The Southern Ocean Environment: Anthropogenic Impact and Climate Change

Abstract

The Southern Ocean, one of the largest, oldest and coldest deep-water marine systems, encircles Antarctica in a 2,500-km-wide, semi-closed belt. Like the Antarctic continent, it has distinctive physico-chemical and biolog-ical features and is important to global processes. Waters with temperatures near the freezing point (about –1.9 °C), produced along the Antarctic conti-nental margin, contribute to the formation of bottom waters and spread through the global ocean. In winter a belt of sea ice with an area larger than that of the continent itself rings Antarctica, with profound effects on the sea/air exchange. The surface albedo, lack of heat and moisture exchanges between the ocean and atmosphere affect the formation of clouds and the stability of the atmosphere over large areas of the Southern Hemisphere. Sea-ice extent and thickness are very sensitive to global changes and may provide an early indication of warming due to increasing greenhouse gas concentrations. Zonally uniform water masses with unique physical characteristics began to develop in the Southern Ocean in the Early Miocene, when circum-Antarc-tic currents largely unaffected by landmasses began to flow around the conti-nent. Marine organisms able to adapt to the new environment evolved for more than 20 million years in a semi-closed system, and several taxa assumed a circumpolar distribution. The upwelling of warmer waters from lower lati-tudes affects the stability of the water column; in spring and summer it favours the growth of phytoplankton, with the establishment in the seasonal pack-ice zone of the characteristic short food chain diatoms–krill–whales (or seals or penguins). Although ocean phytoplankton mostly consists of unicel-lular organisms with a one-day lifespan, it seems to play a very important role in controlling global warming through the uptake of CO 2 and the production of dimethylsulphide (which affects the number of cloud condensation nuclei available in remote regions). There is evidence that a positive phytoplank-ton–climate feedback system developed in the Southern Ocean during the glacial–interglacial transition. Contrary to organisms in Antarctic terrestrial ecosystems, those in the Southern Ocean do not experience extreme variations in temperature, water availability and solute concentrations. The most important changes are due to the formation of the sea-ice cover, which determines marked seasonal varia-tions in light penetration and primary productivity. Sea ice provides a plat-form on which birds and mammals can live and breed, and from which they can make foraging forays into the water. Compared to land ice, sea ice is usu-ally warmer, with a much more heterogeneous and variable structure. Biotic communities may develop within the sea ice and on its underside. The bound-ary between sea ice and the open sea is one of the ecologically most interest-ing regions in the Southern Ocean because, like ecotones in terrestrial ecosys-tems, the sea-ice edge has a variety of niches, with great species diversity and abundance. Reduced competition and long isolation in the Southern Ocean presum-ably provided notothenioid fish and several taxa of marine invertebrates with the opportunity to speciate and fill ecological niches occupied by other species at lower latitudes. Owing to their unique evolutionary history, Antarc-tic marine biota show a much higher percentage of endemic species (e.g. up to 97 % for notothenioid fish) with respect to organisms in continental terres-trial and freshwater ecosystems. However, like cryptogams and terrestrial microinvertebrates, most marine organisms have slow growth rates and can survive long periods of starvation at negligible metabolic cost. In the neritic province (near the coast), owing to the low zooplankton biomass, most phy-toplankton and ice algae sink in the summer, becoming food for the very rich benthic fauna. Benthic invertebrates are mostly suspension feeders such as sponges and echinoderms, and are important sources of food for fish. The lat-ter are eaten by other fish, birds and mammals, which are in turn the prey of leopard seals or killer whales. Thus, in contrast to the short food webs in the oceanic province, those in coastal ecosystems are rather complex and long. This chapter examines the main characteristics of the Southern Ocean, interactions with global atmospheric and oceanic circulation, and the struc-ture and functioning of the main biological systems. Contrary to popular belief, the Southern Ocean is far from being a pristine environment. The exploitation of living marine resources during the last two centuries has had dramatic impact on seal and whale populations and provides important lessons for regulating the exploitation of current and future marine resources. Antarctic marine organisms have unique ecophysiological adaptations due to their long evolutionary history in isolation; compared to related species from lower latitudes, they are probably more sensitive to the potential effects of anthropogenic pollutants and to climate and environmental change. Although several scientific reports on the Southern Ocean were made by naturalists who accompanied several 19th century expeditions, the modern R. Bargagli 84