Nitrogen in Dutch freshwater lakes: trends and targets

A trend-analysis of eutrophication variables revealed that concentrations o f total-nitrogen, total-phosphorus and chlorophyll-a decreased over the per iod 1980-1996 in a large majority of the Dutch freshwater lakes. Relative t rends of nutrients compared well with emission reductions, which were much lower for nitrogen compared to phosphorus. Despite the decreasing trends, o nly 35% of the lakes met the water quality standard for summer mean total-n itrogen of 2.2 mg 1(-1). Based on protection against ecological deteriorati on or possibilities for recovery of eutrophic lakes, a promising goal is a summer mean total-nitrogen of 1.35 mg 1(-1). However, atmospheric dinitroge n fixation by blue-green algae will occur in lakes with a growth limiting n itrogen concentration when sufficient phosphorus, trace metals and light en ergy are available. Therefore, a combined lake-specific approach of nitroge n and phosphorus emission reduction and top-down control measures may be th e best strategy to combat eutrophication.