OOSTERSCHELDE ESTUARY (SW NETHERLANDS) - A SELF-SUSTAINING ECOSYSTEM

The Oosterschelde estuary has a special position among the Dutch North Sea estuaries. With relatively unpolluted water and high transparency it combines high biotic diversity and high primary and secondary production. Before 1970 a major part of the water of the rivers Rhine and Meuse flowed into the Oosterschelde. The building of a permeable storm-surge barrier (1986) decreased the exchange between Oosterschelde and North Sea. The construction of two additional dams (1987) reduced the very limited fresh water discharge on the estuary even further. The total effect of these changes was a decrease of the nutrient levels in the water column. The estuary is used intensively for culture of mussels and fishery of cockles. Both zoobenthos groups use together up to 60% (in the western part) of the organic food available in the Oosterschelde estuary. The storm-surge barrier resulted in reduced current velocities and increased sedimentation, accompanied by higher transparency, hence accelerating primary production. This phenomenon is counteracted by the lowered nutrient concentrations throughout the year. In the seventies a preliminary carbon budget study indicated that a substantial import of organic carbon was required to sustain the ecosystem. Later studies did not confirm this hypothesis. In the present paper three different methods are presented to answer the question, if internal production suffices to sustain the foodweb. At first a steady state model is applied, secondly, calculations with actually measured process rates are carried out and, finally, a dynamic simulation model is used. The conclusion is drawn that, before the construction of the storm-surge barrier, primary production of organic matter by phytoplankton is much more important than import, although the latter can not be neglected as additional food source for mussels, cockles and zooplankton. The simulation model predicts that this conclusion will not have to be changed in the future. The Oosterschelde will remain largely a self-sustaining ecosystem. © 1990 Kluwer Academic Publishers.