UPTAKE OF DISSOLVED NITROGEN BY PHYTOPLANKTON IN A EUTROPHIC SUBTROPICAL LAKE

Uptake rates for ammonium, nitrate, urea and dinitrogen by phytoplankt on in Lake Okeechobee ranged from 0.58 to 1.52 mu mol l(-1) h(-1) amon g four representative stations during a short-term study period. Ammon ium accounted for 53% of the uptake rates, followed by nitrate (19%), urea(16%) and dinitrogen (12%). Half-saturation constants for nitrogen (N) uptake ranged from 8.70 mu mol l(-1) for ammonium, 2.07 mu mol l( -1) for urea and 2.21 mu mol l(-1) for nitrate at South station. This study reveals spatially varying N uptake rates, particularly N fixatio n, within a large eutrophic lake.