Hydro-ecological relations in the Delta Waters

The Hollands DiepJnaringpllet is part of the northern Delta basins,
also called the lower area of the peat riiuers. This is the area where
two large Western European rivers, the Ehine and the Mewe, discharge
into the sea. The river vater enters the area from the east along thtee
river channels and can. in principle, reach the sea via two routes.
When the Rhine discharge is less than 1700 m3/s
tbe Haringvliet sluices
ip the west rermtin closed and all the river water enters the sea via the
New Waterway. At Rhine discharges abwe 1700 ms/s the Hazingvliet
sluices are also used to discharge the riaer water. These sluices are,
haqeaer, ~nly opened at low tide. This management of the hringvliet
sluices obviously has repercussions on the water movement in the
Hollands DieplHaringvliet and henee also on the eutrgphicatios
sitnatLon.
The eutrophlCation situation of the basins ~im he characterized as
falLows. Large amounts df nitrogen and phosphate are brought in by the
rivers, thns the concentrations of nutrienks are high. Howevet, it is
striking that in these waters a low biamass of algae U usually formd.
Rlsa. tlie bbmass of algae usually decreases fxom east torest; as a
~le, the £eves* algae are found near the Wingvliet ailuices.
Generally. fev blue-green algae oceur in these waters and the
trausparency is reasonable on the whole; near the Karisgpliet sluices a
twsperencg of 1 to 1.5 m is regularly found.
The relatively low biarsasses of algae are often attributed to the
influence of toxic substances. It will be shown below that the watez
movemat provides a plausible exalanation. Figure 2 shows the chasges
in mean snmaer Level of chlorophyl along che traject from Lohith to
the Earingttliet aluices for the pear 1982. Chloropbyl is an indicator
of the amaunt of algae in the water.
From Figure 2 it can be seen that there is a clear decrease in the
biomfass of algae along the traJect. However, the picture changes if we
look at figures from other years. Figure 3 shows the mean smmer levels
of chfwrdphyl between Lohith and ehe Earinpliet slaices in 1976, L982
and 1987.