Hydro-ecological relations in the Delta Waters

Breeding areas for pike can be provided by the large-scale installation
of defences in front of the banks and by -g
areeh suitable.
Xf d tor$ng shows that the numbers of bream threaten to become
excessive, then predatory fish could be introduced, or whitefish could
be qetted and removed.
Stimulatiu~ the growth of freshwater mussels is a third type of measure
appropriate to active biological management. These mussels are net
shown ia Rigure 10, but the^ fulfil more or less the same role as the
zooplankton: they fflEer suspended matter from the water and thereby
also contribute to the grazing pressure on the algae. Mussels require a
hard substrafe. This could be provided by dumping large amounts of
shells in suitable plaoes.
5 CONCLUSIONS
In general, the dwming off and comparcmentalizatim of the Delta
wters leads to stagnant or semi-stagnant waters whose very location
m&w them ausceptihle to eutrophication prohlw.
In she Bollwds Diep/Bazingvliet eutrophic+tion problem can be kept
within limits by feeustug management on preventing long residence times
fn the saer. The critical level seqms to be arousd three weeks.
Ln tbe Volkerak-noom lgke system the startiag pobt is favourable: the
water is transparent. At present the phosphate level is too higb to
lMt eutrophication, hut active biological magement offers good
prospects for keeping the water transparent. The phosphate load needs
to be reduced fnrther. It is likely that in the next few years bluegreen
algae will occur regularly in the autumn. It will probably be
possible to tackle the symptoms of thie in areas tlar suffer grerrtly
from the problem.