The systematic study of Southern Ocean ecology was initiated in the interest of the whaling industry and was hence directed at the food supply of the great whales, known at the time to consist largely of krill (Euphausia superba). The presence of enormous animal biomass in what appeared to be an inhospitable environment was explained by an exceptionally high transfer efficiency between trophic levels: whales fed on krill which in turn fed on diatom blooms. This concept of a short and simple "food chain of the giants" was accepted as a plausible explanation till about the early eighties, by which time it was realised that the data did not support this concept. By the late eighties, it was recognised that productivity of the Southern Ocean was fairly low, that diatom blooms were the exception rather than the rule, and that phytoplankton generally consisted of a thin soup of small cells. The factors enabling maintenance of the enormous krill stock were no longer obvious.

In the eighties, the thrust of ecological research shifted to elucidation of the factors constraining primary production: a) hydrography via its influence on mixed layer depth, b) availability of micronutrients, in particular iron and c) grazing, by both protozoa and metazooplankton. The ecology of sea ice and the impact of seasonal ice retreat on water column biology also received special attention. The results of these studies indicate that the pelagic ecosystem is far more complex and structured than previously assumed and that partitioning of organic matter within the pelagic food web varies both regionally and seasonally. Sea ice, far from being an inhospitable environment, has emerged as an important winter refuge in the life cycles of many pelagic organisms including krill and processes occurring during ice melt have been shown to enhance productivity of the pack-ice zone. In addition to krill, copepods and salps are recognised as important metazoan grazers in the water column, whereby the occurrence of krill and salps appears to be negatively correlated.

The studies on krill distribution and population dynamics to date have still not resolved the factors enabling maintenance of the enormous krill stock. Various hypotheses have been broached to explain this unique feature of the Southern Ocean. These are not mutually exclusive and none of them alone is sufficient to account for available observations of krill occurrence. Hence it appears that krill is an animal of amazing versatility that is able to utilise a wide variety of food sources in habitats ranging from open water to sea-ice and benthos. The regions where high krill concentrations have been frequently observed share common features, for instance their proximity to frontal zones separating major water masses. However, the reasons why krill congregate there and the underlying mechanisms of swarm formation and dispersal remain obscure.

For various reasons, research on top predators has been generally conducted independently of studies of lower trophic levels. However, in recent years the application of new technology has resulted in rapid advances in understanding of bird and seal ecology and it is now feasible to integrate studies of plankton ecology with those of their predators.

Biogeochemical studies carried out ocean-wide within the framework of the international Joint Global Ocean Flux Study (JGOFS) programme have demonstrated that interoceanic comparison can significantly advance our understanding of ocean ecosystems. Thus, a spring phytoplankton bloom terminated by nutrient exhaustion is known to occur only in the North Atlantic. In the other two high-latitude oceans--the Southern Ocean and the Subarctic Pacific--spring blooms are the exception, despite high nutrient concentrations throughout the year. Yet, with the exception of krill in the Southern Ocean, the composition and annual biomass patterns of zooplankton populations in these three oceans do not differ significantly. The conclusion to be drawn from this comparison is that seasonality and magnitude of food supply is not the only factor governing the dynamics of zooplankton populations.

The major challenge facing GLOBEC is to determine the factors and mechanisms that regulate animal populations in the ocean pelagial. Whereas food and predation are proximate causes for growth and decline of animal populations, recruitment is influenced by ocean circulation patterns and mesoscale hydrography, which may serve to concentrate or disperse breeding populations. The dependence of Antarctic organisms on sea ice renders this ecosystem particularly vulnerable to global climate change.

The wealth of information on the physics and biology of the Southern Ocean pelagial now available has set the stage for a new understanding of the Southern Ocean ecosystems. The time is now ripe for construction of bold new conceptual frameworks based on available information that can be tested at sea and with numerical models. The recent developments in observation and sampling technology coupled with significant advances in computing and modelling capability provide the powerful new tools necessary to achieve the ambitious goals of the SO GLOBEC International programme.

With due consideration of the limited resources in terms of ship time and personnel available to SO GLOBEC it was decided to concentrate effort on selected target species in specific study sites and to draw up a set of key questions to guide research plans. These tasks were accomplished at the Norfolk meeting in 1993 and the reader is referred to GLOBEC Report No. 5 for a discussion of the underlying rationale. Furthermore, GLOBEC has profited from the experience and knowledge gained from earlier international ventures in particular BIOMASS, and close cooperation with the ongoing related programmes, in particular APIS, CCAMLR, EASIZ, SO JGOFS and WOCE, will be of mutual benefit. /