In contrast to xenobiotic compounds, trace metals occur naturally in all terrestrial and marine environments. The unique physico-chemical characteristics of remote, pristine Southern Ocean water masses and the unique eco-physiological adaptations of Antarctic marine organisms which have undergone long evolutionary processes in isolation enhance bioaccumulation of Cd and other trace elements in some species of Antarctic invertebrates, seabirds and marine mammals. Bioaccumulation is independent of human activity or global processes and probably occurs naturally in marine organisms which grow and reproduce slowly, are long-lived, and have longer moult cycles than related species from other seas. Cadmium concentrations in Southern Ocean surface waters are higher than in other seas, and the efficient uptake of the metal by phytoplankton determines the transfer of Cd to primary consumers such as pelagic crustaceans or benthic invertebrates (in the neritic province). Seabirds, marine mammals and other organisms feeding almost exclusively on pelagic crustaceans or other primary consumers may accumulate very high concentrations of Cd in the liver and especially in the kidney. However, as the enhanced bioavailability of Cd is a natural characteristic of the Southern Ocean,Antarctic marine organisms likely adapted to this environment during their long evolutionary history. There is evidence that, beginning from phytoplankton cells, almost all Cd-accumulating marine species have efficient detoxification mechanisms. The uptake of Cd occurs mainly through diet, and the metal does not usually accumulate according to the age of organisms or their trophic level. As Cd cannot be eliminated through eggs, feathers or hairs, higher vertebrates release most of the metal through their urinary apparatus. The kidney of Antarctic (and Arctic) seabirds and marine mammals may thus contain impressive Cd concentrations; their kidneys can probably tolerate much higher concentrations of this metal than those associated with heavy damage in many species of terrestrial birds and mammals, including humans.
Mercury, like Cd, is one of the most toxic metals; in the environment, however, it behaves differently. The most toxic and bioaccumulating chemical form of Hg is MeHg, which is produced naturally in the environment by bacteria or abiotically. The chemical properties of this compound are more similar to those of POPs than to those of other metals. The uptake of MeHg by marine organisms mainly occurs through diet but in contrast to Cd, MeHg elimination processes are very slow; concentrations consequently increase in larger, long-lived species at the highest levels of trophic chains. There is no evidence of enhanced MeHg availability in Southern Ocean waters or sediments, and total Hg concentrations in phytoplankton, primary consumers and most Antarctic organisms are generally low. However, the highest-ever reported concentrations of Hg were measured in the liver of some species of albatrosses and petrels from the Southern Ocean. These are large, long-lived species which feed on squid, fish or carrion, have low reproductive rates and take years to replace feathers (i.e. compared to other bird species, they eliminate less MeHg through egg laying and feather moulting). The huge accumulation of Hg in the liver of some species of albatrosses and petrels seems a natural process determined by their life-history characteristics; it is therefore probable that, as in the case of Cd, these birds adapted to enhanced Hg accumulation during their evolution.
In contrast to metals, POPs are ubiquitous toxic compounds which are not natural in origin. They are released around the world and transported to polar regions by air masses according to global distillation and fractionation processes. In winter, Southern Ocean pack ice behaves as a sink for persistent contaminants, which are subsequently released during the spring and summer melt. Persistent airborne contaminants are then transferred to water and organisms, where they can accumulate in tissues and biomagnify in food chains. Antarctic seabirds and marine mammals are exposed to the potential toxic effects of some POPs because they have developed few metabolic detoxification pathways for xenobiotic compounds. The potential risk to organisms breeding in Antarctica from POP exposure could also be enhanced by the extreme variability of physiological conditions during the short, intensive breeding season.
Overall, available data for DDT and derivatives in Antarctic seabirds and mammals indicates that levels increased from the early 1960s to the end of the 1980s. Despite the reduction in the input of "new" DDT and DDE from outside Antarctica, the widespread occurrence of DDE in Antarctic organisms is indicative of the high persistence of this compound. Hexachlorobenzene is another POP with a long environmental half-life, and its concentrations in fish tissues from the Antarctic Peninsula are in the same range as those measured in fish from the North Sea. Concentrations of HCB in preen oil of seabirds such as the cape petrel wintering in sub-Antarctic regions were much higher than those measured in preen oil samples from European seabirds. With the exception of benthic organisms collected from a few polluted coastal ecosystems adjacent to scientific stations, concentrations of PCBs and other POPs in seabirds and marine mammals endemic to the Antarctic region are generally lower than those in comparatively widely dispersed sub-Antarctic species. However, available data indicate that the liver of Adelie penguins does not efficiently detoxify PCBs and chlorinated pesticides. Among seabirds nesting in Antarctica, the South Polar skua is a top predator and scavenger which migrates to more polluted marine areas of the Northern Hemisphere during winter. Mercury and POP concentrations in its eggs, feathers, internal organs and tissues are therefore among the highest recorded in Antarctic organisms. Although some detoxification systems such as the activity of mixed function oxidases or the co-accumulation of Hg and Se in the liver are more effective than in other Antarctic seabirds, there is evidence that concentrations of 2378-tertrachlorodibenzo-p-dioxin equivalents in its eggs are higher than in the liver of polar bears and are only twofold lower than toxic-ity threshold values reported for other birds.
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