Hydro-ecological relations in the Delta Waters

4 WAGBEET MEdSIIRaS TO PEEVBT OR LTXIT TAE EUTROPEICBTION OF THB
VOLKERAK-ZDOM LAKE S%STEiT
A large nurober of goals have been formulated Tor the management of the
Volkerak-Zoom lake system. They include the prevention or reduction of
eutrophicatian. Two types of IMma@ement measures could be used to
achieve this goal:
- reducing the nutrient load;
- acteve Biological management.
It mnst he stressed Ulat these are nat afternatlves. Active biological
management eaanplemencs the reduction of nutrient load.
4.1 Dealing tdfh nutrients
When reducins the nutrient load the accent will lia on phasphate.
becatwe in the fresh waters in the Netherlands this Es the natural
limitfug faetor to the grovth of awe. For lakes like Lake B~rieLle and
the Volkeritk--Zoom lake system che pfissphate is only limiring Bar algal
growh if the phosphate load is below 2-4 $Em2 per year.
Frm Table 4 it cen be seen fb% the phosphate load of Lake Bridle has
been drasticallp reduced since 1972. Uovever, chia has not yet led to a
reductian of the eutrophieation preblem. The fact that the bed of this
lake has meanwhile been accumulatFog phospliatea that, in prineigle,
could again cause an iaterna-l lead, is ullquestionably impartat hers.
Zn this lake, largo-seale occurrences of drifting mats can be limited
ts some extent by 5nterim eera flushing in the sumer period.
In the Volkerak-Zoom l&e system the phosphate load has turned out
10- than the foreoast, but in l988 it was still far too high eto he
limiting £er the growth of algae. The option of extra flushing to
cwbat drifting mats is not considered hero. because tt conflicts with
anothar goal of water management, i.e. trying to prevent the water
systw~ from being loaded with toxic Substancee from the Rollands Diep.