) ;
- Oostecschelde: a basin with a half open dam at the seaward side and
closure dams at the lahdward side, with reduced tide and
disconnected from fluvial inflnences;
- Westerschelde: an estuary 13 the true sense, with both tidal and
fluvial imflueuces.
Construction of rhe seaward closure dams has disrupted the former
morphirlogical uncity of estuary and ebb tidal delta into two distinct
subsysrems: a basin and a (relict) ebb tidal delta.
In general, tidal currents and -ranges have been reduced and fluvial
influencee limited, while wave impacts have remained unchanged.
3.2 Category Haringvliet
Within the Haringvliet hasin fluvial currents and waves are the only
geomorphelogicalIy active forces left after fiiaishfng of the
Harfn~vliet nam in 1971. The tidal Emge has been diminished from ca.
2 m to a negligible 0.2-0.35 m. The ahst total disconnection from
tiaal iniluences has kmcked large scale sedimentation in tba channels
(Pi&. 6) and erosion of shoals and mudflafs; the shorefaces of several
Figure 6 Channel sedimentation in the Haringvliet-Hollandsch Riep
basin over the period 1970-1983 (Van Otterloo, et ill. 1987)

mudflats have retreated some 100-200 m within 10 yeare (Van Otterloo.
et al. 1987). Cenerally these developments agree with the hypothesized
changes according to Figure $B.
Sedimentation of tke channels aainly is due to sand and mud transported
by the rivers. The change in chnnel depth (Fig. 6) indicates that the
channels .successively silt up frbm East co West. The siltation process
will contfnue until all over the basin a new equilibrium has been
estahlibhed between the discharge volume and cross seotional area of
the thannels.
Erosion of the mudflats is caused by waves. Now that tidal waterlevel
variations M longer exist, we attacks are concentrated in a narrow
zone; at the same time this erosion no longer is compensated by
sediments deposited by tidal currents during high tides. The result is
an increased net erosion. Dnril 1988 the shore* of two areas (Beniager-
and Korendijksche Slikkep) effectively have been protected against
erosion (Van Otterloo, et al. 19871. A progaam has been defined to
protect all remaining dais and raudflats before the year 1994.
Since 197L tidal currents on Baringolfet's ebb tidal delta mainly are
detefnined hy the North - South directed tidal wave on the Narth Sea.
East - West direefed tidal currents, previcusly flowing into and out of
the estuaq, haoe reduced dramatically. Bluvial currents nowadays are
restricted to periods around low water slack, when excess meter from
Meuse and mine is being discharged through the Haringvliet sluices.
Wave activity has not changed significantly over the last period.
According t~ the hypotheses of Section 2.2. a reduction in tidal velume
of rhe estuary generally will induce emsion of the ebb tidal delta and
sedfmentgtion of the tidal channels. Obeervationa of geomorphological
developmenrs since L971 (Fig. 7) generally confirm this picture.
ErosiQn of the ebb tidal delta is nest obvious aloqg the deltafront
aroudd the NA? -5 m depth contour (Fig. 7). Here the landward directed
Vdve 'induced sand transport no longer is cmpensated by sand traneport
in a seaward direction induaed by ebb tidal currents: the result is a
net erosien of the deltafront. The eroded sediments are deposited

partly in longshore sandbars at the edge of the ebb tidal delta, and
partly in t?a? channels, especially where these have lost their role as
distributaries of ehh currents £ran the estuary.
It is to be expected that these processes will continue in fhe future
at a reduq rate, until a new equilibrium has been established. The
longshore bare, stagnating at a height level of araund mean sea-lwel
(NAP), will continue to stretch in a direction parallel to the coast,
at the same time moving further landward (Kohsiek, et al. 1989).
Figure 7
Development of the Haringvliet ebb tidal delta over the
period 1970-1980 (Van der Spek, 1987)
3.3 Category Grevelingen
After constructian of the Grevelingetl Dam (1965) and the Brouwers Dam
(1971). the G?3?Tellngen basin mat be charracterized a a stagnaat
saltwater basin, without any tidal ot flwiel influence. Waves are the
only ge~morphologically active drfving force that has been left.
Theoretically changes of such nature will induce erosion of the sandy
shoals and mudflats, and sedimntation of tbe ehannels (conform
Fig. &B). DevelDpmOnts over the last 20 years broadly canfirm this.

aq811~33a.e~ pue 3pTatlJS uxaJsafi

Dry shoal with gully

Erosion by waves of sandy shoals (conform Section 3.2) was very oomon
from 1971 onwards; shore faces retreated at a rate of maximum
10 mlyear. After most of the shores successively have been protected
against sroeion, Che total loss of shoals and &fiats in L988 has been
calculated as 54 ha or 6 percent of
(Leeuwestein and Schoot. 1988; Fortuin. 1989).
the original total area
Although the fomer tidal channels tend to silt up, hardly any
sedimentation is observable since the only soure of sediWut,
represeated by the ssudy shoals and mudflats, laqely hae been
protected against erosion. Comeqnently future changes of the basin's
tidal landscape vul be very limited.
The sbift in driving force on GrsYelingen'~ ebb tide1 delta since 1971
largely is comparable to that previously described at the Hatingvliet
(see Section 3.2). As to be espected a similar motphoLogica1
development initiated by similar mechanisms has been pbserved. Erosion
of the deltafront related ts an expansion ~f sandy shoals and silting
n~ Q£ foruer tidal channel ever the last 20 years clearly is
fllustrated by Fiwes 8 and 9.
The tendencies ~f future changes of the ebb eidal delta are comparable
to the Barievliet case.
Figure 8 Cross sections of the deltafront on the Grevelingen ebb tidal
delta (1960-1980), Uwstrating the impact of closing the
estuary in 197 1 (mhsiek and Mulder, 1989)

Figure 3 Developaent of lengshore bars and tidal channels on the
Grevelingen ebb tidal delta 1970-1980 (Kohsiek and Mulder.
1989)
3.4 00s terscbelde
The Oasterscbelde basin was directly influenced by the construerion
works in other estuaxies. Due to the b~lding of the Greuelingen Dam
(1965) and Volkerak Dam (1969), the tidal prism of the Oosterscbelde
increasad cirna 6 - 8% aver the period 19.60 - 1983 (Van den Berg,
19861. A considerable erosion and widening of the channels has been
observed over this period (Van den Berg, 19861, which is in accordance
with the expected geomorphological reaction (conform Fig. 4A).
Construction of the storm surge barrier (1986) and af the landward dams
(Desterdam, 1986. and Philipsdam. 1987) has disiurbe* these tendencies.
Since 1986 the tidal volume of the Ooscerschelde has decreased by 302;
tidal current velocities have dimiaished by a simLlkr percentage and
the tidal range by some 1ZZ. This reduction in tidal influence, most
probably bas initiated a process of channel sedimentation and shoal
erosion (conform Fig. 41)). However, no gevmcrpholdgical absematirms
are available yet to confFnn this.

Lt has been estimated that the sedimentation process Of the channels in
order to establish a new equilibrium will involve some 500 millions lU3
of sediment over the next cedtnries. The total erosive effect over the
next 30 years h& been elculated as a lass of shoal- and mu-t area
amounting to circa l500 hectares i.e. 10 - 15% of the present total
area (Rohsiek, et al. 1987).
The total area of salt marshes very drastically has been reduced after
caepletion of the Delta Project. Since 1987 only 24% of the originally
total area of salt marshes in the kingvliet, Grevelingen. Volkerak
and Oaaterschelfle is rewining in the Oosterschdde. It ia estimated
that these saltslarshes are subject to sn increased rate of erosion.
VegetatioO mid erosion resistance most probably have been weakened
considerably as a result of a proloagued period of extrae tidal
reductiod duritrg, the final constmctiDn phase of the Oostersahelde
works.
The Oosterechalde's ebb tidal delta has shown a continuing exwinsion
over the period 1960 - 1983 as a result of the tikl voluae inerease
and resulting basin erosion over this period. Van den Berg (1986) has
ralculated a net sedimentation over these years of ca. 38 millions m'.
This is i n aceordance with the expeered trend as stated in Section 2.2.
The decrease of tidal volume after 1987 indicates an opposite
gesmorphrtls~ical trend for the future. It is expacted that the
deltafront will erode. the tidal channels will silt up and longshore
sandbars will develop a< the edges hheiek end Nulder, 1989). No
geoolorphological data are available yet to confirm this.
Tidal end gwmorphological changes af the Wastersehelde basin ove-r the
last 30 years are mainly due to large $tale dredging. Because of it's
importance for shippfng traffic to the harbours of Antwerp, no closure
daar has been defined for the Westewchelde. Wifhin the fra81evork of the
Delta Projeet security against flooding around this estuary h- been
odtained by strengthening of the dykes. The intensified shipping

~raffic, however, has irldueed a rfiatiner increase of large sale
dredging activitias fnm about 1970 (Pig. 10).
Figure 10 Impact of dredging and dumping in the WeBtefsGhelde 0% the
development e-f sam&
shod8 {Ww. 198'3)
The drtifieiar deepening of the chhmels hy dredging llae oontributall ta
an -larrmse of tidal frolume in rhe basin of 3 - 73 over the period 1970
- 1985 (Van den Be~g. 19871. %keoreticdly (Eis. 4A) these hydraulic
and gwmcrpholo@cal chaqes should invoke an ecqanaion of the sandy
shoals, Ohserufftkms over the period 1955 - 1987 largely coDfim this
(Pia. 187. Future developnte of the tide1 landscape will d+psnd
mainly on the actual policy Bf dredging and dwfs&.
Westee&elde4s
ebb ti4a1 delta skeuld haye shown aa expanding cen&er,cy
over the pert98 I970 - 1917 in reapwse ts an ine.reashg ci.dal valume
km&??a 3eietio-a 2.2). A moz~hoiwgt~al dweveSopuient in this sense.
bowever, has bees trmd co &seme over ~h*. past yeam. tidal
laadseirpe eeema W have been aetemhed to if greets* -&end by dredging. .
sad bovipUg aartiv&ties 6n titre eb21 tidal delta ir~lf. Deepeatris 6f
shfpp&q C&nnB&e and expaturion of Ze@br.aggc ~. harbour roffiated the
&zedgin$ md dwzng &f sbae 4a0. 0;l1IImi mj sededbent otr the
WasZeec&elae ebb tidal de&a between 197B arrd 1984 [Van den Ems,
1987.). mhss ettriicieal pi~l
.26-bntf1~* to dhtarafnO futuik a&ve1iip*es.

Implementafion of the Delta Project has inLtiated a process of large
scale changes in the tidal landscape. Generally these changos may be
described in relation to the specific changes in tidal valme of the
basins. The dimensions and rate of th. changes only partly have been
foreseen the outset qf the Project. DetaiIed process studies over
the last years have p~wvided =ore insight. &B a result of these studies
adequate mam~ewatt wssures have been defined to stop erosion gf
shoals and mudflats in the tideless baeins. A po1l.o~ plan incwrporating
the developarents of tk ebb tldd deltas - the Voordelta - is fa
preparation, The definitfon 0f adequate measuremeats to mwge
developments of the tidal landscape fa the 0ooster- and Westers&d.de ie
subject of study.
L am grateful ta L.R.M. Kohsiek Por his nseful advise in preparing thls
paper, to J.W.M. Kuijpers (Rilfkswaterataat, Directorate Euid-Holland)
and D. wan Malde~em for pravidimg data, and to J. Polderman and 3. van
den Broeke for sonstructian of the figures.
REFERENCES
BBRG, J.R. VAN DEN.
BEW, J.R.
1986. Aspects of sedi6Pent- and lnorphodynamics of
subtidal deposits of the Ooatezsehelde (the Eletherlds). &VS
Co~ca~iolll m 43, Den Has.
VAB1 BEE. 1987. &@anation of the isallobaths map of the
Voordelta L975 - 1980 (in Dutch). Rota ZL 87.0020, EM Dir. Eld,
49 p.
DMS, 1930. Prelude (in Dutch). Drie Hnd. Ber. Deltmr., 54: 17'2 - 177.
DMB,
1975. Hydrographical research into the impact of the storm surge
barrier on environmant (in Dutch). Drie Mnd. Bet. Deltaw.. 72:
@4 - 92.

