Nitrogen uptake by summer phytoplankton in Lake Balaton

Abstract

The contribution of ammonium, nitrate and N 2 to the nitrogen demand of summer phytoplankton has been estimated in two basins of Lake Balaton. The uptake rates of ammonium and nitrate were determined using 15 N methodology. N 2 -fixation rates were measured by the acetylene-reduction technique. The light dependence of uptake and N 2 -fixation was described with an exponential saturation equation. The decrease of I k values from mid-summer to autumn coincided with an increasing contribution of cyanobacteria to the algal population, suggesting a relatively lower light requirement than that of other species. The dark uptake velocity of ammonium and nitrate were 30-63 % and 1-40 % of light saturated uptake, respectively. The contribution of ammonium to the daily nitrogen supply of algae varied between 50-90%. Nitrate uptake was much less important with a maximum of 40 %. Nitrogen fixation was significant only in the western basin at the time of highest primary production and greatest N 2 -fixing cyanobacterial contribution. Nitrogen uptake experiments and loading data suggest, that nitrogen regeneration is by far the most important process in establishing and supporting the standing crop of phytoplankton in the lake during summer.