Given its extreme conditions, the Antarctic comprises a perhaps surprising diversity of ecosystems. Antarctica is the coldest, windiest, driest and highest continent. Only about 0.4% of the continent is ice-free. Since plants and the invertebrates associated with them and most seabirds require bare rock as growth or breeding habitats, the ice-free areas are important for their survival; consequently, many species are often found breeding close to each other. Antarctica and the Southern Ocean, distinct in their physical properties from other parts of the world, have a large number of endemic species. For example, 100% of the nematodes, 50% of the lichens, and more than 30% of the terrestrial invertebrates on the Antarctic continent are found nowhere else,87 and the dominant fish species in the Southern Ocean are endemic to the region.88
One of the likely results of climate change is an alteration in the distribution of species, as those adapted to warmer climes expand their ranges south. For example, crabs are not found in the ocean around Antarctica-they became extinct there some 15 million years ago.89 Many invertebrate organisms, such as brittle-stars and molluscs, evolved to have only soft shells in the absence of predators. Others, such as some marine snails, have lost their shells altogether. These creatures are no match for shell-crushing invaders, such as king crabs. There is already evidence that king crabs are expanding their range and moving south. In 2004, four specimens of a king crab that is abundant in the coastal waters of New Zealand were found in the northern Ross Sea and near the Balleny Islands (approximately 67°S).90 More recently, thousands of these creatures have been found on the shelf slope of the western Antarctic Peninsula.91 Their presence alone has the potential to extensively modify species diversity in the region.
Marine microorganisms form the basis of Antarctic food webs. They include vast numbers of bacteria, phytoplankton (single-celled plants) and zooplankton (single-celled animals). Bacterial communities occur throughout the water column of the Southern Ocean, as well as in the sea ice. These tiny, single-celled organisms provide food for zooplankton, krill, fish and other vertebrates. They are exceptionally numerous and comprise around 90% of the living matter produced in Antarctic waters. The biomass of phytoplankton is estimated to be 5000 million tonnes; there are also about 1200 million tonnes of bacteria and some 600 million tonnes of protozoa. 92 The number of species for many groups of organisms is still unknown. The Census of Antarctic Marine Life found many new species that are still being identified; this is particularly true for bacteria.93 The phytoplankton (diatoms, dinoflagellates, cilliates and other protists) in the Southern Ocean comprises 560 known species,94 but only a few are widely dominant; their community structure is not constant throughout the Southern Ocean.95 One group, the diatoms, is responsible for most of the primary production (fixing of inorganic carbon into organic molecules). Their level of productivity varies greatly with season, being highest in spring and early summer.96 Most of their production is consumed or recycled by bacteria and protozoa.97
Intense phytoplankton blooms occur in Antarctic waters during spring and summer when increasing sunlight melts the sea ice and warms the ocean. The high light conditions and high nutrient content in the surface waters are ideal conditions for the growth of phytoplankton cells. During photosynthesis, phytoplankton take up carbon dioxide that dissolves in the ocean from the atmosphere. They also produce dimethyl sulfide, a natural aerosol, which is released into the atmosphere. Here it helps cloud formation as it acts as a cloud condensation nucleus,98 and increases the reflectance of the sun's heat from Earth. Thus, these single-celled organisms not only support the food web, they also influence the biochemistry of the ocean and play a vital role in affecting global climate by reducing carbon dioxide in the atmosphere and altering global heat balance. In turn, they are affected by anthropogenic changes to the atmosphere. Ozone depletion has increased the damage they experience due to increased ultraviolet B radiation (Box 7.2). Anthropogenic environmental changes are likely to have far-reaching impacts on the Antarctic marine ecosystem.