DMB. 1976. Hydraulic arui hydrog~aphical alpact on the Oosterschelde
basin (in Dutch). DrieMnd. Ber. Deltaw., 763 312 - 324.
PORTUIN. A. d . 1989. The development and protection of shores in
former tidal hasins (in Dutch). Nota RWS Dienst Getijdewateren.
KOBSIEK, L.B.M.. PIDLDER. J.P.M., LOUTERS, T. and BERBEN, F., 1987. The
Ooeterschelde; twrds a new tidal landscape (in Dutch). Nota
WgO 87.029. RW8 Dienst Ggtijdewateren, 48 p.
KOItSIEI(, L.H.M. and MULDER, J.P.M. (ed5. ), L989. me VoVaoelelta, a
water system is changing (in Dutch). Nota m Dienst
Gecijdewareren. 24 p.
LEE-, K.A.H.W.. 1986. 2000 years of coastal development from Cap
Cris Re2 to Book of Holland (in Dutch). Nota NZ-N-88.19 BUS Dir.
Noardzee. 4S p.
LEEUWESTEIN, W. and P. ECBOOT, 1988. hraluation shores; report m the
project Shore erosion (in Durck). Rep. 73 Delft, Vakgr. Wtkk,.
90 p.
OTTEBLOO, R.B. VAN, BEEGAEM, J.W. VAN, KlJIJPER3, J.W.M., BROMUIZEN.
A., DREUMEL, B.F. VAN and MOL, J.W., 1987. The water soil of the
Nortkm Dalelta Basin (in Dutch). Nuta RWS Dir. bendenriv., 29 p.
Spa, A.J.F. VAN DER. 1987. Deectiption of the development of the ebb
tidal deltas of the Xaringvliet and Crevelingen (in Dutch). Nota
GWAO 87.105 RWS Dienst Getijdewateren, 77 p.
VECBOEL, R.H.W. VAN, 1977. ffirphalagical development in the
Oosterschelde (in Dutch). Nota W-77.009 RWS Deltadienst, 12 p.
W, 1989. Policy plan Wasterschelde (in Dutch). Werkgr. Waterbeh.
W.Sch=l$e, RWS Dienst OetiJdewaterenIDir. Zeelanri, 4 reports.
2 , W . . 1997. Geolmgy of the Netherlands I : The Netherlands
during the Halocene (in Dutch). Staatsuit$everij Oen Raag.

ECOLOGICAL DEVELOWdENT OF SALT MAKSHES PrND SO-
SOUTB-WEST NETBBUANDS
TIDa PLUS IN TBE
A.M.M.
van Eaperen
Both in the remaining and in the embanked estudries of tfie South-West
Netherlands there are good possibilities for conservation and
development of (semi-)terrestrial ecosystems. In the Oosterschelde and
she Westerschelde canservation of the estuarine salt marshes has
highmc priority. They are characterized by cyclic dynamic processes
that largely depend on the tidalwater system.
On fewer cldal flats and salt marshes that are no loager inuadated
em$zitasis Lies on the developmeat of new ecasyeteas. Already in the
first years after ernbhnt rare and threatened bird and plant species
colonized the new habitats. Environment was, hopever, dominated by
nan-cyclic adapcative processes (e.g. desalihation, pkiysical and
chemical ripening af the soil, etc.) causFng a xag14 change of flora
and fauna. Probably it will take mare than a Hmdred years before the
new ecosystems will be stabilized.
kkmgemcnt of water and land is of great importance for the type of
nature tbat will result ultimately. When making a olanagemant plan it is
important to get a go~d idea of the abiotic potentialities of the
diftexent habitats. Tagether with the valae of the diffexent types of
nature for namre conservation, it is the main eriterion to come to the
defisian what type of nature should be developed on the new land.

TIDAL FLAT
....
M HW
MLW
LOW SALT MARSH .......................... .:.:,.
........................
h:'.' .... .,>.. :
......
M HW
MLW
v
........I....
......... ......
..:
HIGH SALT MARSH A :::::U:::;.:
......... :.:;.: ,":.:.:. MHW
........ ..... .;:. ...... ....
:.... .:,.:' '
..: .: .: :"::' .,.v
Figure 1
Geomorphologifal differentiation of tidal flats and salt
marshes in the estuaries of the South-West Netherlands.
MW: Mean Righ Water, KLW: Mean Low Water
Climate

fn tidal water systems sedimentation and erosian of sediment continue
above mean low waterlevel (KLW) and even mean high waterlevel (HXW).
The lowest parts of the tidal zone have no vegetation except algae and
some water plants (Zostera spec.). These tidal flats are m important
feeding habitat for birds (Meire, et al. 1989). Some decimetres below
MEW the ffrst smi-terrestrial vegetation starts to grow (Salicornia
spec., Spartina spec.). This vegetation is usually inundated Cwice k
day. %?hen accretion contintles to above MEW, vegetation develops to a
variecy of plant communities. Geomarphology and vegeration of these
higher salt marshes are much more difgerentiated than on the mud flats
and the lower salt marshes.
The above-mentioned pattern of succession (see also Fig. 1) represents
the situation it! the sdine tidal are= of the South-West Netherlands.
In the fresh and brackish tidal water systems this pattern is not
essentially different, but there is a difference in vegetation types of
the marshes (reed, rushes and willars).
The constru~tian of the Delta Works reaulted ia an ewrmous chanse of
the estnariae environment. In most areas a more or less fixed
wacerlevel w s realized somewhat aound NAP (butcb Ordnanoe Level). The
consequence was that the lmr parts of the tidal land were pennwently
inundated, while the higher parts were hardly iufluenced by the
flooding water. Large parts of the latter could develop as (semi-)
natural ecosystms. Thfs development and ite future perspectives for
nature conservatLon are the subjects of this paper.
2 ECOLllGICBI. DYNdMtCS I N VARIOUS WATERS
The processes influencing the development of animal and plant
communities cm be ordered in an hierarchical model (Fig. 2, derived
from Bakker, et al. 1981). In this model the outer compartments
dominate the inner ones. Though there is some influence in the opposite
direction, this is of minor importance becsuse of its lower

Figure 3 Distribution of some estuarine marshland types in relation to
0
average chlorinity ( 100 CL-) . Open circles: salt marshes;
black circles: brackish marshes with reed and rushes; hatched
circles: fresh marshes with reed, rushes and willow coppices.
This figure gives the situation before the construction of the
Delta Works
1962 SWEOA MARITIMA
Figure 4
Cyclic vegetation dynamics in a salt marsh after severe
waterlogged conditions (after Beeftink. 1979)

hierarchical position. Tn the egtuarine situation the water regime is
one of the met essential factors detemising processes in soil.
gegstation and fa-. The relation between the average salt
concentation and the distrlbotion of some vegetation types is a sood
example wf this dominance (Big. 3).
Ae saon ae great changes occur in the outer cmpartntents of the made1
/by human interference of other), this has impertaut consequences for
the hierarchically lowar parts. Cholllges in the lower camgartments have
Lesa cons+quenees on the system a5 a whule. Figure 4 (derived from
Reftlnk, 1979) gzves a good exwle nf such a situation. In the wet
autnmn of 1961 severe waterlogged conditions occurred on a higher salt
marsh in the Grevdingen estaary. This was the primclry cau6e for a
die-bmk of the vevetation of Halimione pnztulacoides on that WSh. In
the long run there was, however, no chenge of the saltmarsh ecssyatem
as a whole and after sonre years the orfgmal vegetation could recover.
The erosion of a tidal flat or a salt marsh and the developmt of new
onea elsewhere in the same tidal system are an exmple of the same
e).clic dynaunica on a larger scale of space and tb.
The con$truction af the DelCa Works resulted in quite different types
of ecological dynamics. The water syetm a5 a whole changed t~tally and
resulted in a cmplere new and irrevexsihle development of water and
lad (Beeftink. 1987). After the fidehtx~g of rhe dmh many of the
estuafkre plants and animals died and nerv species had to celonize the
disturbed ecobystems. Id Che beginning the daminaring species in the
vegetation changed from year tcr pear. Later on changes took place much
more gradually. There were also marked differexices between habitats.
The vegetation of the lowar salt mrshes died totally sithin two or
three years. The higher salt marshes were well adapted 60 less frequent
inundations and vegetation changed gradually after the disappearance of
the tides. In tha reed marshes and willow woedlands of the brackish and
fresh estuaries the same pattern occurred. Theae vegetattons actually
did not die, btrt durtng some S to l@ years after damming their species
composition and structure changed very mufh.

NUWBER OF BREEDING PAIRS
70
Fieure 5 Development of three breeding bird populations on the
Hompelvoet, a newly created island in Lake Oreuelingen.
Crosses: Kentish Plover (Charadrius alexandrinus); triangles:
Redsbank (Tringa totanus); circles: Willow Warbler (Phylloscopus
trocNus). Data kindly submitted hy 6.J. Slab,
National FQrest Service in the Netherlands, Goes.
HEAN SEA LEVR
FRESH BRACKISH SALT
Figure 6 Distribution of some vegetation types in relation to height and
tidal flooding before the construction of the Delta Works. The
following vegetation types are indicated (Westhoff and Den Held,
1969) :
Fresh: a. Scirpus, b. Phragmites, c. Salicion alhae;
Brackish: a. Scirpus. b. Phragmitas, c. grassland (Lolio-
PotentFUIon):
Sdt: a. Salicarnion, b. Spartinion, c. Puccinellion,
d. Armerion

Of course. tke environmental changes have not only influenced the
vegetatian but also the fauna. Figure 5 gives an idea of the dynamics
of the breeding bird populations on pne of the newly emergod islands in
Lake Grevelingen. &er the constmction of the dam in the
Broursershaveuse Gat some 400 hectares of tidal flats and a wll salt
marsh were exposed to the air and wind. &specially the species of sandy
beaches and pebble coasts (We the Kentish Plover) began to breed on
the new land. The population of Bedshank, also a coastal bird, did not
increase before some vegetation was existing. airily because the h-
needed this vegetation to hide their nests. At that time the species of
bare beaches were alrea* deelinhlg. The fimt breeding birds of shrlibs
have colonized the island only recently and a common species like the
Villow Warbler is still increasing. Woodland species do not breed on
the island, because vegetation structure has not yet been
dfiferentiated enough and there are no nestiag sites for these birds.
It will take at least some 50-1Wl years before the bird communities
begin to stabilfae.
G-rally
speaking the foll4ng factors influence the developnent of
vegetation and animal wildlLfe an the former dt marshes and tidal
f late :
* the initial ecosystem present at the moment of dw constrnctiod;
* processes of adaption ta the newly created situation;
* mahagement of water and Land by m=.
3.1 Initial ecosystems
The newly developed ecosystems have their starting point in the old
estuadne waters. These Mtial ecosystems are strongly influcn-d by
the origins1 waterrepime. This Fs illustrated by the distribution of
the vegetation series qf Figure l in relation to height and tidal
floo6ing (see Fig. 6). p e vegetation types inasated hy an open
rectangle died within rwo or three years &ter embanlaaent was finished

and this resulted in an almost complete ba~e soil. The hatched
vegetation types changed much more gradually, without dying off
totally. After the enclesure in most areas %he water was pemnently
fixed more or lee$ at mean sea-level (%L).
The following aifferences
occuted. In the fresh and brackish tidal areas the orisinal marsh
vegetation c m at a mu&
MLW)
lower level (on certain places even below
than in the saline systems. Coneequently a larger area of here
soils eraerged after the completion af the dams in the ffaltwater systems
in comparison to the upstream tidalwaters. This difference is enlarged
bp the dying off 09 the lower saltmarsh species in the first years
after the enclosure. Vegetation development on the former salt marshes
and tide1 flats uas dominated by colonisation of the new habitets by
mnual pionier species. On the contrary, in the fresh and brackish
areas the new species (that had to invade existing vegetatians) were
m&dy tall peremial herbs and grasses, shrubs and trees.
3.2 Processes abimie eavironntent
After the welesure ~.f an estsarg a nlrmber of precesnes started
thanging ahiotic environment of the new tcrrestr-ial environments (e..g.
A
PERtOUTINB SOIL
--
B YllL WITH IMRRWE8BLE M W
Figure 7
Twa main groundwater regimes on the newly emerged land in the
former estnaries (after Drost and Visser, 1981)

wind erasson, desaliuation, aeration, physical and chemical ripening
and decalcification). The time scale of these processed differs from
some manths or years lwind erosion) to tens or hundreds of years
(decalcificetion). In the beg-ing t?e effects of theae individual
processes are quite clear. Wen the new ecosystems grow older it is
getting more and mors difficult to separate them from each other and
from the internal aynamics of the developin& ecosystems theirselves.
Figure 7 (acbording to Drost and Visser. 19111) gives a pod fdea how
the abiotic envixo~meut of different soil types behaves in relation €0
desaUnation. On well percolating soils desalination goes vezy fa&.
Mter some years there is already a rathez large freshwater body in the
soil. Glycophyloue hmbs and grasses can soon caloniee the bare sou
and mrwal shrub and then woodlmd deve,loplaent follows. CLay Layers in
the wail profile stagnate desalination and then this process can take
tens of years. In autumn and Winter the sail is satiuaced fast and most
of ttle rain water will stagmac@ and ran off supetficially. Aere only
the upper soil layers will have E freshwater body and this will
evaporate cempletely in stmmer. In tthis type of habitat halophylous
plant species dominate in the primary vegetation development. !Frees and
shrubs can hardly colonize the vegetation giving it tbe character of
open grassland for a long time.
3.3 Management of water and land
Uatex manasement is especially important on flat shores, where emaller
or larger areas can be inmdated during some time. This is of great
importance for €he suraival and germination oppnrtunitiea 4f certain
plant species. A good exanple how this can influence the fauna is fouad
n Lake Creuelmen. Aere the bird species of scarcely vegetated shell
shores and beaches axe declining (e.g. Kentish Plover, see Fig. 5).
bcaose ef continuing vegetation development. Ffaybe there is a possi-
bility to maintain a larger area of this type of breeding habit& by
raising the waterlevel OS the lake in autumn or winter time during
several weeks. Then a lareer part of the shore is inundated by saline
water, slowing down further vegetation development.

