ECOSYSTEM RESPONSES IN THE RHINE-MEUSE DELTA DURING 2 DECADES AFTER ENCLOSURE AND STEPS TOWARD ESTUARY RESTORATION

Changes in hydrology, and developments in morphology, water quality, a nd ecology of the Rhine-Meuse estuary after its enclosure in 1970 are analyzed on the basis of existing monitoring data. Annual averages of ammonium, nitrate plus nitrite, total phosphate, total lead, and chlor ophyll-alpha concentrations as well as transparency of the water are p resented. Abundances of some water bird species are given for the peri od 1970-1993, and the relative fish biomass for the period 1971-1988 i s discussed. The gradual evolution led toward the dominance of bream. The area has developed toward a system with generally low natural ecol ogical values. Ecological impacts of present sluice management are dis cussed and include the accumulation of contaminated sediments, disappe arance of intertidal areas and nursery grounds for fish, disturbance o f fish migration, and less mixing of river and seawater. Recent policy developments have brought the present management of the Haringvliet s luices back into discussion. A recent policy document has presented se veral management alternatives, including partial and complete reopenin g of the sluices to permit saltwater intrusion. Three management optio ns are compared in terms of costs and ecological benefits. It is concl uded that a complete reopening, and thus a partial restoration of the estuarine characteristics, is most beneficial for the ecosystems of th e area itself, for the upstream Rhine and Meuse rivers, and for the ad jacent North Sea, but costs amount to about 600 million US $.