Restoration and recovery of shallow eutrophic lake ecosystems in The Netherlands: epilogue

Abstract

During the symposium ‘Restoration and recovery of shallow lake ecosystems in The Netherlands’ studies on restoration of eutrophic lakes were addressed and discussed. Many Dutch shallow lakes have received high external loadings of phosphorus through supply water that is influenced by the River Rhine and loadings in The Netherlands. Two important Action Plans (the Rhine Action Plan, the North Sea Action Plan) are now in operation to reduce nutrient emissions. The targets set are not likely to be fully reached, so that supplementary reduction of phosphorus supplied to inland fresh waters will be required. In several shallow lakes such a reduction has been achieved recently, but without leading to discernible recovery. The main causes of delay are phosphorus storage and its subsequent release from sediments and foodweb; however, the remaining extraneous phosphorus supply is often still too high. Supplementary actions are, therefore, called for. A further reduction of phosphorus inputs is suggested, besides supplementary measures proposed, viz. dredging, flushing, biomanipulation, chemomanipulation. Restoration to the past situation via upwelling groundwater appeares to be feasible in some cases. There is a common consensus that each lake behaves differently depending to its morphology, hydrology and history of eutrophication. Therefore each lake has to be studied before restoration measures can be applied. Besides, the ecosystem should not only be studied as a separate entity, but as a part of systems of a higher integration level.