In the terrestrial habitats it is of weat impartance how the nutrient
cycles of the ecosystem and the structure of the vegetativn are
influawed. The nutrient cycles detenatee growing and feeding
conditions for plant and animal species, whLle vegetation structure is
of great iaqrortance for their survival. Rmtghly speaking we can
distinguish three ways af nature managemeqt: spontaneous developdent.
rcewing and grazing. Each of these managerneat types has its om results
and (financial) costs. Sponfaneous development will lead to woodland
ultinatelp, a l e mowing keeps areas open and rich in herbs. Orazing is
in between these two types and the rcanlt depends on gra~ing intensity.
It is an attractive type of management to create a habitat with a
~aried structure of grassland and shrubs in a semi-natural way. Grazing
anlmaLs l~ndigenrms herbivorous herds as well as domestic cattle) we
large areas in traditihsl patterus. Some parts of the area are (year
after year) more intensively grazed than others. In this way Fazing
can enlarge the differentiaties of the abiotic envirpmenc creating
additional stimuli for mre diversity in developing ecosystems. In the
lens run it will not only result in grassland. Even for natural
woodkd development grsing (of course in very low densities) can be a
good cyse of management.
4 BALBNCE OF ECOSYSTEM DETFELBFXE4T UP TO BOW
The fitst embankment of a large estuary in the South-West Netherlands
tbok place some 30 years ago (Lake Veere, 1961). Since that time
several othsr areas folloved. with difTerent abiotic enviroweuts and
different types of water systeqts. As indicated above ecosyste& i3evrlop-
ment is still going on and eves in the oldeat aress it will take tens
or even hunrtreda of years before the new t~rresrriai habitats will be
stabilized. Notwithstanding these facts we ri1.l try to make up some
balance fxom a pine of view of nature conservativn and manaseoee.
Firstly we wst natice that chere is a great loss of valuable (semi-)
terresrrial habitats in the esttmfes of the South-West Wetherlands as
a consequaRce of the Delta Work$. (The lv,ags of aquatic ecosystems and
tidsl flat6 is leet out of oonsideration.) After the enclosniuze of the

Fonner tirtal flats with saltmarsh vegetation
Grazed £amer tidal flat Vith dense g~assland vegetation and small
*uba

Former tidal flats with changing vegetation
Crazing as a tool for vegetation management

Grevelingen. Lake Veere and Krammer-Volkerak, the area of salt marshes
Fn the South-West Netherlands has been dwished and is at the moment
no more than ca. 30% of that in 1990. If we Look at the area of higher
salt marshes thi. decline is even more dramstical. Especially these
higher salt marshes are a valuable and rare type 05 ecosystem, with d
number of endangered animal and plant species. For this reason kigh
priority must be given to conserve the remaining 5aSt marshes in the
Oosterschelde and Westerschelde. In the Oosterschelde the sediment
balance indicates that there are hardly any perspecti~es for the
development of large new salt marshes. On the other hand the largest
part of the remaining saltmarsh area is formd in the eaptern part of
the Westersdtelde (ca. 80%). Enviromnent is under great pressure here
hecause of chemical pollutiw of water and soil (Bijlsma and Kuipers.
1989) and the predicted eroaion as a consequence of the proposed
enlargwnt of the shipway to Antwerp.
However, there are also a lot Of positive effects as a consequence of
the works. In the first years after the r~mplation of the dams, in the
new habitats impostant populations of breeding birds have been
established, including several rare species. Many of these were
formerly quite connnoo, in our country on shores, in marshes or
extensively used agricultural Land. They have declined iII recent years
because of drabage or more inteneive hu)aan use of their original
habitats. For. some species of terns and waders (e.g. Sandwich Tern.
Little fern, Kentish Plover, Avo~et) the Delta area is now one of the
wst important breeding grou&
Eurape.
in the Netherlands or even in Western
For wintering buds the situation is quite swlar. Large numbers of
herbivorous waterfowl like geese and ducks forage now on the grassland
of the former tidl flats and salt mrshcs. The West-European
population of the Barnacle Goose winters almost completely in the
Kefherlands and this species visits the grasslands along the
Haringvliet since long. However, its original habitats strongly
diminished here because of the public water works and the change of
agricultural grassland in arable fields. Thanks to a nature management.

that chwged farnor reed mrshes and tidal flats fnto wet grasslands,
the numbers of these wintering geese conld stabilize and even rise.
In the new areas also important botanical values arid. The most
striking example Is the developcent of: a wet dune-slack vegetation with
rEre species like Parnassia painstris, Blackstonia gerfoliata and
several species of orchids. This type of habitaf originally occurred in
the lower parts of the coastal aunes. There it dieap~eated, however,
almost totally because of the lowering of the greundwator level.
Especially on sandy soils with a stehle groundwater regime in the Lakes
Be- and Grevelingen hundreds of hectares of this particular
vegetatipn type could develop and in the future probably also in the
Krammer-VolLerak.
However, the new ereas are still in a dmamic state and the above-
mestioned values mag be temposaq. The decllne of shote birds in the
Grevelingen (Fig. 5) is a good exgmple M thSs. We also see that shrubs
and trees begin to caJoniee the dune-slack habitats and displace the
characteristic herbs. In a late* phase parts of the actual duna-slack
enviro~nt will probably change because of decalcif4catiofi. In certain
cases it is possible to stop or slow down natural succession by grazfng
or mowing, thua conserving certain development stages by appropriate
management methadv. However, we hme to realize that it ia not possible
to keep the former estuarine landscape in a young stage, because there
are ne cyclic dynamic processes to renew abiotic envimnment. In the
long run the ecosystems will grow to a more mature stage and this will
inevitably lead to a decline of certain species and the appearawe of
others, AL the moment it will be possible to keep certain species by
grazing or mowing Eor a certain the. However, in the longer run theg
will bisappeaz.
Anyhow. it is qnite eLear that a grazing management will lead te
ecasystems totally different from those of spontaneous development or a
wing regime. And from that point of view the decision lies with the
etologists and the nature management: what type of nature do we wane on
the new emerged land?

As mentioned above aa important question is wh~type of nature w want
to &evelop. Do we wwt open easy grassy areas where geese can forage
and waders can breed, or vast m?.rshy woods with breeding colonies of
rare and threatened birds like the Swonbill Bnd the Bightheron? D?
maybe we prelex a mare patchy landscape with grassland vegetation
alternating with Shrubs and aoadlmds, inhabitated by threat&&
invertebrates, amphihia and reptiles. All these types are possible in
large parts of the nw land. %at will be realized depends on t?ie
choice how to manage nature. To make such choices the following
criteria can be used.
Firstly there are the potentialities of the area that may determine the
choke. It is good to poiat out several elements of this criterion,
like:
* The abiotic enviromental onditions, that may differ from place to
place. On a salty soil, for instance, it is rather easy to develop
a grassland vegetation, but a woodland will nor grow there hefare
the soil is desalinated to a depth of several metres.
* New plant and animal species will have to ~oloaize the new ateas.
?herefore the proportion of the area, its position to the acraal
distribution of the species ~imed at and the presence of suitable
migrating zones are very Impe~tant.
* Ecosystem& must not only be realisable. They must also be
maintainable, especially on the long m.
Another important criterion to formulate the goal for ecosystem
development is the significence of the nev type of nature for nature
conservation. It wfll be clear that the highest priority must be given
to species and commities that are threatened on an internatbnal
scs%e.
A fine1 criterion ia the technical and financial ability to realize the
management that is required.

6 SPATIAL VXW
If we tmslate the things rienti~ned ab~ve spatially the Eollowfng
pattern reeult-s (Fig. 8) .
Figure 8 Priorities for futural ecosystem development in the (former)
estuaries of the South-West Netherlands. Open circles: salt
marshes; hatched circles: former lnarshes and tidal flats to
develop as open landscapes; black circles: the same, but to
develop as woody landscapes; crosses: agricultural areas that
can he set aside for ecosystem development
Firstly there is a strong reduction of the salmarsh area. On the one
hand the greater part of the remaining salt marshes in the
Westerschelde are under great mvironmental pressure and on the other
there are hardly any perspectives to develop new marshes in the
relatively clean Oosterschelde. This loss can partially he
counterbalanced by setting aside parts of some wet agriculture areas in
the polders along the Oosterschelde. In those areas, with a lot of
saltwater seepage, there are very good conditions to create inland salt
marshes and brackish grasslands or even to abandon polderland to tidal
influence. Especially the plants and animals of the higher salt marshes
can find a habitat here.

A seaond important perspective for ~cosystr?m development is lging
outside Che new dams .tn the North Sea, the so &led
"Vwrdelta". Eere
are on several places good possibUities to aevelop dvne aateas with
gradients of fresh dme slacks to salt marshes and breeding place6 fox
shore birds.
Mnally there are the new terrestrial habitats on the Parmer tidal
flats and salt marshes in the closed estuaries. In the estuaries closed
first, like Lake Veere, Lake Grevell%en and liaringvliet, large
grasslands ate the dominating type of ecosystem. Prm an international
poLat of view spontaneous woodlands and patchy environnents with shrubs
can also contribute very much to the conservation of endwered species
and cmunities. For this reams it is necessary to rnake room for shrub
and woodland develepmnt %a the wre ~ecently closed estuaries like the
WrIcieearstsftieer aad the Krmer-Volkerak.
~n conclnsion I wonld lika to stress the differences between ttie
develsping ecosptems of the famr sslt marhhes and tidal Eliits and
the nature reserves on the "old land".
Firstly there is a gmat df3ference in size, Mast nature reserves of
the pld land ar*, in general, muoh smaller than the Gees in the former
eswanies. In the Ntetherlanas there are very Fw nature reserves with a
size ef 500 to L50D hectares with undl.stwM sradients in height ad a
eatural groundwater regime. This gfm special perspectives in the
former wstuaries for those epecies and ecosystems wbzrh depend m
large-sale prsc6eses and lwk of disturhitnee. P%$ LbZs readqn
splitting up ~f B@ new areas mst be prevented and nature ~wia~w~ent
must rrg to cmso~idaze their large-scale character.
s.eehdljr we muse mylise that m the ffimr estuatiee W* um deaelop5ng
axlsritirig snes.
new -sal&, dhila on. the $&d we &re W~\rfn.$
Ilecauw of the non-cymc dynamri&s ern the ecosystem* dn tbe famsr
tiaal flaTs and salt m;prslies., eoMervation 62 rhe aettial flora Bnd

favna often is the wrrmg policy, whatever rare the species may be.
It is more important to get a good idea af the (abiotic) processes in
aad the ecologioal potenzidries of the atea. 0x1 the basLs of this
knowledge we must formulate a management plan with a goal for ecosystem
development a~ the long run. The long-term goal must be used to
evaluate what short-term measurements can be taken. For instmce, with
the Long-term goal to st5uslate woodland and shrub develbpment it will
be difficult to accept short-term mowiq to canserve breeding habitats
for rare shore birds. Mren the lerrg-term goal is the development of
grassland ewsysfems a mowing regime during some gears may be quite
acceptable, particularly when endangered species are involved. Even in
rhe last case, however, it is possible that the breeding habitat gets
more and more unattractive.
The author is ~uch indebted to J, van Baalen, C.
Bisseling,
A.R.L. Huiskes, D.J. de JMg and espeeiallx W.@. Beeftink for valuable
tornenta on earlier dr&ts of the manuscript.
J. Visser (Bi>kewaterstaat, Directorate Flevoland) and G.J. Slab
fk'atiofuil
Forest Service in the Netherlands) kindly provided some data.
BEFERENCES
BAKKFIR, T.W.M., RLIJN. J.A. and ZAUELHUFB. F. J. VAN, 1981. Blederlandse
kustduinen; landschapsecologie. Pudoc. Wageningcn. 144 p.
BEEPTLNK, W.G., 1979. The structure of aalt snarsh cornunitlee in
relation to environmental disturbances. Tn: R.L. Jefferies and
A.J. bavy (ed.). Ecologisal prowasses in coasCal eavirenmeats,
p. 77-93. Blackwell Scientific Publications. O~ford.
BEEFTIEIR. W.@. 1987. Vegetation responses to changes 19 tidal inundation
of salc marshas. In: 3. van Andel et al. (eds.), Disturbance in
gr@Salan&, p, 37-11?. Dr W. Juolt Fublishers, Dordrecht.

