NITROGEN CYCLING ACROSS THE SEDIMENT-WATER INTERFACE IN AN EUTROPHIC, ARTIFICIALLY OXYGENATED LAKE

Processes controlling the nitrogen (N) exchange between water and sediment in eutrophic Lake Sempach were studied using three different independent methods: benthic flux chambers, interstitial water data and hypolimnetic mass balances. The sediments released NH4+ (1.1-16.1 mmoles m-2 d-1), NO2- (0.1-0.4 mmoles m-2 d-1) and dissolved organic N (<0.25 mmoles m-2 d-1). A net NO3- consumption (2.4-11.1 mmoles m-2 d-1) related to the NO3- concentrations in the overlying water was observed in all benthic chamber experiments. The flux of the reactive species NO3- and NH+ was found to depend on hydrodynamic conditions in the water overlying the sediment. For this reason, benthic chambers overestimated the fluxes of inorganic N compared to the other methods. Thus, in short-term flux chamber experiments the sediment may either become a sink or a source for inorganic N depending on the O2 concentration in the water overlying the sediment and the stirring rate. As demonstrated with a (NO3)-N-15 experiment, nitrate-ammonification accounted for less than 12% of the total NO3- consumption. After six years of artificial oxygenation in Lake Sempach, a decrease in hypolimnetic total inorganic nitrogen (TIN) was observed in the last two years. The occurrence of dense mats of H2S-oxidizing Beggiatoa sp. indicated micro-aerobic conditions at the sediment surface. Under these conditions, a shorter distance between the ecological niches of nitrifying and denitrifying bacteria, and therefore a faster NO3--transport, can possibly explain the lowering of TIN by enhanced net denitrification.