Hydro-ecological relations in the Delta Waters

are taken. The first year after the desalinikation proved mmre
favourable thm predicted (the himas8 of algae was relatively small
and transparency was ghod), but the input of nutrients meds to he
limited still further in order to arrive at an enduring Iavouzable
water quality, In addition, "active biological management" should be
considered, i,e. the applicatimn of biological management raeasnres to
obtain a healthy water system wfth clear water, an abundance of aquatic
plant@ and a stock of predatory fishes that wrll keep the whitefish
within bounds and hence allow sufficiefit eooplankton to remain to graze
on the algae.
Eutrophication is the enriehmerrc of a water system r*ith plant nutrients
(also simply called nutrients), of which nitrogen and phosphate are the
mast important. Eutrophication is partly a natural pzoeass: erosiofi and
leaching wash nutrients inta zivers, wkich transport them to the lower
reaches. Therefore, delta areas are natur.%lly wtrophic, and the Dutch
Delta area is a good example.
In the last 100 years humanity has greatly exacerbated the natural
process of eutrophication. The insrallatien of drains ad sewers, the
use of detergents and fertilizers and the increasing intensfficstion of
livestock fanning have led t0 the amounta of nutrients in the rivers
being increased many times over.
Tn sttagnant water the enrichraent with planr nutrients can lead to m
increase in the growth of algae, which can reault in high 4ensities of
algae. Large numbezs of algae 14 the water results in a lev
transparency; when the algae dh aeaerobim can occur and thie In turn
can lead to fish mortality and a bad emell. Furthermore, toxic species
of algae can occur (blue-green algae). Some species £am floating mts.
Sa~tions 2 aad 9 of this paper describe the eutrophication situation of
two Serni-stagriant water systems in the Delta area: the nollands Dlepj
Haringvliet ad the Vohrak-Zwm lake system. The wtrophtcation state