Nitrogen fixation, ammonium and nitrate uptake during a bloom of Cylindrospermopsis raciborskii in Lake Balaton

N 2 -fixation and 15 N-labelled ammonium and nitrate uptake were measured during a bloom of C. raciborskii in two basins of Lake Balaton. In the eastern basin when the chlorophyll-a content reached 70 μg/l the N 2 -fixation was low (59 mg N m -2 day -1 ) and the N uptake was dominated by ammonium (393 mg N m -2 day -1 ). The nitrate uptake was of minor importance in both basins (43 and 55 mg N m -2 day -1 , respectively). In the western basin, the chlorophyll-a content was 160 μg/l and recirculation of regenerated forms of nitrogen could not support the demand. Although the ammonium uptake was high (357 mg N m -2 day -1 ), the fixed nitrogen (407 mg N m -2 day -1 ) exceeded it. The daily integrated carbon and nitrogen uptake ratios were 5.2 (weight/ weight) in both basins.