Hydro-ecological relations in the Delta Waters

Table 2 suonna*es the mast importmt results of the research on
eufrophicath in the Hollanda DiepIIlaringvliet. From thia table it can
be seen that in 1976 rhe residence time in the Ballads Diep/
Haringvliet was Long; 60 days on average. The smtem behaved like a
lake. The algae in the IEollands Diep settled out, but in the
Haringvliet they were able to grow well.
The residence time was much shorter in 1982: apgroximately 6 days on
average. The system behaved Like a transition between a river alld a
lake system. Algae settled wt both fxi the HolLands Diep and in the
Earingxliet.
In 1987 the residence dm. was vely sbrt; the system behaved like a
slow-flowing river. The algae transparted with the pates -re egrried
along rwards the sea.
l'he water movement thus offers a very acceptable explanation for the
behaviour of algae in this system. kence. other reasons, such as
t~xological effects, seem less probable.
The VoUcaak-Zoem lake system eame inta being in April 1987 after the
closure of the Philips dam. Ptsm a saltwater tidal area this area has
gradually chaaged, first ints a bSacld8h lake. During 1987 fhe
desalinieation was acaelerated bg allow* an ex+ra intake of fresh
watez via t&e Volkerak slaices. The Dintd, a small river that draing
much of Wear Brabant end z& smell part of Belgivm, is annthiir important
smrce of frbsh water for this lake.
Before the l&e was created predirtions were made about the
eutrophication that could be expected. Table 3 gives an impression of
this and also shows the eutrophicatima observed in 1987 and 1988.