More than 99.5% of continental Antarctica is covered by ice 7 that is, in places, over 4 km thick, and open areas are restricted to the continental margin and mountaintops. Even though there is significant melting of ice in some areas in summer, very few sites have habitable aquatic sites. However, in the maritime Antarctic, along the Antarctic peninsula and offshore islands, significant pools and lakes exist that are inhabited by microbial, algal and invertebrate communities, some of which can be substantial. These pools and lakes are usually seasonal, being frozen in winter, although some of the larger water masses only freeze at the surface. The extent of the period of open water, and temperatures that any of them experience in summer are a function of many factors including air temperature, altitude and aspect. Pools that sustain significant populations of fairy shrimps, Branchinecta gaini, occur near Rothera station on the Antarctic peninsula (Fig. 1). In these pools summer temperatures are positive, but fluctuate between 0°C and 20°C, whereas in winter temperatures fall to around -20°C, but are more stable. The fairy shrimps living there thus experience an annual temperature range of 40°C, or more, and in summer can experience daily temperature fluctuations of up to 20°C. Similar, or more extreme temperature regimes are common in maritime Antarctic terrestrial systems 8.

Ecological conditions in maritime Antarctic (which includes coastal regions of the Antarctic peninsula and several offshore islands 9) freshwater environments vary markedly over short distances, with nutrients such as orthophosphate and ammonia, as well as water alkalinity varying by more than an order of magnitude between adjacent lakes 10. Algal communities also vary markedly between localities, with Chlorophyll a concentrations varying by factors of 5 or more between lakes on Signy Island 10. These marked variations between pools and lakes over short distances are the product of a range of factors including: aspect effects on insolation; small variations in temperature; hydrology and erosion in the catchment area; and in-lake biological activity and biogeochemical cycling.

Thus over the majority of the Antarctic continent there are very few sites where aquatic systems are available for colonisation. However, at the continental margins and in the maritime Antarctic significant numbers of pools and lakes exist where biological communities are established, and these environments are generally both cold and frozen in winter and highly variable in ecological characters.

Species inhabiting pools and lakes in Antarctica exhibit great physiological flexibility in coping with variations in environmental temperature. B. gaini populations can survive temperatures up to 10°C for 1 week with no mortality, and temperatures up to 25°C for 48 h with only 50% mortality 11. Oxygen consumption rates also rise in an expected fashion with temperature, doubling for every 10°C rise in temperature in this species, with rates at 20°C four times higher than those at 1°C 11. This indicates a metabolic scope for B. gaini of at least × 4 (where a metabolic, or physiological scope refers to the maximum ability to raise a function over the minimum required for long-term survival). Ventilation rate on the other hand only increased by a factor of 2.2 between 1°C and 20°C. This mismatch between capacity to raise ventilation rate and oxygen consumption suggests that, although oxygen delivery does not appear limiting between 1°C and 20°C, the upper lethal limit may be dictated by an inability to raise oxygen supply mechanisms at a sufficient rate, as seen in marine ectotherms 12.

Summer temperature variations are survived as adults. Adults cannot, however, survive freezing, and winter periods are passed as eggs, or cysts, which remain unfrozen to temperatures around -25°C. Antarctic terrestrial species, predominantly Collembola and Acari survive even wider temperature fluctuations as adults, using antifreezes, cryoprotectants and supercooling to maintain viability at temperatures down to -30°C, or below 13. These groups often endure annual temperature ranges in excess of 50°C 13. Terrestrial invertebrates in Antarctica appear to have very wide physiological flexibility, and show cold adapted metabolic rates that are faster than metabolic rates of similar species from lower latitudes at any given temperature 14. Growth rates are also reported to show some temperature compensation, and the optimum temperatures for feeding and growth are low 15.

Terrestrial and freshwater species, therefore, exhibit large physiological flexibility, giving them the ability to function normally at temperatures between 0°C and 15–20°C. They also possess adaptations allowing short-term survival in the range -25°C to +25°C for freshwater species and -30°C to +30°C for some terrestrial Acari and Collembola.

The Southern Ocean is characterised by low but highly stable temperatures. At the most variable sites, such as Signy Island in the maritime Antarctic, temperatures usually range between -1.8°C in winter and around +1.0°C in summer (Fig 2). At the highest Antarctic sites, typified by McMurdo Sound, temperatures range between-1.7°C and -2.0°C yearly 16, although recent data suggest temperatures may be slightly more variable than this at McMurdo, with temperatures approaching -0.5°C in some years 17. Thus the total annual fluctuation in sea temperature rarely exceeds 3°C in the region of the Antarctic Peninsula and Scotia Sea and is half this or less at high Antarctic (continental coast), making this one of the most thermally stable environments on earth. Temperatures have been low and stable around Antarctica for a long evolutionary period, at least 10 million years.

