This book contains the proceedings of a two-day symposium held in Amsterdam (18 and 19 April 1991). There are 26 scientific papers divided into three parts. Part One presents the results of a ten-year study carried out by the multi-disciplinary WQL working group (Water Quality research Loosdrecht lakes) on the Loosdrecht lakes. The central theme of the studies was the quantitative phosphorus flux in the various compartments of the ecosystem (including the food chain), paying special attention to supplementary measures to combat eutrophication. Several papers portray the state of the different aquatic environments, pertaining especially to ecological assessment of their water quality, complex hydrology, and interactions at the sediment-water interface, as well as between phytoplankton, detritus, zooplankton and fish. An overview of the food chain in the Loosdrecht lakes is given, in the light of relationships between the physical-chemical environment on the one hand and between the phytoplankton and zooplankton on the other, using multivariate analysis. The vast amount of data collected on the structural and functional aspects of the ecosystem enabled a mathematical eutrophication model, with which to simulate additional measures. The logistics employed and the infrastructure developed by the WQL facilitated ground-truth verification of the remote sensing studies. The research studies and conclusions derived from them were critically analyzed from the points of view of the national eutrophication policy and the regional water authority. A list of WQL publications, the last chapter in Part One, is preceded by a brief synthesis of WQL studies by the editors. Part Two contains more papers with studies carried out on other lake ecosystems. Here the emphasis is a critical analysis of eutrophication. These studies were directed at additional restoration measures, such as flushing, dredging, chemomanipulation and biomanipulation, either in isolation or in combination. In Part Three, the Epilogue, the editors briefly review the causes of water quality deterioration, lake eutrophication and problems of resilience and delay in recovery of these shallow, eutrophic systems in the Netherlands.