BEEEXNX, W.D.
and 80ZBf&. J. L9M. The nature and funceioning of salt
marshes. In: W. Salomaes. B.L. Bayne, E.K. Dun~sma and U. Btirstner
(&S.). Pollution of the North Sea, p. 59-87.
BerliTniNev York.
BIJLSPIb. L. and EVIPW. J.W.M.,
the Delca Waters. This volume.
Spzinger Vedag,
1989. River weter and the quality of
DBQST. A.3. aad VLSSEK, J.. 1981. Bet' grandwaterregime als
structererende factor voor de begroeiing in afgealoten egtuaria
het een toepa~sing in het GreveLin$eh%&km. In: Ministerie van
WrkeeE eh Waterscaat, Vijftig faar andsraeek, p. 201-216.
Lelystad.
MEIRE, P.M. SFYS. J., YSEBAERT, T., BEEiICElh, P.L. &ld BAPTIST, B.J.N.,
1989. A Bhanging Delta: effects of large eoagtal eqineering
works an feeding eaclogical relationships as illuatra%ed bp
waterbirds. This volume.
WZSTftO@, V. and RELD, A.J. DEX, 1969. Plantengemeenschappcn
,in Nedeslwd. Thieme, hrtphen, 314 p.

A CMANGING DEtTdl; EFFECTS OF L&E
COASTAL ENGINF5RING WOW ON FEEDB'G
ECOLOGICAL RELATIONSKIPS A5 ILLUSTRATED BY WAlXlWIBDS
P.M.
Meire, 3. Seys. T. YsebaerC. P.L. Mekinger and H,~.hl. Baptist
In the Delta area of the South-Wwt Netherlands the majrrrity of estuarine
eaoagstems has disappeared due to large coastal engiaeeriag works,
creating new artificial saline Emd Freshwater lakes. Thege
hydrodynamical changes had enormous consequences for plants and
animals. In this paper the changes in the avifauita are desccikd.
The Poordelta is charrtcterized by a La~ge number of diving ducks and
gulls. In comparison with the tidal basins, where the benthivormus
species (mainly waders) dominate, numbers of h~rbiubres and piscivoTes
increased snbstantially in the new lakes, while waders near17
dtaappeared. Herbivores dominate the avLfaona of the lakea. This
differentiation is even more obvious in the distribution of individual
species, since same species prefer fresh over saline conditions and
vice versa, In order to understand the observed distribution patterns,
the densities of different bird grows were compared with estimates of
the available food. In the tidal azeas and the saline lakes a clear
relation beween piscivore bird density and D~biid density (their main
grey species) was cbserved. BLeo wader densities in the Cidal basins
*ere clearly related to their food supply. Densities of herbivores are
comparable between various basies. Thetr food saurce is very variable
and hence impossible to estimate. The distribution a£ div*g
ducks.
feeding a$Iy on macrobenthos. is vety puzzling and .seems not at all
related to food supply. This indicates factcrs other thaa food supply

are determining the dfstrlbution af birds in che Delta r2s well. NeM to
ecolo@cal factors mch es ppulatiob. size, arlgracian partem and diet
selecriob. human arrivities affect bir&i, often negatimely, in a
varietg of nays. Disttubailce, poLlution artd habitat loss are Lnnoediate
threats. Several tomagament options are available and discussed in
Drder Ca inpmve the acologlcal cwditions.
Gver Ehe l46t tws deeaaes the wealled Delta ,a-
?n fhe asu~hwsrern
part of the Netherlands has changed profgunLUy. 14 this area $he rivers
~Wn. W and Sehdde £lea ineo B& sea thrangh e cosplelt s.p%em of
estuaries.. Is respanwe to the disastms etcrro £l@od of 1953, in whim
.aaSut l:&O.O people were killed, as BBlbfti~als plan @>E aalm@
up all ~ht
me e$tcr&i~e$ was deedoped. In 1-967 the &.xec&ion of this plan,
&Ithod& @rptte8 id the b&Se
Df tHe yeais, s s mleted. ThLs
~ ~ u h tn e d ~profalrnd chaages kn thil eelbgieal relatims, as lilrge
't:idal .&reds di&p$ear@d and ~ e habitatir, w such as stagnant saline wd
bi&.kish lekes, were cr-ed.
5ie.c~ d$.scxibieg all these eeolsggtcal
chrtng*~ is fmpossible. In $his paper W@ shall reetrict eu~selves to
descLlibe tibw tha oeeu.s.r?nce and d~i.@%ribation o* birds wer zhe
aiffereat basin9 Changed and xe isdieate reia%imxv with food suppigi and
otber faceors.. W.@@, the area is? very imparranr for btsds apt is^,
et al, 198&; Saeijs a~d Baptigt, 1~91771 d., as the gentgal pwblle is
sepsAtive tg birds, the7 haye p.byzld a relatfvely ~ortanc role in
deci.s:i@ndiug (ens. in eke disewasian sht at spen or closed
b,mte~sBhel@e: d~riqg the Uo6sre-s #f the Philips- and OeatettBni; in
tbe eurrent dbcusriori abwt rhe &its to be @et or reeteatilbn in
Oasfemchelde erc.l.
Xn the fiei.wt part of thfs papixr ttie Wbitati in the De'lyta area
and the ohsewed chaQgesia tht?. occnrt-ence ef waferbird% are descritred.
In the: saEBnil p&
some fwtoes inflmencing tiiis obse.md distTibutan.
bein$ Toad BUatLability and ~nat?Xp btber fac&ors as migration pattwns.
pdpwlatton bize and thlaran fiite&ereuce are Wyxed.

The preseat knarledge of birds in the Delta area and the data presented
in this paper result from the wtrrk of many people. Eluch is due to
Rijkswaterstaat, Tidal Waters Division (the f~mer Delta Department)
which sponsored a project in which Riikswaterataat (Hen!+
Baptist,
Eris Narceijn and Peter Xeiniager) carried out the monthly counts and
breeang bird surveys, the Delta Institute for Eydrobiplogical
Research. Yerseke (Xob Lambeck and eolleagnea) which started a ringing
pragramme 19 co-operation With Eijkswaterstaae, end the Uaiversicg of
Gent which stuared same aspects of the feeding ecohgy of waders
(Patzick mire, Jan Seys, Johan Stuair. Fred Tesk and Tom Ysebaert).
2 DEGCRDTION OX THE AREA
As the ecological conditions are so different between the basizis and
because they have changed oonsiderably, a brief description of the
Delta area (Rig. 1) is even. More details can be found elsedere in
NETHERLANDS
Pigure 1 !4ap lapof
hasins
the Delta area with the location of the different

this volume or in Duutsaa, et aL, 1982. The situation is descriBed up
to 1984 since the data on birds presented in thia paper are restricted
to the period 1976-1984.
The major physical and chemical characteristics of the afferent basins
are given in Table 1. For each OS the tidal areas, the surface of
salt marshes, tidal flats, tidal waters (the area permanently covered
by water) and adjacent wetlands (creeks, "inlagen" and grasslands which
are used by birds either as resting or as additivnal feediqg sites) is
given. For tbe lakes the surface of deep water ( > 1.5 m), @hallow
wager ( 1.5 m) and sbwes is given. Shores are the very shallow areas
( F 0.2 m) of the lake of the majority of the areas perraanently exp~sed
after the fomatlon of the lake and now used by weterbirds
Kagrieultural land, nature reserve etc.) as well as the adjacent
wetlands.
The Voordelta is that part of the North Sea just off the Delta area. It
consists bf a system of chwnels and shallow areas. Salintry atrd
turbidity are high. The sandy beaches are subject to hem wave action
and su£fer from a very high ~ecreation pressure. More details of thia
area and the changes occurring here ate described by Mulder, 1989. The
Westerschelde is the only rema*ning true estuary in the Delta. The
freshwater input frqm the river Schelde is small (on average 100
m3/sec), hence the braokish part is limited to the area beMwn
Ancwerpen and Hansweert. From the mouth tmards Antwerpen a gradient of
decreasing salinity, oxygen and transparency. and of inereasing
suspended meter and nutrients is found (Humel, et al, 19881. Large
mid- and sandflats occur all over the eatuary. "Bet Verdronken Land van
Saeftinghe". in the brackish part BE the estuary, i5 the largest salt
marsh (8250 ha) of the whole Delta area. The esnurry is characterized
by a large anthropogenic stress due to discharges of inorganic and
organic eontamfnants, mfnly by the river Sahel.de, but alsa from othar
effluenrs slew the Westerschelde itself, and due to intensive dredging
activities for shipping. In the Oesrerschelde rhe input Q£ fresh water
is so small, that the area is better characterized as a sea arm than as
an estuary. Salinity is high and constant; turbidity, concearrations of
nutrieets and pollutants are low. Before the constructioo of the

Table 1 Some important physical and chemical characteristics of the
different beins in the Delta area. Voordelta (W). Mammer-
Volkerak (m). Oosterschelde (OS), Westerschelde (WS),
Orevelingen (W), Veerse Meer (W). Haringvliet (W).
Eollands Diep (W). Bieshosch (BB). (' VM tidal range is
difference between winter and summer level.)
VaLkerak sluices, the Oosterschelde had a much more estuarine
character. TBe relatively Eew and small d t 81a+ahe8 are restricted to
the eaatern part, whereas extensive tidal flats occur all aver the
estoav. Tbs nornleastern branch of the estualy (Keeten, Mastgat,
Zijpe) continues as the Hrrrrmmer-Wolkerak. The freshwater input here was
regulated by the Volkerak sluices at about 50 m9/s. In both
Oosterschelde and Krammer-Volkerak there is d very intense mussel
culture. Although most of the musselbeds occur snhlitoral, the
intertidal ones are extremely important for hirde. The Grevalingen Weer
was created in 1971 after the closure of the Brouwersdam and is a
saline lake. Water ephange with the North Sea was re-established in
1978 when a sluice in the Brouwersdam becae operational and an

artificial waterflow was created after the eonstruction of a sluice itl
the GreveUagendaw in 1985. This had profound ei5ecee on wa.arerqsalitp
(Bannink, et al. 1986). Large musselbeds and extensive fields of
Eelgrass (Zostera marb) occvr in €he Lake. The Veerse Meer was
created in 1961 after the closure of the Veerse Gat Dam and the
Zandkreelrdam. To imptooe trater drainage frnm the surrounding poldezs
the warerlevel is lowered about 70 cm Yn winter and raised again in
sprins with water frbm tbe OosterseheLde. This managemt reaulted in a
bnackish, wy errtf~phic lake- As a consequence searreeds. especially
UWa sp., are very common and in the deeper parts of the lake qnoxic
eonaitims prevail during most of the year (due to stratifieation). Be
Earingvliet, formerly a typical estuarine area, and the Bollands D+ep
and Biesbosch, formerlg freshwater tidal areas, became freshwater lakes
rsith every small tidal influence (abmut 20 cm) after the cpprrtructiou
of the Valkerakdm in 1970 and the BaringvUetdam fn 1971. These basttls
are important for the diseharge of the rivers Rijn and Maas.
Fluctuations of the WrtterleveL are accordin&ly related to the river
discharge. The reduction of current velocities after the closure
resadred in a very high sedWn

The monthly bird-counts in the whole Delta area between 1976 and 1986
have been summarized in PleLninger, et al, 1984. 1985; Meininger asd Van
Haperen, 1988, and Baptist and ~ ~ g e1989. r , The average number per
species per month pez basin was calculated asd used in further
wal~sis. The total number is the sum of these average rnofitbly cows
of each spedes in the period 1976-1984. It is the best measure of bird
use of w area siace it ie not biased by seasonal patterns.
In rrlnter up to 700 000 waterbirde cm be present in the Belta area.
Much maller numbers Cccur in summet @ig. 2). Approaimately 80 species
MONTH
Figure 2 Pattern ~f occurrence of waterbirde in Khe Delta area. The
average monthly totals of the period 1976-1984 are given.
(Black, waders; hatched, ducks; wbtte, ether wacerbirda)

of waterbirds occur regulsrly. The wst important groups are ducks,
waders, geese, gulls and tern; less abundant are grebes, cormorants
etc. For all these bim3 species, occurring regdarly, the Delta area
has a very important funetion in several stages of their life cycles.
Figure 3 Map W£
the World, showing the major migration routes of birds
occurring in the Delta area. (Horizontal hatching: major
fptertidal areas; vertical hatchins: breeding a d wintering
range qf birds using the Delta area)
Figure 3 suomlari~qs Chs migins and destinations of birds occurrim in
the Delta. In summer important numbers of breeding bads a£ several
speeies are present (Meiniqger, 1986). After the breeding season most
of these birds migrAte south, some to tPle we6.t coasts of Afrioa, lJfrile
other speties are more or less resident. Birds breeding in the .boreal.
Subatetic and arctic zone of Europe, Asia. Greenland and North America
use the Delta either as B refuelling site on migration between the
breeding areas and the wintering grounds [in spring and autumn), or as
a wintering site. Many species also moult in che heea. All bhese
species have in common a large energy denand a d are hence dependent on
an abundant food supply md low levels of disturbance. Their wcurrence
show that the Delta area acts as a turntable in large scale movements

of birds and this: immediately stresses the internatianal importance M
the Delta. Criteria have been agreed upon intsrnatiooally to &press
the importance of a wetland area CSzijj, 1972): if more than 1% of a
population (either 20 000 waders or 10 000 ducks) r-ularly occurs, the
area is considered to be of international importance. In the Delta area
the numbers of /+l species exceed these criteria.
Figure 4 Proportion of the tetal number of waterbirda in the Delta
occurring in each of the basins. (Vmordelta (M);
Westerschelde (WS); Oasterschelde (OS) ; Krammer-Volkerak (KV) ;
Grevelingen Meer (GM) ; Veerse Meer (W); Haringvliet (HV);
Bollands Diep (BD) ; Biesboseh (BB))
The bir& are not evenly spread over the whole Delta area. In Figure 4
the relative importance of each basin is given (expressed ss the
percentage the total number of birds in each basin represent$ of the
ove~all total number in the Delta). The oatstanding importance of the
Ooster- and Westerschelde is very clear. With less than 50% af the
total vetland area they accmmodate 67% ef the birds of Khe whole Delta
area. KQV this distribution is related to rood supply will be analyzed
in the fallowing bections.

