NITROGEN-FIXATION, AMMONIUM AND NITRATE UPTAKE DURING A BLOOM OF CYLINDROSPERMOPSIS-RACIBORSKII IN LAKE BALATON

N-2-fixation and N-15-labelled ammonium and nitrate uptake were measur ed during a bloom of C. raciborskii in two basins of Lake Balaton. In the eastern basin when the chlorophyll-a content reached 70 mu g/l the N-2-fixation was low (59 mg N m(-2) day(-1)) and the N uptake was dom inated 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), respec tively). In the western basin, the chlorophyll-a content was 160 mu g/ 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 da ily integrated carbon and nitrogen uptake ratios were 5.2 (weight/weig ht) in both basins.