Hydro-ecological relations in the Delta Waters

model calculations reveal that a nitrogen loading of approbimately
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10 g W m y or more unwuples the eutr@hiulti@n controlling
processes (Qe Vries, et al, 1988).
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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