. DISTRIBUXIOBI OF WATEJBIRoS OVER TEE DIFFEREMT BASINS
RELATIOIIS WITK THE FQOD SUEPLY
Before relating the occurrence of birds to the distribution of their
food it is necessalry to describe the position of the birds in the
estuarine or lake ecosystems. Weretore a simplified model of the
energy fXmes in the different basins is giuen in Figure 5.
Figure 5
Schwatic represeatation of the majar links in the foed veh
in the basins of tke Delta area
flurrients are used by phytoplankton, mi~rophytobenthos and macrwalgae.
The latwr, together with saltmash plants, their seeds or the
ve&etarion along the banks a d adjacent agricultural areas are the main
food source for herbivores, being geese and most specles of dabhlwg
ducks. Plicropbytohenthos can be usecl as a food source by a faw birds,
especially Sbelducks. The mrozoohenthos, large sized I . I mm)
inaertekates living in the sediment, can be divided into filterfeeders,
depositfeeders, scavensers and predators. Filherfaeders.
mainly molluscs such as Cockles (Ceraetoderna edule) an& Wssel.

(M~tilus edulis) in salt water, Cerastoderma glaacum and Mya arenaria
in brackish water and the Zebra Mussel (Dreissena polymorpha).
Pisidiidae and Anodontidae in fresh water, feed mainly on
phytoplankton. Under normal oonditiona filterfeeders dominate the
hiomass. Uoarever, they are often the first species to decline or
disappear under poilution stress. In salt and bracWsh water several
polychaetes (e.g.
the Lugworm Arenicola mariua) , molluscs (e.g.
balthica) and crustaceans feed on detritus, others are scavengers or
predators (e.g. the Shorecrab C a r d s maenasS. Especially in the
saline lakes the epibenthos must be very i!qpartant although available
data are scarce. It consists of shrimps (Grangon sp. and Mysidaceae)
and nany other species of Crustacea (Isopoda and Amphipoda) . In fresh
water Polychaeta are replaced by Oligochaeta and Chirononddae.
All these organisms form the prey of the different species Of
bentbivores such ss waders and d%ving ducks. Shrimps and mysids are
also important pzey for gulls (e.g. Black-beaded Gull) and terns dnring
the breeding season and for some species of grebes in winter. The diet
of the Pochard, a diving duck, consists in the Delta mainly of the
seeds of Eelgrass (Boudewijn and Mes,
1986) and this species is
consequently included with the herbivores in our fuxther calculations.
Benrhas and zooplankton also form the food source for many fish
species, which in turn are eaten by the piscivores as Cornsrant,
mergansers and grebes. This basic food web is quite smlar for the
different haslns, although - mainly depending on the salinity -
different species are fouad and depending on the hydrodynamical
conditions different links of this food web are more imporrant.
The various functional groups of birds described abuve (benthivores,
piscivores, herbivores) are used in further analysis and called bird
groups. Benthivores were divided into waders and diving dncks as these
groups differ so markedly in the habitat they use (tidal flats versurr
open water). Gulls are mainly omnivores and are also treated
separately. Terns, typically piscivorous birds, are nor included in the
analysis, as this species is not very well covered in the counts. In
addition their feeding areas are not well knovn. Since they are rather

small bird$ extludiag them vill net si~cantly change the
co~clusions.
DISTRIBUTION OF DIFFERENT BIRD GROUPS
based on numbers
IN THE DELTA
Figure 6
Oecurrewe of different bird groups in the different basins.
7er each group the percentage tm the total number of birds is
given. (For abbreviations see FLgwre I; the bird grbups are
defined h the text: and in the Appendix)
To gaia insight into the distribution of waterbirds over the different
basins the proportim of each bird group on the total number per basin
is given in Flpre 6. The Voordelta is clearly different from the other
sites with a very high proportion of gulls, few waders and quite many
diving ducks. The three estuaries, Krammer-Volkerak, Dosterschelde and
Westerschelde, are very similar in having a very high proportion of
waders and - to a lesser extent - herbivores and gulls. The relatim
importance of the different bird groups in the Lakes is quite similar
aod characterized by a dominance of herbivores. Corepared to the
estuarine situation the importance of waders in the lakes decreased

subitanteally and thac 6f piseimres and divhg ducks inereased. The
distribution of individual species omer the bebins is even mare
pronounced than that vf the bhlrd groops. In Figure ?, as an example,
the proportion of the total number in the Voordelta, tbe tidal areas
(Krammer-Volkerak. Oosterschelde, liesterschalde), the saline
(Grevelin~en. Veetse Meex) and freshwater lakes tltaringvlier.. Hollands
Mep, Biesbasoh) for aome important auct species is given, Common
Scoters are found almost exclwively in the Qoerdelta. OBldeneyes in
the salhe lakes and Tufted Ducks and Piachards in freshwater lakes.
Digeonrc d Pintail$ occur whly 14 the tidsl azebs, Mal,larb are
coxmen everywhere with the exception of the Voordelta and Csots elearly
prefeh saline lakes. eimilar preferewes e-xists ia the ctbr species.
B~wewfa
Wiidal areas
=$aline
lakes
@~reshw&er lams
a~oredelta
HT~U~I area
S5allne lakgs
B~Ms~~w&~@P
lakes
Figure 7 DiGtrfbiltt6~l 6f msie @re& dyer the basins Cl[ecup& in tEe
Vwordelta, ttie estuaries Per tidal araa's CW.steFs~chelde.
Oastersobelde. 'ranmiecVoUter&>, s;lline lakes (Bees8 Weri
Gre-in@% Heer] and f re~hwater Lake5 (IlarinmlLet, Roll&
Diep, BLeebasch).
The percatwe of bhe tstal sumber in each tatesprzy is
p,Lotted.

In order to lsok for relatiouehips between prey and bird abundance and
to cry to understand the distribution described abmve, the density of
each bird group was calculated (the wan of the 12 average monthly
c m s of the period 1975/76 - 1983184 was used). For each group the
surfaces of t4e follwing habitats (for the tidal and stagnant basfns
respeetlvely) were u ~ s t calculate o the density: pi8civores: tidal
water and tidal areas er deep and &allow warer; waders: tidal flats or
shores; 6iVing ducks: tidal wata and tidal ateas or deep a4ld shallow
water; herbivores: sale marshes and adjacent wetlands ar $here6 aad
shallow water. Of course this is a very crude approximation hut for
such a large scale comparison no greater detidl can he used.
Table 2 Winter density (N/1000 me) of Gobiid fish and densities of
Piseivores (h11100 ha) in the different aquatic s$stems of the
Delta arm. (Source of fish data: Cl) Ewrlynck (pen. mm.)
(2) Bamerlynck, et al. (in press) (3) Dopmhos and Tviek.
L987 (4) Waacdenburg and Meyer, 1988) (Voordelta (W);
Westerscbelde (TB); Oosterschelde (05); rame er-Volkerak
(KV); kevelinaen Meer (CH); Veerse Mser (W); Baringvliet
(W); Bollaads Ddep (!.DJ; Biesbosch (BB)]
Fish
Bird
Basin Density Seurce Density
(~11000 m*)
(e/ioo ha)

The mein diet of piscivores in the salbe waters consists of Cobiid
fish (Pomatosehistus microps, P. minutums, P. loaanei). In the
Grevelingen Meer the diet of Great Crested Grebe consisted for more
then 60%: (by weight) of Gobiidae (Domrnbos, 19$4). Other important
species were Herring (C1,u~eq harengus) and Shrimps (Grangmn crangon) .
The Three-spinod Stifkleback (6asterosteus aculeatus) is very abundant
in the Veerse Meer (Waardenburg and Meiler, 19881, but it is not known
ta &€it esent they $re eaten by birds. As GmbFid fish fenus the
majority of the diet and more PT less comparable data are available,
the average winter density is even h Table 2 together with tha
densities of pibcivofes. Clearly the highest densitieg of bath fish sad
birds are found ib the Veerse Meer. In the Voordelta the nobiid
densities ate varg varia%lo and prehably transparency. wMth is very
low, end wave action prevent Birds from feeding here. In the tidal
areas and the saline lakes a clear relation bstwesn ptscivore densities
and Gobffd densities is found. In the froshwatar lake6 ratber high
densities of piscivores are found. Perch (Perca fluviatilis), Boaeh
(Rutilus ~tilus) and other Cyprinidae as well as Gasterosteidae ere
inportant prey species in these lakes. Howevr;~, no density eqtimations
are available.
The bibmass of macrozoobenthos end the dessities of waders and diving
ducks ere given in Table 3. Waders occur -My on intertidal areas
here they feed on *any dzffetent species @f izverteh~afes. Same waders
are very speciUred, for instance Cystercatchers which feed mditrly 6zl
Mussels and Cackles; others suCh as Dunlin feed on a verierp of
polychaete worms (Rerefs sp.; Scoloplos armfger =Cc.) and small
crustaceans (e.g. Corophiua sp.) whereas Bar-tailed Godwits feed on
both worms, crustaceans and molluscs. Sometimes the diet ohansea with
the seasons. In autumn the diet of Curlew oonaists mainly of the
Sharecrab wbereas in winter, when the crabs move to deeper water,
Curlews feed l~iiinly on large worms (Lugwonus and Nereis sp.). In the
tidal basins an obvious relation between the densities of waders and
the available biomass extsts. A very high bkmass and density is fomd
fn the Ooeteyschelde, much lower valnes in the Westerschelde and
Kramer-Volkerak. The densities of waders in the other basins are much
lmer and as they arzt i?ee-g these in a veriety of habitats, the diet

Table 3
Biomass of macroroobenthos and densities of waders and diving
ducks in the different hasins af the Delta =ea. The average
autumn biomass (g ash free dry weight/m2) of the whole basin
and of the intertidal area and the density Q£ waders and
duoks (birds1100 ha) are given. (Source of macrozoobenthos
data: (1) Craeymeersch (pers. cam.) (2) Coosen and Van den
Dool, 1983 (3) Coosen and Smaal, 1985 (41 Meire and Develter,
1988 (5) Lamheck, et al, 1985, 1986, 1987. (6) Seys and
Meire, 1988 (7) Fortuin. 1985.) For abbreviations see
Table 2.
Basin biomass hiamass source waders diving
overall intertidal ducks
is not lmam and hence it is not possible to calculate the available
food supply. Diving ducks like Goldeneyes and Tufted Ducks feed mainly
on molluscs (Zebra Mussels, Mussels, Cockles etc.) as well as on
different species of wow and crustaceans. A relation between
macrobenthic biomass and the density of diving ducks is not clear at
all. The highest densities of diving ducks are found in the Veerse
Meet, although the biomaes is lower than in other areas. Additionally
in the freshwater lakes high densities of diving ducks are found
although the benthos biamass is very Low. On the other hand in both the
Qosterschelde and the Krammer-Volkerak a larpe benthos biomaas is
present but remsrkably low densities of diving ducks are found. Clearly
other factors must influence the distrihutfon of these birds. This will
be discussed later.

