Nitrate and ammonium uptake by plankton in an Amazon River floodplain lake

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Abstract
Uptake of ammonium and nitrate by plankton was measured in tropical Lake Calado, Brazil. Nitrate uptake was strongly influenced by light and was light saturated at ˜340 μE m−2 s−1. In contrast, uptake of ammonium was less influenced by light, and saturated at ˜250 μE m−2 s−1. Uptake rates of both forms of nitrogen were inhibited by up to 80% at light intensities higher than those required for saturation. Concentrations of ammonium and nitrate also had a strong influence on uptake rates. Half-saturation constants (0.3–5 μM) were usually greater than ambient concentrations (0.1–0.6 μM), indicating that uptake rates at ambient concentrations were less than one-half of the saturated rates. Ammonium is the more important type of inorganic nitrogen for plankton of Lake Calado because nitrate concentrations remain low to undetectable except during periodic inputs of nitrate-rich water from the Amazon River. Using the observed dependence of uptake on concentration and light, maximum uptake rates per unit chlorophyll α were computed to be in reasonable agreement with rates derived from PBm values for carbon uptake.

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