Hydro-ecological relations in the Delta Waters

(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