Zooplankton excretion and NH ⁺₄ cycling in near-surface waters of the Southern Ocean. I. Ross sea, austral summer 1977–1978

Summary

Ammonium (NH ⁺₄ ) is a preferred nitrogen source for Antarctic phytoplankton growth and the principal nitrogenous waste excreted by zooplankton and micronekton. In austral summer 1977–1978, NH ⁺₄ was present at average concentrations of 0.1–0.3 μg-at liter^-1 in the upper 50 m of the Ross Sea and was excreted by resident mixed zooplankton at rates of 1.6 μg-at NH ⁺₄ g^-1 wet weight h^-1. Zooplankton biomass sampled by vertically-towed 0.5 m nets averaged 0.06 ml m^-3 in the upper 200 m over the Ross Sea shelf and 0.09 ml m^-3 in the upper 200 m north of the shelf-slope front. Olson (1980) has reported that phytoplankton nitrogenous requirements were 110 and 170 μg-at NH ⁺₄ m^-3 day^-1 in these respective regions. A synthesis of these data predicts that average summertime standing crops of zooplankton might regenerate only about 2% of the daily NH ⁺₄ requirements of Ross Sea phytoplankton. For comparison, the potential NH ⁺₄ excretion impact of microheterotrophs, seabirds, and local aggregations of zooplankton are discussed and contrasted with non-biogenic inputs of NH ⁺₄ associated with basal meltwater from the Ross Ice Shelf and the seasonal melting of the sea ice pack.