Trend-analysis of eutrophication variables in lakes in The Netherlands

A trend-analysis of eutrophication variables was performed for a large number of lakes in The Netherlands. Data of in total 231 lakes were available. Data on chlorophyll-a, total phosphorus (total-P) and total nitrogen (total-N) were analysed over the period 1980-1996. Summer-averaged concentrations for chlorophyll-a, total-P and total-N decreased in respectively 65%, 73% and 75% of the lakes with at least eight years data between 1980 and 1996. Results for winter means were comparable (a negative trend in 54%, 77% and 69% for concentrations of chlorophyll-a, total-P and total-N respectively). Since 1980 the median decrease in the summer averaged concentrations of chlorophyll-a, total-P and total-N is 2.61 mu g l super(-1) y super(-1), 0.008 mg l super(-1) y super(-1) and 0.046 mg l super(-1) y super(-1), respectively, illustrating the effects of (inter)national and regional measures to combat eutrophication.