The herbivores form a rather beterogenous group of swans, geese apd
ducks. Some species like the Brent Geese feed nrainly on Eelgrass and
salt marshes, although in inoinrer semizkatural grasslands and cereal
crops form an important part of phe diet (Ebbinge, er 81, 1987). Mute
Bwens are strongly attached to water for feeding, whereas other species
like the Mallard feed mainly on adjacent wetlands and agricultural land
and use the basins mainly for resting, d~inking and preenins. Ad the
food source used by these birds is so variable (@.g. agrimitural
wastes and crops) it is impossible to estimate the wailable food
supply. Therefore in Table 4 only the biomass of macroalgae (mainly
Ulva sp. and Zosrera 8p.)
and the denaity of herbivores are given. With
the exception of the Voordelta the average density of herbivoree is
quite similar in the different hasins, although somewhat hisher in the
Tahle 4
Occurrence of macrqphyta (tong ash free dry weight at martmum
stwding stock) and herbivores CbirdsIlOO ha) in the different
aquatic system of the Delta atea. (Souree of macrophyta
datat (1) Craeymeersch (pers. cm.) L2) De Jong (pers.
corn.) (3) Nienhnis (pers. corm.) (L) Hannewijk, 1B88.) For
abbreviations see Tahle 3.
System 'Lotal Gran algae Source Density of
biomass
macrohyta
herbivores
W smell 0 mall 1 87
KV 360 80 280 2 199
OS 1833 S67 1457 3 334
ws smdl 0 sncg21 2 399
QV ? 1110-3300 ? 3 302
W 630 9.6 680 4 562
W small 0 0 415
BD small 0 0 418
BB
small

freshwater basins. Clearly the presenoe of these macrophyta haw no
influence on the density af all herbivores as they form only a 4 1
part of the diet pf only some species. Notwithstanding the huge
differences in food supply it is rewkable no larger diffexenees in
densities exist.
Table 5
Comparison between the consumption (C) (kg ash free dry
weight/lQO ha3 and deqsity CD3 (birds/lQD ha) of different
bird groups in the tidal areas, the saline and t&e freshwater
l&s (far further explanation see tm).
TIDAL WINE FKESH
Herbivores
C
D
Diving ducks
C
D
Waders
In Table 5 the dara on densities are summarized together with estimates
of the food emsumption a£ the different bird gzoups. This was obtained
by standard methods (see e.g. Nienhuis and Groenendijk, 1986). It i8
obvious that the densities of herbivores are high in cwtpsrisun with
those of other groups, their consumption even mueh higher. This is what
we expect from primary consumers. Waders and diving ducks are secondary
omsumess and their densities as well as their oonsmptian are much
lewer. The on17 exception are waders in the tidal areas. It is obvious

that the predation pressure here is very high. This is also confinmd
by the review paper of Baird, et al, 1988, and the tidal flats form a
v- distinct ecosystem. Densities and comumptions of piscivores
(tertiary consumers) are on average lower. Eemarkable is the high
density and consumption in the saline lakes. These results fit more or
lees in the concepts of ecological pyramids (Odm, 1872). From Bigure
5, however, we see that a food web is a better description of the
reality, hence explaining some deviations from the expected pyramid
structure.
5 PAIXOBS COMPWWING TAE RELUTON BETREEN DlSTRIBUTIDN AND FOOD
SUPPLY
From the previoudi section it is obvious that relatiens hetween bird
density and food supply are hot always obvious. First of all the data
of many spedes have been analysed rogecher. If wader densiries ii~
gener&l are related to grey densities, it ia very likely that this
holds also far most of the abundant species. However, if no rerrelatian
is found for all birds of a hitd group, this does not me= that the
densities of iddividuaL species ma), not be closely link& ta their food
supply. It is clear, however, that mt only food supply hut m), other
variables may influence the distribution and density of birds. These
factors may be purely ecological or resulting £ram human activities. We
will try to briefly describe somg of them in order to show the
qomplexity of the whole system and the difficulties one encounters in
predieting effects of impacts oq the ecosystem. We hope this will make
clear that in planning we shvuld he very conservative and use very
large margins concerning enviromntal marters.
5.1 Ecologital factors
Migratien, large-scale distribntion patterns of a species and climatic
oonditions are very important in determining bird numbers. Indeed for
several species the Delta area forms either che northern or the

scmthem bottueary o-f the winter distribncion (e.g. Lapwing, Avocet,
Blaek-tailed Godwit and Barnacle Gomse, Bewick's Swan respeewvely). In
cheee cases the severiq of the winter may strongly influence bird
occurrence. In mild winters much larger numbers of some species,
sensitive to frost, may be present; in severe winters large numbers of
Birds, norntally winterfrrg.more to the north, are forced to &rate
further southwards and peak numbers of wme species can be counted.
Duriqg a cold spell the presence of open weter is extremly important
and may attract birds from the large eurfoundin@.
Pnrthermore bird riders in .n area are not ohly depertdent on the food
supnly but also &ad on the qnality of other areas. Indeed if, for
whatever resaon, the food supply at the begirming a£ the winter in an
asea is eXUegK1y high or low, many mtroe or less birds may stay there,
influencing the numbees in Other basu, where nethin$ in the fmed
supply niisbt have changed.
MIDWINTER NUMBERS OF GRNLAG GOOSE IN THE WESTERN SCHEUIT
1976-1 987
year
Pigore 8 Evolution of the number of Oreylag Geee in tke Wesrersohelde

Overall population size Fs very irpporCane as well. Indeed in the last
decades a few species, lac the Greylag Gowe (Pig. 8) and the
Coxmurant, Wreased in numbers in the Delta area. phis is not due fa
any ehanges in the fbbd suepl~ hut to an overall increase in the
population. In Europe the Greylag population ha* increased from 30 DUO
to 1%
000 bemen 1974 and 1984 (Madaen, 1987). The total population
size of arctic breeding wecies eau vary substantially between years
due to the varying breeding success. Of
ether spactes (e.g. DuW)
pspulatiwn siae decreased dwriqg the last years, a declfae which is
obvious in the couunte from the Delca area.
Tlvese were all Eacrors ac~tiqg on a lazge sea&. M the DeLlt area
itself, several z~pccts rYf prey reLettfon may fafluence the
diatribwtion af a species. In she asalf,t~s prasent-a ad1 prey 8pedea
were lrrmped ed4?aca large categeiesi Same species are, hCmemr, uee
specXaLiz&. IeeakrD, vh@reas Others feed a* a large vazlaty of p?*
species. Tlris may affect the occurreace ti£
birds qutte subgtantliaf.ly.
In the Veerse Neex Dlva sp. fo- a v.&y important prep item far
hehivor~e Such ns eonss aud Br*nt Geese.. nse to predarlqu and *be
natural mortality of the plant* the aviril&le amount ,of Qlve q.
declines rapidly *u.tumn vwdnter. bnring that the BanL Geese
switch eo *Chef pray available in ehe area. d d y pastures .W$ asaMe
land, a& their nmbers replain stable wnrii April-Hay (Fig. 91. Owte,
kqwevez, do net change diet and amhers start te decline aireaey in
Jmwarp, mch earlier tkaa in other part6 of the De~lta.
Even for birtl4 Se4ddfs$ aa me prey fssp.eei&s, t&ce mi@t be facmts
compli~wzing the relative prey abondande m hird dasiff. The
i2ystet"aatehn is the most nwraus %del ,gp.eel&s in €he Wd*refm&el~&
and ie a.lmst edcrusivelg faeds m 'Cockle* and Mussels,, %irB back
Inbtvlda2 cinly feeczing on One of tW&L? %~&Les. %-he selertiw of
mnssels ha8 W& s~turliQd in detail lHerr& w d ErySnck. 1986.3 Mere,
1B871 sad it is ~mtnd thde there is n6t mily selectiim foi laper
miwels but even Eo? the chin%er-shelled i@ivfd+ls wishin a lengch
cLies,. It has h~en shoran that these selection patterns are etuced by
energeti'f constraints. Wile the pp6t .~pening-.&l
eureseL6 %a wt
rewartB.ng due to very -U ylela of fl,osh, tha thick sirelied mussele

OCLURRENCE AND DIET QF B ~T
.tUQ
80 -
a
2 40'-
U
(D
8
-20
SBP w'r N~SY 6x0 J& F.XS APE ,,g.
I,.
1
m
0
066URANCti AND LEET DF IFT
108. -- - l00
W
U
L
W
- 60
G,
9,
DI
2
D
2QL
0 b Q
SBP QdT NOW S2SW JW FFsB AQE
>,
-ZQ @
are too hard to open. Tbis selection pattern implies that only a part
of the prey population is really available to the birds, caused by the
selectitin-behaviour of the birds themselves. Eowever, prey availabilicg
is often influenced by the prey itself. In intertidal areas many
maerobenthic spec%es may be buried deep in the substrate, well beyond

each of the birds' bill. Different age classes of burying bivalves.
such ae! Scrobiacularia plmb andMacma balthica. can be successively
eaten by different- species of waders depending on their bill ldngtb as
the older individuals lie deeper in the substrate (Zmuts and wdnk.
1984). Similarly dabbling ducks can reach aquatic plants only whitin a
small depth range. Mute Waas can reach down to 1.5 meter, Cwts oaly
to 0.5 meter. Bentbic feedins ducks are also limited by the water depth
as they can stay rmdet- water only for a certain time.
However, the availability of the prey population can also he dependent
on its own activity. Corophium volutator, a small crustacean Living in
If-shaped burrows, is detected by its predators, e.g. Redshanks. when it
is feeding on the mudsurface by quickly moving the antennae. After a
Bedshank passed, all C. volutator retreat in their burrow, due to the
vibrations in the sediment caused by tbe walking bird, aad become
unavailable to the predators for nearly a quarter of an hour (Gsss-
Custard, 1970).
gnother very important factor iufluencing bird distribution is
salinity, not only by its effects un the prey populations, but by its
influence on the osmoregulat1an of the predator. B i r h feeang on
marine prey organisms have to excrete the excess amount of salt intake,
while feeding, throngh the dasal glands. If the salinity of the water
where thep are feeding is too hlgh, they need to drink fresh mater in
order not to dehydrate (Nysrrtim and Pehrsson, 1988). It is very well
documented that the Pochard, which feeds on Eelgrass (Zostera marina)
in the saline Grevelingenmeer, rest on the fresh Haringuliet (Boudewijn
and Mes, 1986). Nystriim and Pehrsson. 1988, linked both distribution
and diet of swsrel coastal waterfowl species to their uptake and
ability to excrete salt. It is most probably that the very high
densities, especially of diving ducks, in the Veerse Meer, in
oompirrison to the Grevelingen Meer where the food supply i~ much
higher, might be due to the much lower salinity in the Veerse Meer.

5.2 H- factors
Next to all fartors intrinsic to animals and ecosystem.? the occurrence
of htrds is also strongly influenced by man in a great variety of ways.
Disturbance, pellution and habitat loss are the major factors.
Disturbance by airplanea. beats, walking people, bait-diggiw. etc.
causes bitds to leave the feeding ar rerrfing areas fot some time, For
waders, disturbance distances have been measured. They differ greatly
between species. Curlews are most sensitive and fly away at leaat 250 m
from someone walking, Turnstones can be approacbad up to 1130 meter (Van
der &er. 1985). Disturbance causes a reduction in feeding time and
extra energy losses due to flying. This can cause the birds to leave
suitable feeding sites. Not only disturbance at feeding sides but the
availability of suitable rest sites can alss determine the occurrence
of some bird species. Thia certainly holds for breeding sites.
Pollution cab affect birds in tws distinct ways: rbrougb a
deteriozation of the food supply or through impairment of the
physiological processes in the animal itself. possibly leading CO
death. It is known that pollution sees chaagea the structure of
benthic populations a& cmmamities become daminated by a few small
sized $peCies (Oxay, 1982). Although their densities can be very high.
the biomss is on average quite low. Burthermore, few birds are able to
exploit efficient13 thme very small prey items. When the pollutfon
load increases and anoxic conditions oceur, the macrobenthic faona may
nearly completely dieappear, as is found in the eastern patt of the
Westerschelde CDevelter, et al. 1988; Meire and Kuyken, 1988).
Obviously birds will disappear here as well. It has been &own that
eutmphtcetion can improve benthic productivity (Bewkema and Cadee,
1986, 1987) and Van Impe (1985) associated the increase in bird numbets
In the eastern part of the Westersehelde between the fifeies and the
seventies to the improved feraging couditiells for the-e birds as a
eonsequenee of hlgher preductivity due to eutropwcation. As the
benthix fauna has collapsed in this areas by now the posititre effects
of the wtrophication were only very shartlived (Develter, et al. 1988;
Meire and Kuyhn, 1988).

