Hydro-ecological relations in the Delta Waters

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).