Surface water masses, primary production, krill distribution and predator foraging in the vicinity of Elephant Island during the 1989-90 austral summer

Four successive surveys of the pelagic marine environment were conducted over a 100-by-100 mile study area centered on Elephant Island, Antarctica during January and February 1990. Data were collected to describe the thermohaline structure, the distribution of phytoplankton biomass and primary productivity, the distribution and demography of krill, and the foraging patterns of krill predators. A hydrographic front, north of Elephant Island and aligned southwest-to-northeast, persisted throughout the observation period. Relatively high phytoplankton biomass and production also persisted throughout the observation period, although both were lower to the north of the front and higher to the south of it. A second front was apparent at the eastern side of the study area and was tentatively identified as the western end of the Weddell-Scotia Confluence. Krill abundance increased approximately 5-fold over the 2-month period with highest concentrations north of Elephant Island and apparent immigration from east of the study area. Krill sampled throughout the observation period were reproductively mature and exhibited a general transition from reproductively active to gravid and spent individuals. A size mode of small, reproductively mature females and immature males was sampled in January and associated with an apparent inflow of water from southeast of the study area; this size mode was not evident in February. As a consequence, overall mean size increased from 41.8 mm in January to 44.3 mm in February. Krill composed 99% of the diet of chinstrap penguins and over 80% of the diet of fur seals throughout the observation period. Fur seals, macaroni penguins and chinstrap penguins all foraged to the north of their breeding colonies on Seal Island, but at different distances (18-100 km for fur seals, 20-35 km for macaroni penguins, 11-24 km for chinstrap penguins) and at different depths (25m mean maximum dive depth for fur seals; 43m and 44m for macaroni and chinstrap penguins, respectively).
Examination of interrelations between the various physical and biological components measured in the study area led to the following tentative observations: 1) Five water mass types were classified in the study area, with a frontal system existing to the northwest of Elephant Island. 2) Although there was high variability in patterns of Chl-a and primary productivity, high phytoplankton biomass and rates of primary production were found south of the front, low values were found to the north. 3) The different water masses were characterized by distinct phytoplankton taxonomic groups. 4) There appears to be a generally inverse relationship between krill and phytoplankton distribution and productivity, although exceptions were observed and may be expected given the small scale of the study area. 5) Antarctic fur seal diving effort was not closely correlated with krill abundance; the reason for this may be a problem of the sampling scale of the ship compared to the predator. 6) Krill found in chinstrap penguin stomach samples was found to have a different age, sex and size distribution than that found in net samples, implying fine-scale patchiness in krill distribution patterns and/or penguin feeding selectivity.