The biomass of euphausiids and that of zooplankton may be negligible in shelf waters around Antarctica (Jarre-Teichmann et al. 1997). At Terra Nova Bay, for instance, during summer the zooplankton biomass in shelf waters down to a depth of 200 m usually consists of a few hundred individuals per m3 of water. Dominant species include copepods (above all Metridia gerlachei, Euchaeta antarctica and Calanoides acutus), Euphausia crystallorophias and the pteropod Limacina helicina. During summer most phytoplankton and organic detritus from the euphotic zone settles to the bottom where it supports rich benthic communities. The involvement of benthic organisms in the transfer of energy and persistent contaminants from phytoplankton and other autotrophic organisms to benthic invertebrates, notothenioid fish, seabirds, seals and top predators such as killer whales lengthens coastal food webs and makes them much more complex than the relatively simple krill food chain.
Birds breeding along Antarctic coasts are almost exclusively marine species, and each species is uniform across very large coastal regions. As they mainly feed on zooplankton and fish larvae, food availability is limited to the summer after the break-up of pack ice. Seabirds have a short and intense breeding season in this period, after which they must migrate northwards. About 90 % of the avian biomass in Antarctica consists of penguins. There are 18 species of penguins, of which seven breed south of the Antarctic Convergence and only four breed on the continent: Emperor, Adelie, chinstrap and gentoo penguins. Only Emperor (Aptenodytes forsteri) and Adelie (Pygoscelis adeliae) penguins are true continental birds,because the other two species are found exclusively in the northern Antarctic Peninsula. Other birds which breed successfully along Antarctic coasts include the South Polar skua (Catharacta maccormicki) and some species of procellariforms such as the snow petrel (Pagodroma nivea), cape pigeon (Daption capensis), Antarctic petrel (Thalassoica antarctica) and Wilson's storm petrel (Oceanites oceani-cus).
During the breeding season Adelie and Emperor penguins concentrates in dense colonies along Antarctic coasts between 60° S and more than 77° S (e.g. the penguin rookeries on Ross Island). According to several population estimates (Knox 1994),there are about 2x106 breeding pairs of Adelie penguins in Antarctica, while the Emperor penguin (about 200,000 breeding pairs) is one of the rarest penguin species. Together with Weddell seals, they are the most important biotic components of the sea-ice zone, and the distribution of sea ice affects their travelling and foraging activity, with strong implications for chick production and population growth (e.g. Croxall et al. 1988; Fraser et al. 1992; Watanuki et al. 1997; Cherel and Kooyman 1998; Wienecke et al. 2000; Wilson et al. 2001). Although penguins mainly feed on crustaceans and fish (especially juvenile specimens of P. antarcticum),their diet shows large spatial variations. On the South Shetland Islands Adelie penguins feed almost exclusively on E. superba, while in the Ross Sea they ingest P. antarcticum during periods or years with little pack ice and E. crystallorophias during periods or years of heavy pack-ice cover (Ainley et al. 1998; Olmastroni et al. 2000). About 90 % by mass of the stomach content of Emperor penguins along the Ross Sea coast consists of fish (especially P. antarcticum; Cherel and Kooyman 1998). However, the diet of emperor penguins along the Mawson coast is dominated by squid (mainly Psychroteuthis glacialis and Alluroteuthis antarcticus; Robertson et al. 1994), while Emperor penguins in the Weddell Sea feed not only on fish and squid but also on significant amounts of krill (Klages 1989). Spatial variations in the diet of penguins from different colonies makes it difficult to compare between concentrations of persistent contaminants in their tissues. Annual and seasonal variations in sea-ice extent and prey availability can even determine temporal changes in the diet of birds from one and the same colony. Males and females show some differences in feeding behaviour (Clarke et al. 1998), and moreover very little is known about the distribution and feeding behaviour of penguins during non-breeding seasons.
