NUTRIENT CYCLING IN SHALLOW, OLIGOTROPHIC LAKE KVIE, DENMARK .2. EFFECTS OF ISOETIDS ON THE EXCHANGE OF PHOSPHORUS BETWEEN SEDIMENT AND WATER

The importance of isoetids for the exchange of phosphorus between sedi ment and water was studied in the shallow Lake Kvie, Denmark. Vegetate d 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. T he vegetated littoral sediment showed highest conc. of S RP just below the surface and decreasing conc. with sediment depth. The SRP release was low on all stations (<40 mumol m-2 d-1). The observed release of SRP on the vegetated station was much lower than the calculated diffus ional flux probably due to assimilation of P by isoetids and binding o f SRP to oxidized Fe. The high redox potential in the rhizosphere was probably caused by oxygen excretion from roots. The annual P release f rom vegetated sediments were only 5% of the P requirements of the macr ophytes. The efficient plant assimilation of SRP from the porewater re sulted in a small SRP pool with a very fast turnover of P of 500-900 t imes 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 p lant group is important for keeping lakes in an oligotrophic state.