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 Peti t Saut Lake, French Guyana, to examine nitrogen cycling in this recently fl ooded equatorial forest environment. Ammonium regeneration rates were extre mely high (mostly in the range 1-6 mu mol N l (1) h (1)), and are due to th e very high grazing rates of the microzooplankton, which consumed between 5 6 and 95% of the phytoplankton production in any given incubation. These re generation rates were about an order of magnitude higher than the net ammon ium uptake rates. This imbalance is probably due to dissolved organic nitro gen release during grazing. At the bottom of the euphotic zone (4-5 m), pho tosynthetic bacteria are responsible for ammonium uptake. Diffusion-driven ammonium fluxes are an order of magnitude lower than biologically driven fl uxes. Therefore, ammonium fluxes are dominated by biology rather than by ph ysics in this lake.