Carbon fixation, ammonium uptake and regeneration in an equatorial lake: biological versus physical control

Ammonium uptake and regeneration were measured in the euphotic zone of Petit Saut Lake, French Guyana, to examine nitrogen cycling in this recently flooded equatorial forest environment. Ammonium regeneration rates were extremely high (mostly in the range 1-6 mu mol N l (1) h (1)), and are due to the very high grazing rates of the microzooplankton, which consumed between 56 and 95% of the phytoplankton production in any given incubation. These regeneration rates were about an order of magnitude higher than the net ammonium uptake rates. This imbalance is probably due to dissolved organic nitrogen release during grazing. At the bottom of the euphotic zone (4-5 m), photosynthetic bacteria are responsible for ammonium uptake. Diffusion-driven ammonium fluxes are an order of magnitude lower than biologically driven fluxes. Therefore, ammonium fluxes are dominated by biology rather than by physics in this lake.