About 20 shallow lakes form, together with interconnected canals, one hydrological water system in the province of Friesland. Total water surface area meets 14,000 ha, of which the lakes cover 10,000 ha. The regional catchment area is about 305,000 ha. In summer periods inflow of water from Lake IJsselmeer keeps the system on a constant water table of 0.52 m below sea level.

At the end of the sixties the lakes became hypertrophic. Algae were dominating the biocenoses and submerged water plants disappeared. Systematic investigations started in 1970, and biological data were included from 1976 on. During twenty years a stable, but highly eutrophic situation, passed. Blue-green algae, especially Oscillatoria agardhii, were dominating the phytoplankton. Submerged vegetation was absent and bream dominated the fish-stock.

From 1991 onwards there is a slight improvement in water quality. Transparency increased, phosphorus concentration and algal biomass decreased. Also a shift in phytoplankton species occurred. Dominance of O. agardhii decreased and species diversity increased. First submerged water plants recovered in 1994. Improvements are induced by a combination of measures and a culmination of small effects. Results of 14 years of water quality research are presented. Results are given as trends in time. Attention is paid to the spatial gradient in the province as well. From south to north the chain-arranged lakes represent a remarkable pattern of increasing trophic state, caused by regional loading of nutrients.

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