Ammonium (NH4+) is a preferred nitrogen source for Antarctic phytoplankton growth and the principal nitrogenous waste excreted by zooplankton and micronekton. In austral summer 1977–1978, NH4+was present at average concentrations of 0.1–0.3 μg-at liter-1in the upper 50 m of the Ross Sea and was excreted by resident mixed zooplankton at rates of 1.6 μg-at NH4+g-1wet weight h-1. Zooplankton biomass sampled by vertically-towed 0.5 m nets averaged 0.06 ml m-3in the upper 200 m over the Ross Sea shelf and 0.09 ml m-3in the upper 200 m north of the shelf-slope front. Olson (1980) has reported that phytoplankton nitrogenous requirements were 110 and 170 μg-at NH4+m-3day-1in these respective regions. A synthesis of these data predicts that average summertime standing crops of zooplankton might regenerate only about 2 of the daily NH4+requirements of Ross Sea phytoplankton. For comparison, the potential NH4+excretion impact of microheterotrophs, seabirds, and local aggregations of zooplankton are discussed and contrasted with non-biogenic inputs of NH4+associated with basal meltwater from the Ross Ice Shelf and the seasonal melting of the sea ic
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