Nitrogen in Dutch freshwater lakes: trends and targets

A trend-analysis of eutrophication variables revealed that concentrations of total-nitrogen, total-phosphorus and chlorophyll-a decreased over the period 1980–1996 in a large majority of the Dutch freshwater lakes. Relative trends of nutrients compared well with emission reductions, which were much lower for nitrogen compared to phosphorus. Despite the decreasing trends, only 35% of the lakes met the water quality standard for summer mean total-nitrogen of 2.2 mg l−1. Based on protection against ecological deterioration or possibilities for recovery of eutrophic lakes, a promising goal is a summer mean total-nitrogen of 1.35 mg l−1. However, atmospheric dinitrogen fixation by blue-green algae will occur in lakes with a growth limiting nitrogen concentration when sufficient phosphorus, trace metals and light energy are available. Therefore, a combined lake-specific approach of nitrogen and phosphorus emission reduction and top-down control measures may be the best strategy to combat eutrophication.