NUTRIENT CYCLING IN SHALLOW, OLIGOTROPHIC LAKE KVIE, DENMARK

The importance of isoetids for the exchange of dissolved inorganic nitrogen (DIN) between sediment and water was studied in shallow Lake Kvie, Denmark. Vegetated sediments from the littoral zone (55 % of lake area) were compared to unvegetated sediments from the littoral and profundal zone. Clear effects of the isoetids were found on DIN in the porewater. At the vegetated station, NH4+ showed the highest concentrations just below the surface (< 40 muM) whereas NO3- was dominating below 5 cm depth with concentrations up to 100 muM during the spring. The unvegetated littoral sediment showed a distinct change between winter where NH4+ dominated and summer where NO3- was most abundant. NH4+ dominated in the profundal sediment and showed increasing concentration with depth. The Eh was high (> 400 mV) in the vegetated sediment, indicating isoetid release of O2 in the rhizosphere. A low DIN uptake was observed at the vegetated station while, based on porewater data, a diffusive release from the sediment was expected. This difference was due to plant assimilation. In general a release of NH4+ and an uptake of NO3- was seen in all sediments. The denitrification rate calculated from the mass balance for the entire lake was 0.4 mmol m-2 d-1 and accounted for removal of 77 % of the annual N-input to Lake Kvie.