@ 1980, by the American Society of Limnology and Oceanography, Inc. Nitrate and ammonium uptake in Antarctic waters1

Nitrate and ammonium uptake rates were measured by the 15N tracer technique in Antarctic waters. The ratio f= NOs- uptake NOs- uptake + NH4+ uptake averaged 0.54 in the Scotia Sea in early spring and 0.40 in the Ross Sea in summer, indicating that NO,- and NH4+ contribute about equally to primary production in these regions and that a large proportion of the primary production is exported from the surface layers. Uptake rates in laboratory experiments increased with increasing temperature and the relative uptake of NH,+ was positively correlated with increasing mean ambient NH,+ concentration and depth in the water column. A common image of the Antarctic ma-rine ecosystem is one of large herbivore and carnivore stocks supported by vast pastures of phytoplankton, these in turn made possible by the massive upwelling of nutrient-rich waters around the edges of the continent of Antarctica. Early workers emphasized the richness of the phytoplankton crop (Hart 1934), but more recent work with 14C uptake measure-ments has shown that the rate of primary production is quite low, being compara-ble to oligotrophic areas such as the cen-tral North Pacific gyre on an annual basis (El-Sayed 1967; Holm-Hansen et al. 1977b). The idea of a short and direct food chain (e.g. diatoms-krill-whales) was proposed to account for the prodigious production of higher trophic levels in the Antarctic (Murphy 1962). The clarification and interpretation of these observations is complicated by a lack of knowledge of the fate of primary production. Turnover rates for the higher trophic levels are unknown, and hence their impact on and utilization of the pri-mary production is also unknown. The trophic levels may be coupled tightly so that primary production is recycled quickly; alternatively, losses by sinkin