Hydro-ecological relations in the Delta Waters

Table 3
Eutrophication of the Volkerak-Zoom lake system and
Lake Brielle
Lake Brielle
Volkerak-Zm lake system
predicted 1987 L988
total P
(mgfl)
transparencp approx. 1 approx. 1 1 2.2
(m)
!t%e general tendency of the forecasts was that the Volkerak-Zoom lake
system, with 8 residence tinle of 2-3 months, would develop into a
eutrophic water hody with a chlorophyl level of 60-100 mg/ms, a total
P-level of 0.20-0.35 mgll and a transparency not exceeding l m. It is
not possible to compare the actual situation in 1987 with the
predictions, because iri that year there was a sharp fall in the
chloride level during the growth season and hewe there was no
equilibrium situation. Rowever, the values measured in 1987 fall within
the range of forecast values.
The situation ia 1988 was clearly different. That was the first year in
whi& the lake was largely Eresh water: the measured values were far
more fwontahle than predicted. The mean sunmeer chlorophyl level was
approzimately 20 %/m3, tbe tQtal P-level was 0.17 mg/l and the mean
transpan&ncy was 2.2 m.
For cqarison, the values measured in Lake Brielle have been included
in Table 3. This lake is reasonably comparable with the Volkerak-Zoom
l& system in terms of morphology and residence time. The meam
chlorophyl Xevels in Lake Brielle are relatively low, but the