NUTRIENT CYCLING IN SHALLOW, OLIGOTROPHIC LAKE KVIE, DENMARK

The importance of isoetids for the exchange of dissolved inorganic nit rogen (DIN) between sediment and water was studied in shallow Lake Kvi e, Denmark. Vegetated sediments from the littoral zone (55 % of lake a rea) were compared to unvegetated sediments from the littoral and prof undal zone. Clear effects of the isoetids were found on DIN in the por ewater. At the vegetated station, NH4+ showed the highest concentratio ns just below the surface (< 40 muM) whereas NO3- was dominating below 5 cm depth with concentrations up to 100 muM during the spring. The u nvegetated littoral sediment showed a distinct change between winter w here NH4+ dominated and summer where NO3- was most abundant. NH4+ domi nated in the profundal sediment and showed increasing concentration wi th depth. The Eh was high (> 400 mV) in the vegetated sediment, indica ting isoetid release of O2 in the rhizosphere. A low DIN uptake was ob served at the vegetated station while, based on porewater data, a diff usive release from the sediment was expected. This difference was due to plant assimilation. In general a release of NH4+ and an uptake of N O3- was seen in all sediments. The denitrification rate calculated fro m the mass balance for the entire lake was 0.4 mmol m-2 d-1 and accoun ted for removal of 77 % of the annual N-input to Lake Kvie.