Nutrient cycling in shallow, oligotrophic Lake Kvie, Denmark

Abstract

The importance of isoetids for the exchange of phosphorus between sediment and water was studied in the 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. Porewater concentration of soluble reactive phosphate (SRP) was in general low, however, different distributions were found in the three sediments. The vegetated littoral sediment showed highest conc. of SRP just below the surface and decreasing conc. with sediment depth. The SRP release was low on all stations ( < 40 µmol m-2 d-1). The observed release of SRP on the vegetated station was much lower than the calculated diffusional flux probably due to assimilation of P by isoetids and binding of SRP to oxidized Fe. The high redox potential in the rhizosphere was probably caused by oxygen excretion from roots. The annual P release from vegetated sediments were only 5% of the P requirements of the macrophytes. The efficient plant assimilation of SRP from the porewater resulted in a small SRP pool with a very fast turnover of P of 500–900 times per year. Both the effects of isoetids on the P cycle in littoral sediments and on the P budget of the entire lake indicate that this plant group is important for keeping lakes in an oligotrophic state.