Planktonic regeneration and uptake of ammonium was measured in Lake Calado on the floodplain of the Amazon River (Brazil). In the epilimnion of the lake, regeneration and uptake averaged 0.86±0.15 and 1.9±0.4μmol NH41−1h−1respectively. Uptake exceeded regeneration under the ammonium-enriched conditions of our experimental incubations, but uptake and regeneration are in near-balance atin situammonium concentrations of<1μM. In experiments that simulated lake overturn, samples from the meta- and hypolimnion had higher rates of regeneration (1.4±0.6) and lower rates of uptake (0.89±0.53) than epilimnetic samples. During inflow of water from the Amazon River and a local stream, ammonium regeneration rates were similar to those measured in the epilimnion (0.84±0.22), but the uptake rates were lower (0.49±0.13). Although regeneration always exceeded uptake in the overturn and inflow experiments, ammonium did not accumulate. Rapid isotope dilution of the ammonium pool occurred in the incubations, and the effect of not accounting for isotope dilution could result in underestimating uptake rates by a factor of 2–5 in the epilimnion of this
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