THE EASTERN SCHELDT (THE NETHERLANDS), FROM AN ESTUARY TO A TIDAL BAY - A REVIEW OF RESPONSES AT THE ECOSYSTEM LEVEL

In the past 30 years, various estuaries in the southwest of the Netherlands have been closed off from the North Sea to prevent the flooding of the lower drainage basin of the rivers Rhine and Meuse. The tidal characteristics in the Eastern Scheldt (Oosterschelde) estuary have been maintained since the construction of a storm-surge barrier which is closed only under very severe storm conditions. The barrier (completed in 1986) and compartment dams at the eastern and northern boundaries (completed in 1986 and 1987, respectively) have changed some of the abiotic characteristics of the estuary. The total area under tidal influence has been reduced by 22%. The water exchange with the North Sea has decreased by 28%, which has consequently decreased the current velocities and turbidity. Due to this reduction a new geomorphological equilibrium between erosion and sedimentation has evolved, resulting in a further loss of intertidal area. The freshwater load has been reduced by 64%, and hence salinity and water quality have increased while nutrient concentrations have decreased. Former gradients in salinity and nutrient concentrations have disappeared; the estuary has turned into a tidal bay. Shifts in habitats and species distribution and changes in the phytoplankton community structure are the dominant responses of the ecosystem. Minor changes in the productivity of the system at the first and secondary trophic level have occurred and the carrying capacity for wading birds has decreased due to reduced availability of foraging areas. On the basis of four years of observation it is concluded that the Eastern Scheldt ecosystem has shown a resilient response to the changes in physico-chemical environmental factors.