Antarctic Marine Food Webs

Summary

Traditional views on the productivity of the Southern Ocean have changed greatly. High primary production in the shelf region and near the pack-ice and the short diatom-Krill-whale food chain were first taken as indications of a very large renewable resource. However, it is now known that primary production in the open parts of the Southern Ocean is relatively low, and Soviet estimates of Krill, Euphausia superba, stocks have decreased by an order of magnitude. The pelagic Southern Ocean contains 3 major zones which, although they interact, may for convenience be considered as having distinct food webs. These zones are: the ice-free zone of the West Wind Drift dominated by herbivorous copepods, salps and small euphausiids; the seasonal pack-ice zone of the East Wind Drift and adjacent fronts and eddies in which Krill is the dominant element; and the permanent sea-ice zone near the Antarctic continent and its ice shelves where zooplankton biomass is low and, thus, predators are scarce. The Krill system is a product of a long evolutionary process in isolation. It is characterized by high seasonality in light and ice cover but otherwise stable conditions of low temperature and high nutrient supply. Within the Krill-dominated food chain unusually large size steps, and high energy demands for maintenance and swimming, require large concentrations of food to keep the effort for searching and feeding low during the short summer season. This pelagic system running on high energy costs is in contrast to the low energy system of the rich Antarctic benthos, particularly in pack-ice regions. Because survival in the Antarctic is energetically expensive, there is little net gain in the Krill system, despite its short food chain. However, the large biomass stored in Krill swarms and in large vertebrates has made the Krill system attractive to exploitation by man. Antarctic communities typically exhibit K-strategies but paradoxically there is great environmental variability in the pack-ice zone. Any model of food chain dynamics, or any management strategy, must take into account not only the low efficiency of the food chain but also the variability of its key elements.