Multi-lake studies in The Netherlands: trends in eutrophication

The relation between (inter)national programs aiming at nutrient load reduction and changes in eutrophication has been studied for 231 Dutch lakes over the period 1980-1996. Trends in total-phosphorus (P) and total-nitrogen (N) were negative, as determined by analysis of both individual lakes and the complete data set. The relative trends in the nutrient concentrations as well as in the N/P ratio correspond with the significantly reduced P emission and the limited reduction of N emission in The Netherlands since the beginning of the 1980s. Negative trends in chlorophyll-a and positive trends in Secchi-disc transparency may be partly explained by reduced nutrient concentrations. Perspectives for the nearby future are discussed. Lake characteristics had only a minor impact on the trends. The improvement of the water quality was found for all subsets of average depth, surface area, hydraulic retention time and soil type. Furthermore, the effect of restoration measures and meteorological conditions on the trends were studied. Biomanipulation resulted in an additional improvement of several water quality variables compared to lakes that were only subject to (inter)national programs on nutrient load reduction. Specific measures resulting in additional P load reduction resulted only in lowered P concentrations. Severe winters resulted in lower chlorophyll-a concentrations in the following summer and dry conditions in spring were favourable for all eutrophication variables in the following summer.