Nitrogen retention in freshwater ecosystems

Given the prominent role of nitrogen in the eutrophication of aquatic systems, recent increases in nitrogen loading to freshwaters are of concern. A comparison of nitrogen retention in freshwaters in North America and Europe shows that wetlands retain the greatest proportion of their nitrogen load, followed by lakes and then rivers. A comparison of the relative importance of nitrogen retention mechanisms found denitrification to be responsible for the greatest proportion followed by nitrogen sedimentation and then uptake by aquatic plants. A more in-depth examination of denitrification in the littoral sediments of Lake Memphremagog in Quebec, using the N2 flux technique, found an average denitrification rate of 111 mumol N m-2 h-1. Denitrification rates were positively related to the % organic matter of the sediment, temperature and macrophyte biomass density and negatively related to depth. These results, in combination with a review of the literature indicate that denitrification rates are higher in littoral than profundal sediments