Hydro-ecological relations in the Delta Waters

The average discharge of Xhine and Xeuse together is 2538 m3s-I
Ruyter, et al. 1987) and the nitrogen (ammonium and nitrate) load of
these rivers has increased over the peried 1950-1980 with a factor 2-4,
whereas rtre phospkorus load has increased with a factor 5-7 (Van der
Veer, et al. 1988). Contrary tn the rtitrogen load, the phosphorus load
decreased slowly after 1980. The concentrations of plant nutrients
tncreased even nore markedly than the load: a fivefold fncrease for N
and a tenfoLd 3icrease for P, with recently a slight decrease (Pig. 2).
The increased concentration6 of nutriant.8 in the Bhine-Meuse resulted
in a 3- to 5-foLd rafse of N-
(De
and P-concentrations in the Dutch coasta3
waters. The increase of nutrient concentrations an3 -loads in the
Wwterschelde ebtUary showed the same dramtic trend over the past 30
to 40 Bears (Van Buuren. 1988).
Figure 2 Nitrate- and orthopho9phate concsntrations of Ehinewater
near the Dutch-German border, durins the period 1966-1982
(lUWA Bnnual Report 1982. in Slooff, 1983).
Approxisrately 15% of the mine run-off flows Suto nartherly dinecticns,
aYa IJssel and 'IJsselmeea, ad fiirinlb enters the wwtern Wxdden Sea,
causidg oonaiderable wthrophi~ation to chat estuarine area (Van der
Veer, et al, 1988). More than 50a of the EWnewarer mns via tbe Niewe
Uatemeg (Fig. 1) direct13 into €he North Sea. &bout 3QZ flows into the