Hydro-ecological relations in the Delta Waters

Figure 5 show the numbess of Cladocera (an o~der of zooplankton which
includes the water fleas) in Lake Volkerak and Lake Zoom in 1988.
The unusual rransparency can be atfributed to the intensive consumption
of algae bp zooplankton.
- Lake ---- Lake zoom
Votkarak
260
zoo C
1
0 5 l0 16 PO 25 3 0 36
week no.
Figure 9 Numbers of Cladacera in Lake Volkerak ad Lake Zaom in 1988
The fact that ths group of aooplanktm peaks in W/June indicates
that the clear deel%ne in the Biomass of algae may be caused by intensim
grazing by the zooplankton. Although not sham in Biguxe 9, during t b
reat of *he stumer it was striking that e relative17 high n&r vf
large water fleas were present. 3usr these water fleas can create an
effective grazing pre$sure on algae.
In turn. the large nuwbers of water fleas e m be explained by the still
limited numbem of plankton-eating fish.
Floating mats of blue-grem algae accured very locally L& Volkexak
and Lake Zoem in autumn 1988.