Uptake of dissolved nitrogen by phytoplankton in a eutrophic subtropical lake

Uptake rates for ananonium, nitrate, urea and dinitrogen byphytoplankton in Lake Okeechobee ranged from 0.58 to 1.52 µmol1^–1 h^–1 among four representative stations duringa short-term study period. Ammonium accounted for 53% of theuptake rates, followed by nitrate (19%), urea (16%) and dinitrogen(12%). Half-saturation constants for nitrogen (N) uptake rangedfrom 8.70 µmol 1^–1 for ammonium, 2.07 iimol 1^–1for urea and 2.21 µmol 1^–1 for nitrate at Southstation. This study reveals spatially varying N uptake rates,particularly N fixation, within a large eutrophic lake.