Hydro-ecological relations in the Delta Waters

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