LLttle Tern (Stezna albifrons), characteristic atrd threatened bird of
the Delta area
Young Little Tern (Sterna alhifrons)

Bewick's Swans (Cygnus columbariusl
The Delta area is one of the most important wintering areas of the
specie*

As birds are mainly top predators, they accumulate quite a lot of toxic
substances. The dramatic effects of chlorinated hydrocarbons,
insecticides and chlorinated biphenyls on many piscivores in general
and especially on the Sandwich Terns in the sixties, decimating their
pop~lations are well known (Koeman and Van Eenderen, 19721. The effects
of accwwlation of pollutants ere, hmever, not always so obvious,
neverthelens they wy severely affect populations. Pew data on
conoentrations of pollutanrs in birds in the Delta area are avaFLahlc.
However, experiments with Tufted Ducks clearly deman$trafed that when
fed With Zebra Mussels from the Raringvliet their reproductive oatput
was v eq much depressed IMarquenie, et al. 1986; Seholten and Poekema,
1988). Clearly pollution is a major factor affecting birds in the Delta
area and, as its effects are vey diverse and by now not very well
onderstood, much mere attention should he paid to this problem in the
Eutnre.
Babitat loss is ~f course a very impartant factor influencing birds.
The effects are obvious. In the remaining areas bird densities
increase, causing a higher predation pressure and hence a smaller food
supply. This tpgether with an inereased level of social iateractions
between individuals can reduce quite aubsfantiallp the food intake
lead* td death. detailed revietv af the effecrs of habitat loss bn
wader populatione is given by Goss-Custacd, 1985. It shauld be nated
that hahicat lams is not only cased by physical removal of an anea but
also by deterioration of the food supplp e.g. due to changing
environmental condition#. In the Westerschelde the increased dredging
activities cause the intertidal flats to become more sandy shee in the
larger channels higher current speeds occor, influencing the
sedimentation patterns. The benthic fauna and the numbers of waders are
much smaller on send- than on mudflats. This is especially in the
Wesrerschelde a severe threat.

m's activiries do not always have to affect ecoaystw in general or
bir& in particulex in a negative way. Indeed m y management measure4
ean be taken to improve the arituation, measures which often are rather
cheap but very effective. Oe the Hooge Platen, a large intertidal flat
in the Westerschelde, a small part 19 flooded wly at extreme bigb
tides. The constructidn of a small dike hy meane of sadbags made the
site very attractfoe for Little Terns. The population increased from 60
in 1979 to 170 in 1985 (Pig. 10). a threefeld increase. compared rrlth a
NUMBERS OF BREEDING PAIRS OF LITTLE TERN ON THE HOOGE PLATEN
1979-19BB
Figare 10 Eyelutian of tbe number of breeding Little Terns w the
"Woge Plaacen" i~ the Wesrerschelde (data RQQ
R. Beijersbergen, peasonal eonrmunication~
1.3-fold increase in the whale Delta during the same time (Mein&ger,
1966). On many of the artifiaial islands in the Oostersdlelde, f~rmerly
used during the construction of dams, sluices etc., there are

$oseibilities to create ptentially vaiuable breeding sites for
characteristic coastal breeding birds. By meem of changing the
hydralogy, e.g. by (partially) inumlating 8maU polders adjacent to the
OostersmheLde ancl the Westerschelde, the importance far birds can be
improved. In most places the ever increasing pressure bp recreational
activities (boating, wind-surfers, walking, bait-digging, fishing etc.)
is a serious threat to the various functions for birds. In particular
the intertidal areas are highly vulnerable to disturbance, and measures
to reduca this disturbance are required. The same applies to some of
the shallow parts of the Veerse Meer and Grevelingen Meer, which are
increasingly visited by wind-surfers, not only in snmmer but also in
winter! Simple measures (snch as float* cablw) can be takeu to make
at least dtome of these areas inaceessibSe.
6 CONCLUSION
The Creation of new habitats by damning up eetuaries has, beyond doubt,
increased the diversity of bird life in the Delta. QT both breeding and
non-breeding birds several species increased in riders, Lass is known.
havexer, on the changes in population size of the species OriginaIly
present, although there are indications their populations did not
deurease markedly until 1984. Birds quiekly reacted to the MW
habitats
created, and the Voordelta, the estuaries, the aline and the fresh
lakes each have now their more or less tyaical avifauna. The gradient
in abiotio and biotic ~onditions is very well lreflerted by the
distribution of the different bird groups a+ well as by the individual
species, liowever, notwithscanding the attention paid to nature
consemation in the e l y czeared habitats. the estvarise areas still
accomdate the largest numbers af birds. It ehould also be stressed
that all estuarine ec@wetems, and especially brackish and freshwarer
tidal habitats are on a global scale very rare habitats, *hi& should
MW be completely protected. Thelr loss can not be compensated bp the
creation of new habitats.
For several bird species or groups it is s hm that their distribution
and abundance is closely linked to that Oe their food sapply. An,y

measure affecting food supply or available feeding area will reduce the
population of tkse species (e.g. waders in the Oosterschelde). Far
other species the relircion between density and food supply is lese
oboioys and other variables confeuding this relation are discussed.
The influence of reduced food supply or available habitat m these
species is less easy to pre&tct, although the birds will certainly not
benefit from these. Beyond doubt, disturbance has a negative effect
on all species. I£ the international importance of the Delta area for
waterbirds is to be preseraed, this aspect should receive much
attention from the manager. It is very important! to inform the public
about the imeortance of the area fOr birds and their need for rest.
Canalizing people by means of hides or observation places might be one
part of the salntion. It reduces disturbance and the good observation
possibilities may enhance a positive attitude towards the habitat and
its inhabitants. Additionally the accessibility of certain sites should
be sestricted. Man's impact on the habitat, hg means of dredging.
pollution, etc., should be minimized so that naturally functioning
ecesrystem might be' created or preserved.
These few examples imply that management of the Delta area should he
based on a sound ecological knowledge, but in order to understand the
ecology Of a species much detailed research bas to be done and a
general pattmn, as presented in tbls paper, is amly a first step.
The data presented in this paper have been collected by a large umber
of people. Their help was invaluable. E. Wijken and S. BublE provided
a stimulating enviromeat in Gent, and Rijkswaterstaat. Tidal Waters
Division and Directorate Zeeland not only provided the funds necessary
for the research, but were also very interested fm the results and
wed to use chem in the management of wetlands.

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PISCIVORES
Litfle Grebe
Great Crested Grebe
Black-necked Grebe
cormorant
Grey Berm
sew
Red-breasted Merganser
Gooijande r
(Tachybaptus ruficallis)
(Podiceps cristatus)
(Podiceps ni~ricollis)
(Phalaerocorar carbo)
(drdea cinerea)
Wrgug albellus)
(Menergus serrator)
(Mersus merganser)
Mute Swan
Bewick's Swan
Bean Goose
Whire-fronced Wose
Greylag Goose
mrnaele Goose
Brent Gouse
Wigeon
Teal
Mallard
Pintail
shoveler
Gudwall
coor
Pschard
(cygnus olor)
(Cygnus colw6arius)
(heer fabalis)
(Anaer albifrons)
(Anser anser)
(Branta leucopsis)
(Branta berniclal
(Anas penelope)
(AnaB crecra)
(Anas platyrhynchos)
(Anas acuta)
(Anas clypeata)
(Anas strepera)
(Fulica afra)
(Aythya ferina)

(WADERS AND SHELDUCK)
Sheldock
Oystercatcher
dvocet
Ringed Plover
Kentish Plover
Oolden Plover
Grey Plover
Lapwing
K,l@ t
Sanderling
D u n U
Xuf f
Bar-tailed Godwit
Curlew
Spotted Redshank
Redshank
Greenshank
Twtone
(Tadorna tadorna)
(Haematopus ostralegus)
(Recnrvirostra avosettaf
(Charadrius hiaticula)
(Charadrius alexandrlnus)
(Plwialfs apricaria)
(Plwialis squataroia)
(Vanellus vanellus)
(Calidris canutus)
(Calidris a16a)
(Calidris alpina)
(Philomehus pugnax)
(Limosa lapponica)
(Nmenius arquata)
(Tringa erythropus)
(Tringa totanus)
(Tringa nebularia)
(Arerraria interpres)
(DTVIIiG DlJClls)
Tufted Duck
Cornman Scoter
Goldeneye
(Aythya fuligula)
(Melnitta nigra)
{Bucepbala clangula)
OMNIVORES (GLJLLS)
Black-headed Gull
Comn Gull
Herring Gull
Great Black-backed Gull
(Larus ridibundus)
(Larus canus)
(Larus argentatus)
(Larus marinus)

IPYBRO-EGOLOOIOAL BELATIONB ZN THE DELTA WATERS 08 TKE SOUTH-WEST
NFm-s
C.W.
Iedema
The Deltii Pruject ha* taught us a great deal abont the ecologied
relations between the individual waters af the Delta and bemen the
waters bf the Delta wd the Earth Sea md the major risers. In a
number of instances the knowledge gained could directly be translated
into terme of allscation and management measurdtr. In other easa
valuable lessaw were learned for preeent ar future -gemant. In dl
cases it became clear that ie the context of compartqentalization,
allocation plm aud management should pr*mily be gewed to
supervising the reaulcing processes of chqe. The object is to make
optislaL use of the porential of the indiskdual systems as chey relate
to one another. This paper pzwidea a brief suldmary of the lessons to
be learned £rose prwious studies.
One of the most important lessons learned from the Belta Project, and
one which has been learned elsewhere too, is that it is fatal to
attempt to fight nature. Victories often turn out to he Pyrrhic. The
battle sgainat the vaters must he waged. but in =&=h a way tbat osture
is on man's aide. b e should learn from nature, rather than try to
teach nature a lesson. Exsmples can he found in the Delta region of
how, in this way, an Sncrease in ssfety can be ~~mhined with the

develwpment of lasting and valuable systemnr. This should wt be
interpreted as a licence to compartmentalize estuaries and to create
polders in coastal areas. V we have learned one thing, it is that
estuarine systems are precious and irreplaceable, both in ad
ecological and aa ecoMmic sense. Estuaries are by nature highly
productive and dyllamlr systems. Adequate protection of these area*
against pollution, over-exploitation and other detrimental
inflames can provide a lasting guarantee for such assets.
The c~npartmenralization of the Delta had taught us that this is no
matter of course in the case of the thus created waters. The
developments, which this process triggered off, require close
supervision if these newly-formed waters are to achieve their full
potential, and these new Delta waters do indeea have the potential to
develop into valuable systems.
The nacural relacions in che Delta have been upset. However, it has
been replaced by new relations. In some cases this has happned on a
fairly lbited scale, whereas in others an entirely new natural basis
has been created. Natural relsti~ns are no lwger taken for granted.
Instead it must be taken for granted that an attempt will he made
wherever possible to manage the Delta in its present form as a single
coherent whole. The lessons which have been learned up tb now. and
which have been dealt with extensively in previous chapters, can help
ta guide this process.
The build-up of silt and pollutants are processes with which the
Netherlands, as a delta-country, is eminently familiar. This problem
has also affeceed the Delta of the South-West Nerherlands, and the
following lessons have been learned:
- Upstream pollution results in the build-up of pollueants in
d6wnstrsam sedimentatibn areas with serious detrimental effeets cm
ecological development and economic potential (e.g. pollution of
eels). This process is being reinforced by dsuuning up the
downstream areas at one side. Therefore dadng up cannot be seen

apart from upstream purification;
- h natural estuaries, azeas of sedimentation - lnud flats and salt
marshes - are usually the most valuable areea. Theg aw accardingly
exceptionally susceptible to pollution, as in the case of 'Het Land
van Saeftinghe'. Attempts to clean upstream should therdore at the
very leasr be directed at safegwding the natursl richer; of these
sedimntatlon areas;
- Compartmentalizatiun eau be used to confine accumulation of
pollutants to a rescricted, manageable area. This protects othe~
areas from pollution &nd accompanying problems. Wiahent
compartmentalization a camiderable parr of the severely
coutamina0ed silt, n w present as sedimentation in the Hollands
Diep and Haringrrliet, would have entered the coastal waters and the
Wadden 6ea. So compartmentaliration could also be applied
especially for this purpose, harever, as an interim measure in
combination with efforts to clean up pollution.
3 DOWNS'J!XUM FRESH WATER EUTROPRZCATION
Besides being ezceptianally vulnerable EO pollution, the semi-stagnant
freshwater basins sltnated do*astream are also increasingly
susceptible ta eutrophication. iieretw, upstream purification is
necessaxy for responsible compartmentalizatian. At the same time,
however, the scope for management increases:
- A major factor affecting heightened snsceptibility to
eutrophication is the increase in the residence time oi water. The
organization of the watemnanagement infrastructure, in such a way
that the residence times can be reduced to a euffioient degree, is
an important management aid;
- Eutrophication priaariLy resnlts from an excess aE nutrients. One
of the facters affecting the way in which nutrients paas thtcueh an
aquatic ecosystem is the latter's structure, and this creates scope
for esntrolling such systems, partioulazly in the early stages of
development, as in the case of the Zomeer, Ecological parasleters
must be set fsr the establishment and develop~nent of rich, divers
biocoenoses eharacteriscics of clear water rich in nutrients.

