Antarctic zooplankton metabolism: carbon requirements and ammonium excretion of salps and crustacean zooplankton in the vicinity of the Bransfield Strait during January 1994

Metabolic rates (oxygen consumption and ammonia excretion) of zooplankton w ere determined during the austral summer (January 1994), in the eastern Bra nsfield Strait. The study area comprised four distinct hydrographic zones: the ice edge on the western Weddell Sea, the waters of Weddell Sea origin, the southern part of the Weddell-Scotia Confluence, and the waters of Belli ngshausen origin. The objectives were to estimate the fraction of primary p roduction accounted for by the metabolism of salps and crustacean zooplankt on, as well as the contribution of their excretion to the nitrogen demand o f phytoplankton. Biomass-specific respiration and excretion rates of zoopla nkton were measured simultaneously by incubation methods at in situ tempera tures. Zooplankton biomass was measured as organic C on aliquots of samples taken between 0 and 200 m. Primary production was measured by C-14 incorpo ration, using photosynthesis-irradiance relationships. Primary production r anged from 179 to 1612 mg C m(-2) day(-1). Salps were the most abundant zoo plankton group, located mainly in the northern part of the study area, wher e they excluded other zooplankters. Their biomass ranged from 115 to 2930 m g C m(-2) (0-200 m). On average, their metabolic carbon requirements repres ented about 5% of primary production per day, and their excretion of ammoni a contributed 10% of daily phytoplankton nitrogen demand. Crustacean zoopla nkton were of minor importance in the study area, except at the ice edge an d areas of recently melted ice. Their biomass ranged from 14.5 to 494 mg C m(-2) and they required on average 0.9% of daily primary production, while ammonia excretion accounted for around 0.4% of the phytoplankton nitrogen d emand. During this cruise, salps were the most important zooplankton group from the point of view of the transfer of biogenic carbon and nitrogen recy cling. However, the control exerted by zooplankton on primary producers was overall modest, and most of the phytoplankton carbon probably sedimented o ut of the water column.