Other characters of the marine environment vary markedly with season around Antarctica. Annually light varies between no direct sunlight and overall radiative loss from the surface to 24 h direct sunlight and incident radiation levels at or above tropical values over a 24 period in summer. Between 10 and 15 million km² of sea-ice forms in winter and melts in summer. These factors result in intense seasonality of phytoplankton productivity, especially in nearshore waters. Blooms around Signy Island, South Orkney Islands (60°43'S, 45°36'W) and Rothera Point, Adelaide Island (67°36'S, 68°12'W) are both restricted to approximately 2–3 month periods and at peak reach chlorophyll standing stock levels in excess of 25 mg Chl a m^-3 1819, values that would be high for any site on earth.

In the open ocean productivity is often associated with the edge of the sea-ice, and a significant portion of overall oceanic productivity occurs in these areas 20. Micro-algal productivity associated with sea-ice can also be extensive 21, as can blooms of benthic communities on the seabed 22. When taken together these 3 areas of productivity can be a more significant source of resource supply to secondary consumers than open ocean blooms in the Southern Ocean.

Antarctic marine ectotherms are characterised by low rates of growth, development, metabolism and activity with little or no evidence of temperature compensation 2324 (Fig. 3). In general growth rates are 2–5 times slower in Antarctic species compared to similar temperate organisms, and larval development is 5–10 times slower, with echinoderm embryos taking up to 150 h from fertilisation to hatching in Antarctica 2325 (Fig. 4). The metabolic rate of adult animals is lowered by a factor of 2 to 3 times for every 10°C drop in temperature compared with lower latitude species 2627, and metabolic rates of larvae may be even more affected by temperature than adult animals, being around 10 times lower in larvae of Antarctic gastropod molluscs than temperate species 28.

As well as exhibiting low rates for most life history characters, Antarctic marine species are also highly stenothermal, dying in experiments when temperatures are raised to between +5°C and +10°C 26. There is now strong evidence that these low upper temperature limits are set by a limitation to aerobic scope, or poor capacity to raise metabolic rates to perform work 122930. In the bivalve mollusc Laternula elliptica oxygen consumption and heartbeat rate rise with temperature between 0°C and 9°C, where both collapse and animals begin to die. Both oxygenated and deoxygenated blood is obtained from samples at 0°C, 3°C and 6°C, but no oxygenated blood is present at 9°C, and death ensues from a deficiency in oxygen supply to the tissues 32. At around 5°C anaerobic end products of metabolism such as succinate and acetate begin to accumulate, indicating a long-term physiological limit, termed the critical limit 29.

That Antarctic marine species have poor capacities to raise metabolism for work has been shown for several species. The brachiopod Liothyrella uva can only elevate metabolism by × 1.7 over resting rates at 0°C when temperature is raised and the limpet Nacella concinna, by a factor of 2.6 33. In bivalves the value for Limopsis marionensis × 1.8 34, and for Laternula elliptica is × 2.7 32. These values compare with the factor of over 4 that the Antarctic freshwater fairy shrimp B. gaini raises its rate of oxygen consumption with increased temperature. One difference here is that in experiments on the marine species temperatures were only raised to between +4°C and +10°C, where animals died, compared to the 25°C for B. gaini .

Limited aerobic scopes have further consequences for Antarctic marine ectotherms. Their ability to perform activity or work is severely temperature restricted. Thus, 50% of individuals of populations of burrowing bivalves, Laternula elliptica lose the ability to bury when temperatures are raised to 2.5°C, there is complete loss of burrowing ability in all animals at 5°C, and large reproductive individuals lose the ability to bury first 35 (Fig. 5). Populations of the limpet Nacella concinna similarly lose the ability to right themselves when turned over, with 50% loss of ability at temperatures between 2°C and 2.5°C. The scallop Adamussium colbecki is even more severely constrained, with no animals capable of swimming when temperatures are raised to 2°C 35. Essential biological functions are, therefore rapidly lost with rising temperature in Antarctic marine species. Such limited capacity will clearly have marked effects on abilities to obtain and process food, and to reproduce, and hence animal fitness. This factor may be more important ecologically in terms of survival of populations and species than the physiological limits described earlier, although the physiological and ecological restrictions caused by extreme temperature sensitivity will be intimately linked. Current maritime Antarctic summer sea temperatures are usually around 1°C. Reproductive sized L. elliptica and N. concinna lose the ability to perform significant activities at, or below 2.5°C, only 1.5°C above current summer maxima, when animals need to exploit the short phytoplankton bloom. Scallops are even more tightly constrained, with all individuals losing the ability to perform important biological activities at temperatures less than 1°C above current summer maxima. These data are all for bivalve molluscs. However, the effect is likely to be widespread and suggests that a 1°C rise in summer sea temperatures around Antarctica could have severe ecological consequences for populations and communities of marine ectotherms.