In order to evaluate the potential input of xenobiotics through diet, Cor-solini et al. (2003) determined POP concentrations in the stomach content of Adelie penguins at Edmonson Point (northern Victoria Land). They found that the mean concentration of HCB, p,p'-DDE and PCBs from one foraging trip were 1,412, 1,508 and 303 ng g-1 wet wt. respectively. Stomach contents richer in euphausiids had generally higher xenobiotic concentrations, and this was attributed to the effective release of particulate materials containing POPs from melting ice and their adsorption on the body surface of crustaceans. Although dioxin-like PCBs (i.e. those with either meta or para chlorine substitutions or with one chlorine in the ortho position) were detected in all stomach content samples, the estimated toxicity for Adelie penguins was very low. The amount of POPs in the diet of penguins was rather high in comparison to concentrations usually measured in these birds. A significant amount of ingested xenobiotics is probably metabolised and does not build up in organs and tissues. Besides variations in their diet, van den Brink et al. (1998) showed that during the breeding season penguins utilise different fat stores at different times, thereby contributing to seasonal fluctuations of organochlorine levels in blood and uropygial oil.
Despite all these factors causing strong variability in contaminant levels, Adelie and Emperor penguins are useful biomonitors of persistent contaminants in Antarctic marine ecosystems because of their distribution around the continents (exclusively within the seasonal pack-ice zone), their lifespan of more than ten years, and the occurrence year after year of many individuals in one and the same colony. Studies on POP accumulation in penguins began in the 1970s. Risebrough et al. (1976) assessed chlorinated hydrocarbon concentrations in penguin eggs from different zones of the Antarctic Peninsula and compared these with data from penguins on sub-Antarctic islands. The results proved, for the first time, that POPs in the Antarctic environment did not derive from local human activity and that the atmosphere (rather than oceanic water masses) is the main pathway of transport of PCB and DDT compounds to the continent. Tanabe et al. (1986) examined the mother-to-egg transfer in Adelie penguins of PCB isomers and congeners and of p,p'-DDE. Although the transfer rate was low (about 4% of the body burden of mothers), the pattern of individual PCB isomers and congeners in eggs was similar to that in mothers. In subsequent years several papers were published on POP and Hg concentrations in penguin eggs (e.g. Luke et al. 1989; Focardi et al. 1992 c; Court et al. 1997; Bargagli et al. 1998a; Kumar et al. 2002) and in various organs, tissues and feathers (e.g. Subramanian et al. 1986; Focardi et al. 1993,1995a; Inomata et al. 1996; Sen Gupta et al. 1996; Corsolini and Focardi 2000). POP concentrations in penguins are generally lower than in birds from other seas and usually fall below recognised threshold levels for eliciting tox-icological effects in birds. However, toxicity threshold levels for penguins are unknown, and there is evidence (e.g. Court et al. 1997; Wanwimolruk et al. 1999) that the liver of Adelie penguins has a low capacity to detoxify PCBs and chlorinated pesticides.
In 1981 Honda et al. (1986) collected organs,tissues and eggs of Adelie penguins at a breeding site about 18 km south of Syowa Station. A detailed survey on the elemental composition of many samples revealed that the liver and kidney accumulated the highest concentrations of Hg and Cd. The Cd content was much higher than in seabirds from other seas. The Fe content was much higher in the pectoral muscle of adult penguins, which is rich in myoglobin (associated with their ability to dive; e.g. Tamburrini et al. 1999), than in their femoral muscle or in the pectoral muscle of other bird species.
In order to evaluate the uptake and elimination rates of trace metals in Adelie penguins, Ancora et al. (2002) determined Cd, Hg and Pb concentrations in stomach contents, excreta and feathers. High Cd concentrations were measured in stomach content and excreta samples, while Hg contents were significantly higher in feathers, which in birds are an important route for the excretion of MeHg. Bargagli et al. (1998a) measured slightly higher total Hg concentrations in the feathers of Emperor penguins (0.98±0.21 |g g-1 dry wt.) than in those of Adelie penguins (0.82±0.13 |g g-1 dry wt.) from northern Victoria Land. The eggs and muscle tissue of Emperor penguins from certain populations also had slightly higher HCB, DDT and PCB concentrations (Focardi et al. 1993). Schneider et al. (1985) found that the Cd content in the liver and especially the kidney of A.forsteri (48±21 and 382±199 |gg-1 dry wt. respectively) from Atka Bay (Weddell Sea) were much higher than values measured in the liver and kidney of Adelie penguins, South Polar skuas and seals from the same bay. The higher bioaccumulation of Cd, Hg and organochlorines in Emperor penguins than in Adelie penguins is likely due to their different diet, lifespan, and detoxification and excretion capabilities.
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