Crucial here is the presence of extensive, protected shallows.
Manageability of fish stocka should be lrentral to policy. Simply
granting fishing rights to anglers and fishermen, frustrate
atzeqts to combat eutropkieation directly by means of active
biological oiutagEment. Suceess is conditional on the management of
fish stocks being incgrporated into water quality management.
Salwte* entraphication in different frotn freshwater wtrophication,
althongk closed, stagnant salt waters also have a greater
susceptibility to eutrophication. One of the main differences is that
in the case of salt water often nitrogen is the limiting factor and
nitrification is the key process. In response to this the Eollowing
managewant measures can be taken:
- Excessive levels of nutrients in stagnant, saline systems are
chiefly regional in origin. In p~ineiple this provides scppe f ~ r
gearing nlltrient levels to the individual eapacity of a system. At
its present level, the channelling of nutrient-rich polder water
into the tidal systems does not lead to undesirable affects
associated with wtmphicat-lon. The channelling to ttre Ea6tern
Scheldt of the polder water currently teleased into the Veerse
Wer, for example, i4 even expected to hring about a slighr rise in
productivity. The main objective, incidentaliy, continues to be the
improvement af polder water quality, not in the Least because of
present pollution levels;
- Besides adaptirrg nutrient levels to the system, it is necessary to
create sufficient scope for intermingling with salt tidal waters if
stagnant, saline systems are to develop. Besides the significance
of salinity as euch aa m ecological determinant, this approach
permits regulation of the nutrient balance. Surplus nutrients and
organic matter ean be &rained
off and the likelihood of
stratification can be reduced. That this latter phenomenon is
rmportant is illustrated by the Veerse Meer. where srratitiaatien
has been Eamd to exacerbate the current prablenns of
eutropllication;

Storm surge barrier Eastern Scheldt

Recreation on Lake Vee~e
e
Shore protection Haringvliet

- In addition to existing management measures, there may well be
future scope for a creative nespome to the ways in wbicb salt,
stagnant lakes function. Such steps might include increasing
denitrification in erde~ to remom nitrogen from the system. Thi8
might be achieved by influenoing nirrifieation and denitrification
processes in the bed by meam of stratification. Another
possibility Would 6e to make use sf the natural process, whereby
organisms which inhabit the salt lake bed act as an essential
fater in the regulation of nutrients. The passibilit~ pf using
sui3 organisms as an active policy irtstrument could be
investigated.
The water in the Delta sues its major significance to the marpholegy
of the basins. The hydraulia conditions of the tidal waters have given
rise to a m~rphological structure and dynamics, which contribute to
the probuctlvify iard diversity of these regions. Gomparmentalization
hs9 sparked af* a process of ~lorphol~&iical change, which will lead to
mme UdEaoourable donditions in the clesed compartme?ats. On the othsz
hma, the morpholog%eal changes along the coast - the creation of the
Voordella - shwld he regarded in a favortlable light, because:
- The morphhetry Of estaaries provideo an ideal starting point fa:
mmimiiring the developent potential of campartmentalized syst-ems.
This applies paftlcnlarly to tbt peesenee of lac:@ areas of
shallows and their gradual transition to t b hanks. It is essential
to preselnre the morphwetry of such areas as much as posp~ihle if
their potential is to be exploited. Considetable experience has
been amassed in the Delta in the constructian of out@? hank
defences;
- It is possible to create new and valuable areas through influencing
processis of seclireantation and erosion. Tha Voordelta is a good.
albeit unintentional, example of this. Sowever, it shoatd be
poaslble, given present know-har and experience, consciously to
create snch as by working in harmwny with nat.ural processes and
eseahIishFng the right parsmeters. In lipe with this approach, ways
are cuzrently be+
investigated of influensisg mud flat and salt

marsh development in the east of the Western Scheldt.
Just as in the case of morphologi~al developments, natural
developxcents often take place on a time scale which far exceeds ehe
term of the average government. This is particularly important in the
case of natural developments, because O~WOIIS of the desired course
of such developments are subject to change. Bowever, this concerns not
sa ioch the ecelogical conditions, which have been dealt with earlier,
hnt ware the management and policy choices which affeet natural
developaent. This applies particularly to former intertidal areas.
To date natural devel6pmenta in rhe Delta have sometimes faced us with
unexpected situations. Although naru mre is knm of the directions in
which nacure can develop, it is important to be aware that the future
may still hold surprises in this respect. Since hug-term developments
are involved, it is important to guard against phasing the successive
stages too rigidly and against trying to steer them too qnicklr in a
certain direction.
Water meaagemeqt rewbs a key confro1 element iu natural development,
especially tn areas along the banks. It is clveial to maintain not
only the quihity, but particularly the level of the vatar. nspits
this fact, in almost no Delta area wat~rlavelmanageaeut is geared to
natural development. This is due KO orher secid interests having been
taken WO considmation. In the damstream freshwater areas of the
Delta, the potential for natural develapment would be optimally
furthered by waterlevel mmagement, which reflected the nacural rise
and fall of waterlevels in downstream areas. In the case of the salt
lakes of the Dslta this is leas clear-out. The best approach is
probably to maintain a standard level, with brief periods of flooding
in winter. Howeven, in the event of a review of or change in
waterlevel-ent great weight shanld 4e attached so the qatural
development factor.

Nature is pliable. Belattvely young and dyndc areas offer particuiar
scope for pasifive influence of natural. development. Such possibilities
must. however, be recognized. Reference was m&de earlier to the
creation 02 md flats and salt marshes through tbe control of
processes of sed3mentatiorn and erosion. An6ther poasibilitg might ile
in the hydrology of the areas within the dykes. The dyking in OX xh~d
flats and salt marshes 6as resulted in the loss of many saline areas.
At the same rime the consttuction of dykes strongly reduced the
potential for the ra-rgence of new saline aims. Uithont wishiw to
add new land to the list of "Eet Verdronken Land van ........." *l,
use could be made of the scmpe for prowting the estzblishnrent of new
saline areas within the &ykess, for example in poldera which are now
alraady margM for agriculture.
7 FODD ECOLOGY CONNECTIONS: BIDS
The Delta is an important "Filling statlon" for birds, and must reraain
so. In the context of nature coneervation and the wxse use of
wetlands, this is one of our mst important international
responsibilities hexe in the Delta.
Besides supervising khe changisg ecological parrnters, dealt with
atenaively earlier, mttnagement and policy should be geared to
regulate the changing use of such areas bg the general public as a
result of greater a~ce~sibility. Particular atteatien will have to be
gioen to resfric'tisg the disrurbanee caused by recreatirmal
actiulties. Other, relaced points will however eIso require conside-
raeiaq, such as llntiting food competition resulting from fiaing.
On the other haus, public support fm certain rwirsures will increiwe
if people can see for th-ekes haw valuable such areas are.
*) "tbe drowned Land of . . . . . . . . . . ."

The best approach would sew to be to provide restricted but ta~geted
admission, for example by building hides and providing effecrfve
public infornation.
Compartmentaliaation has sharply increased the diversity of habitars.
As a result, the individual system$ are characterised by deferent
bird populatiow. 5-
syeteae are 6tiI1 in a state of transition.
Sandvich terns, fmr example. still breed in €he Greuelingemeer area,
whereas the type of bare ground required by these birds for nesting
purposes is increasingly disappearing fromthe region through natural
causes. Although the presence ol this specias ma7 be extremely important
to local nature conservationists, one mwst try and avoid making
desperate ef3orts tQ fry to keep certain species in areas which are
actually no longer suited to them. A more sens%ble approach would be
to try t6 reinforce the characteristics peculiar to each individual
system.

Ro. 1.
No. 2.
SQ. 3.
Investigations into the water balance of the Rottegatspolder.
The water supply for craps I.
Obsemvations of groundwater levels.
Investigations by $rain gauges in the Netherlands.
me water supply for crops 11.
The problem of the increasing salinity of ground and SutfaCe
vat& in the Netherlands.
Proceedings of Tecbuieal Meetings 1-6 (with s~lthlties in
English), 1952.
The study of precipitation data.
Model reeearch on groundwater flows.
Measurements and improvement works in basin of brooks.
Oeo-electrical research.
Proceedtn@ of Technical Meetings 7-10, and
Report on the evaporatian research in the Rottegatsaolber
1947-1952 (with solmsaries in English), 1955.
The water supply of sandy soils.
pualisy requirements far surface waters.
Proceedings of Technical Meetings 11-12 (with aumoqaxies in
English), and
Reprt on the Lysimeters in t&e Netherlands I (in %&ish),
1958.
No. 4. *) Evaporation Symposim and
Report on the tysimeters in the Netherland5 IL (with eaies
in English). 1959.
*) Gut of print.
No. 5.
No. 6.
Groundwater levels and grsundwater wement in the sandy
areas of the Wethmlwds.
Water in uwaturate4 soil.
Procacdings of Tedmical Meetings 13-1b (with SMsnaries in
English), 1960.
The regime of the Uhine, eh& Psselmeer and Zeelasd Lake.
Proceedings of Technical Meeting 15 (with summaries in EnglisR
and French), 1961.
NO. 7. Tbe dry year 1959.
Proceedings of Technical Meeting 16 (with smtmaties in
English). 1962.
No. 8.
The laws of groundwater Slow and their application in practice,
Proceedings of Technical Meeting 17 (with smmnaries in
English). 1963.

No. 9.
Water nuisanc*.
Proceed%ngs of Technical Meeting 18 (with smries in
English). 1963.
No. 10. Steady flow of groundwater twa~de wells.
Compiled by the ltgdrologisch Colloguilmr (in English), 1961r.
No. 11.
No. 12.
No. 13.
Geobpdrological cartography.
Proceedings of Technical Meeting 19 (with summaries fii French
and Geman), 1964.
Water balance studies.
Prqceedings 05 Technical &eeting 20 (in English), 1966.
Recent trends in hydrograph synthesis.
Proaeedings of Technicsrl Meeag 21 (in English). 1966.
go. 14.
No. 15.
Precipitation data (11) and
Report on the Lysimerers in the Netherlands (111)
Proceedings of Technical Meeting 22, Cvlth suromaSe6 in
English) 1968.
Soil - water - plant.
(h Znglish) , 1969.
#Q. 16. Bpdrological investigations .for masterplan for the future
watersupply in the Netherlands.
Proceedings of Technical Meeting 29 (with sumaries in
English). 1975.
No. 17. Auromatfc processing of h~drological data.
Proceedings of Technical Meeting 25 (in Emlish), 1973.
Ns. 18. Eydraulic research for uatar laanagement.
Proceedings of Technical Meeting 26 (in English), 1974.
WO. 19. The hydrological investigation programme in Salland
(The Netherlands).
Procaedings of Technical Meeting 27 (in English). 1974.
Na. 20.
MO. 21.
Salt dietributian in estuaries.
Proceediags ef Technical Meeting 30 (in English, 1976.
Groundwater pollution,
Proceeding% of Technical Meeting 31 (in English), 1976.
N0. 22. Systems agproach to the managemant of water resources.
Eioceedings of Technical Meeting 32 (vlth suromaries in
English). 1976.
No. 23.
No. 24.
Precipitation and measurements of precipitation.
Proceadings of Tecturical Meeting 33 (in English), 1977.
Urbanization and water management.
Proceeding@ of Technical Meetfng 34 (in English). 1978.

25. The relation between water quantity and water quality in
studies of surface waters.
P~oceedings of Technical PLeeting 35 (in English), 1979.
26. Eeaearcb an possible change* in the discrLhuriou of =line
seepage in the Netherlands.
Prnceedings of Technical Meeting 36 (in English]. 1980.
No.
No.
NO.
27. Water resources menagament on a regional scale.
Prsceedings of Technical Meeting 37 (in Fagfish), 1981.
28. Evaporation in relation to hy4mlogy.
ProceedZngs of Technical Meeting 38 (in English). 1981.
29a. Pality analysis for &e national water mnagement of
the Netherlands.
Background papers for Technical Meeting 39 (in Engliah), 1982.
(Netherhds! contributiorw, related to the PAW-Mudy, for the
ECE-seminar-1980.)
2%. Economic Ustruments for rafiohll utilization of water
resources.
Netherlaads contributions, nm related to the PAW-study, Tor
the ECE-saminar-1982 (in English), 1B2.
No.
No.
30. The role of hydrology in the United Natims Water Decade.
Proaeedings of Technical Meeting 40 (in English), 19S3.
31. Methods and instrumentation for the investigation of groundwater
systems.
Prooeedings of isternatienal ~osiran, Noordwig kerhout .
The Netherlands (in En&lisb, vfrh summaries in Brench). 1983.
32.*) Flaming of Water Resources Management on a Regtonal Seale.
Proceedings of Technical Meeting 41 (with Prefaae in English),
1985.
*) Qut of print.
NO.
Ns.
33. Wacar in Urban uleas.
Proceedings of Technical Meting 42 (in English). 1985.
34. Water management in relation to Nature, Forestry and Landsciyle
knagement.
Proceedings of Teehnlel Beeting 43 CSn English). 1986.
35. Design aspects of Bydrological Networks.
Published with support of the World Meteatological Organiaation
(in Engliah), 1986.
36. Urban storm water quality and effects upon receiving waters.
Proceedings of International tonference. Wageningen,
The Netherlanas (14 English), 1986.

No. 37.
No. 38.
No. 39.
No. 40.
No. 41
Water in the Netherlaods.
Repxint of special issue CHO-TNO 194&I486 with dnnex SeIection
of current topics, (in Engliahl. 198'9.
Vulnerability of soil and groundwater to p0llntants.
Proceedings of the International Conference, Noardvijkerh~ut,
the Netherlands, organi~ed by the National EttstlNte of Public
Health and Envirosmental Rygiene (in Esgltsh) , 1987.
Evaporation and weather.
Proceedings of TecMcai PIeeting 44 tin E%Lish), 1987.
Georhewal emgy and heat storsge in aquifers.
Proceedings o& Tecbical -tin& 45 (Tcl Emish), 1988.
Ilpdro-ecolpgiCg1 relations in the Delea TSaters of th~ South-West
Netherlands.
Proceedings of Techniaal Meeting 46 (in English), 1489.
All repart* are written in Enelish except reports nos.:
l, 8 9 l 4 16, 22, 32.
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