LITMIR.BIZ

      Milieu of terrestrial and Martian biomes. Note the intermediary status of Antarctic biomes, especially away from the peninsula. Redrawn, original courtesy Smithsonian Institution.

      With the exception of far-ranging migratory species, such as some birds and whales, the great proportion of Antarctic species are unique and endemic, confined to the continent or within the convergence. The geography of krill, for example, conforms to the frontal systems of the Southern Ocean. Most krill live within the Antarctic divergence, and the Antarctic convergence segregates the Antarctic from subantarctic species. Similarly, 86 percent of Antarctic coastal fishes are found nowhere else. The isolation and cold of Antarctica—both long-standing environmental parameters—have greatly simplified the Antarctic ecosystem. Marine life in The Ice shows the same traits as the Southern Ocean and the pack. It is mobile and migratory, strongly seasonal, with powerful circumpolar mixing superimposed over regional diversity. Terrestrial life shows an even stronger tendency toward reductionism. There are few species; they experience simple life histories, form discrete populations, and occupy circumscribed sites; their interactions are few and direct. It has been said that Antarctica has the most diminutive continental flora and fauna on the planet. For all this The Ice is responsible. Once ice has claimed an area, there is little opportunity for organisms to recolonize. The species that exist are survivors from the last glaciations.

      The pack, however, does provide a matrix for life. It exerts an indirect influence through its effects on the Antarctic atmosphere and the Southern Ocean. But its biotic services are direct, too, and seasonal productivity closely parallels the annual cycle of sea ice. The pack furnishes a necessary platform for many marine mammals and birds. The heaviest concentrations of krill, squid, and fish are somewhat away from the ice-abraded shoreline; the pack furnishes rafts to carry seals and penguins to primary feeding grounds, small islands upon which they may rest, sleep, and even mate. Paradoxically, unlike the Arctic, where the continuous pack provides a platform for humans, the Antarctic pack has been a primary barrier for human movement into and around the Antarctic. In this respect, the sea ice again functions as an insulator, a filter, not between air and sea but between civilization and terra nunquam cognita.

      Perhaps most spectacularly, the pack features a special biota of microorganisms. Unlike land ice, sea ice is not a desert. Two communities of organisms actually exist within it. A snow community forms from sea wash collected on the snowy surface of floes. More productive by several orders of magnitude is an epontic community of microorganisms that thrives on the bottom and interior of floes. The sudden freezing of congelation ice traps algae, diatoms, ciliates, and flagellates within a crystalline scaffolding. Other organisms are caught up in brash-ice slush and frazil-ice clumps that are incorporated into the larger floe. Thus there is a surface snow-biota and a subsurface ice-biota. There are ample nutrients for each. Nitrates and phosphates gravitate to the lower strata of floes, where biological productivity is greatest, and microalgae migrate along capillaries and through brine channels within the floe—weakening the structural strength of the floe and tinging the floe with a brownish stain. Incredibly, the density of microalgae populations and the productivity of the ice biota are perhaps greater than in seawater. An ice fauna, in turn, grazes on this ice flora. Ice biotas contribute as much as 20 percent of the total primary production of Southern Ocean biomass. Whether or not the nutrients and biota released by the recession of the pack actually “seed” the phytoplankton bloom that occurs at the same time is undetermined, but this bloom contributes significantly to the seasonal cycle of life.

      The trophic hierarchy of Antarctica is comparatively simple. The food chain is characterized by large numbers of a few species, an enormous biomass within a less diverse ecosystem than those typical of temperate or tropic lands. Virtually all of its bird biomass (99 percent) consists of penguins, and they are dominated by one species, the Adelie. Nearly three-fourths of all Antarctic fishes belong to one group, the nototheniformes. The crabeater seal is the most abundant seal in the world; this single species accounts for 85 percent of Antarctic pinnipeds. The baleen whale dominates the Antarctic whale population, the greatest herd in the world ocean. The impressive yields are best explained by an abbreviated food chain. Even compared to the Arctic—with its biotic connections to the land masses of Asia, Europe, and North America—the Antarctic is almost artlessly uncomplicated. The marine ecosystem thus mimics The Ice: great bulk in an equally awesome simplicity. Among the advantages enjoyed by this marine ecosystem, it should be noted that sea temperatures remain relatively constant and that the main seasonal change is restricted to the oscillation of the pack. The tremendous seasonal fluctuations in sunlight and ice cover, however, are vital for controlling the variable productivity of the system. Continental influences act indirectly on the biota through their control over pack ice production.

      Phytoplankton consist principally of diatoms and dinoflagellates. Their abundance is legendary. The figures may be exaggerated, but the primary productivity of the system is often estimated to be the richest in the world ocean, perhaps four times greater than anywhere else. Primary production is greatest along the coastal areas, excluding the nearshore environments that are scoured or coated by ice, and amid the coastal seas, the deep embayments that surround West Antarctica. The nutrient-rich cold broth that upwells from the circumpolar deep water, along with possible contributions of phosphate or other minerals from discharged icebergs, accounts for much of this abundance. Accordingly, productivity is most prominent just below the cold surface waters that veneer the nearshore drift, and it shows pronounced geographic and seasonal variations.

      But it is the next trophic layer that provides a universal link in the food chain. The euphausiid shrimp known as krill is the only significant connection between the primary producers and all the higher trophic feeders. Like the phytoplankton on which it grazes, krill converges around the Antarctic Peninsula; the Scotia Sea, nourished by the cold waters of the Weddell Sea, is especially favored. But the importance of krill to the circumpolar ecosystem depends also on its mobility. It is found everywhere around the continent, though the heaviest concentrations of Euphasia superba are embedded within the east wind drift of the Antarctic circumpolar current. Krill feeds by migrating in a daily rhythm vertically through the water column, exploiting the upward leaching of nutrients, and it migrates around the continent in vast surface or near-surface swarms, with tons of krill to a swarm. Because of its universal importance, directly or indirectly, to all subsequent trophic levels, krill establishes the basic geography and dynamics of biology in the Antarctic. Life is pelagic, migratory, seasonal, abundant in mass and scanty in variety.

      On the krill swarms feed squid and fish; on them feed other fish, birds, and mammals. Although relatively constant in its temperature, the Southern Ocean is bitterly cold. Cold-blooded species, such as fish, adapt by several means to temperatures that would otherwise freeze internal fluids, including the production of several chemical

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