Hydro-ecological relations in the Delta Waters
Hydro-ecological relations in the Delta Waters Hydro-ecological relations in the Delta Waters
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
- Page 84 and 85: osi* rn@8IOIP m .SL 4 Has F~~ULB 3
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freshwater bas<strong>in</strong>s. Clearly <strong>the</strong> presenoe of <strong>the</strong>se macrophyta haw no<br />
<strong>in</strong>fluence on <strong>the</strong> density af all herbivores as <strong>the</strong>y form only a 4 1<br />
part of <strong>the</strong> diet pf only some species. Notwithstand<strong>in</strong>g <strong>the</strong> huge<br />
differences <strong>in</strong> food supply it is rewkable no larger diffexenees <strong>in</strong><br />
densities exist.<br />
Table 5<br />
Comparison between <strong>the</strong> consumption (C) (kg ash free dry<br />
weight/lQO ha3 and deqsity CD3 (birds/lQD ha) of different<br />
bird groups <strong>in</strong> <strong>the</strong> tidal areas, <strong>the</strong> sal<strong>in</strong>e and t&e freshwater<br />
l&s (far fur<strong>the</strong>r explanation see tm).<br />
TIDAL WINE FKESH<br />
Herbivores<br />
C<br />
D<br />
Div<strong>in</strong>g ducks<br />
C<br />
D<br />
Waders<br />
In Table 5 <strong>the</strong> dara on densities are summarized toge<strong>the</strong>r with estimates<br />
of <strong>the</strong> food emsumption a£ <strong>the</strong> different bird gzoups. This was obta<strong>in</strong>ed<br />
by standard methods (see e.g. Nienhuis and Groenendijk, 1986). It i8<br />
obvious that <strong>the</strong> densities of herbivores are high <strong>in</strong> cwtpsrisun with<br />
those of o<strong>the</strong>r groups, <strong>the</strong>ir consumption even mueh higher. This is what<br />
we expect from primary consumers. Waders and div<strong>in</strong>g ducks are secondary<br />
omsumess and <strong>the</strong>ir densities as well as <strong>the</strong>ir oonsmptian are much<br />
lewer. The on17 exception are waders <strong>in</strong> <strong>the</strong> tidal areas. It is obvious
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