Antarctic Marine Food Webs and the Impact of Human Activity

Although the krill system is unique in the world oceans, and although it is undoubtedly a very important component of the Antarctic marine ecosystem, the seasonal pack-ice zone covers some 19x106 km2 (Knox 1994), and food webs in the larger part of the Southern Ocean contain little krill. Human activity, such as the exploitation of marine resources or the bioaccumulation and/or biomagnification of persistent anthropogenic pollutants, will affect the various food webs in the Southern Ocean in different ways.

The area of the ACC (about 27x 106 km2) is free of ice throughout the year; it is rich in nutrients but relatively poor in phytoplankton (mainly nanoplank-ton) and usually has two production peaks each year. Zooplankton is dominated by herbivorous copepods, salps and small euphausiids, with higher biomasses usually concentrating at depths between 500 and 1,000 m. Myctophid fish, juveniles of benthic fish species and, above all, cephalopods are the main consumers in the productive mesopelagic layer. In this zone of the Southern Ocean, sperm whales, elephant seals and several species of albatross are among the main consumers of cephalopods and, probably, the main bioaccu-mulators of persistent pollutants. It has been suggested that Antarctic stocks of cephalopods have increased in the past century as a result of the overexploitation of sperm and baleen whales (i.e. the principal consumers of cephalopods and krill; Laws 1985). However,some major cephalopod fisheries are located in cool temperate waters, just north of the CCAMLR boundaries, and one of the target species (Martialia hyadesi) is also found in the CCAMLR Convention Area. Fishing poses special problems for stock management (Rodhouse 1990), because commercially fished species are fast-growing and short-lived (approximately one year). As recruitment depends on the breeding success of a single generation, cephalopod populations are prone to extreme inter-annual fluctuations and are therefore highly susceptible to overfishing, which also adversely affects seabirds. Populations of wandering albatrosses and petrels are also decreasing because they are caught on hooks deployed by vessels which tow huge, heavily baited lines to longline to catch various fish species, particularly the Patagonian toothfish D. eleginoides.

In the marginal ice-edge zone, spring and summer phytoplankton blooms allow the development of zooplankton communities, and the large stocks of krill directly or indirectly support vast populations of vertebrate consumers. Owing to high primary and secondary productivity, a vertical flux of organic matter, and probably of ad/absorbed chemical compounds, establishes in deep waters and sediments. Along with the relatively short krill food chain, this sedimentation probably helps reduce the potential bioaccumulation of persistent pollutants by seabirds and baleen whales. In this area, however, at first sealing, then whaling and, more recently, the exploitation of krill have determined dramatic perturbations in mammal and bird populations. While the harvesting of species such as baleen whales, at the higher level of the food web, increased the availability of food and enhanced the productivity of competitor species, the long-term effects of harvesting species such as krill, at lower levels in the food web, are unpredictable. Phytoplankton in the seasonal pack-ice zone is the most exposed to the effects of UV-B radiation, and the biological cycle of krill is closely linked to that of sea ice. Thus, climate and environmental change will also affect biotic communities and food chains in the marginal ice edge.

Near the continent, in the permanent pack ice or fast ice zone, phytoplank-ton production is restricted to a brief, intense summer period, and the zooplankton biomass is usually low. Therefore, a large proportion of algae inside or below fast ice, phytoplankton, and zooplankton organisms falling from above become an important source of food for benthic invertebrates, on which notothenioid fish feed in turn. Given the low biomass of euphausiids (E. superba is often replaced by the smaller E. crystallorophias), most seabirds breeding along Antarctic coasts, such as the Emperor and Adelie penguins, and marine mammals (Weddell seals) feed on crustaceans and pelagic, cry-opelagic or epibenthic fish.

Regeneration of nutrients takes place on the bottom, and they are returned to the water column together with persistent pollutants, determining interexchanges between pelagic and benthic environments of the Antarctic continental shelf.

Most scientific stations in Antarctica are located at coastal sites, and marine ecosystems seem particularly at risk because the involvement of ben-thic invertebrates in the transfer of energy and persistent pollutants from phytoplankton, other autotrophic organisms and sediments to fish, nesting seabirds, and seals. The lengthening of the food chain enhances the bioaccumulation of pollutants (Bargagli et al. 2000). At Terra Nova Bay, for instance (Bargagli et al. 1998 c), concentrations of Hg in seawater and marine sediments are very low, and there is no evidence of metal inputs from the Italian Scientific Station. However, the total body content of Hg progressively increases from primary consumers (zooplankton and sponges) to benthic organisms feeding on algae and/or detritus (e.g. sea urchins) and more opportunistic feeders such as starfish and gastropod molluscs. Metal (in the form of methylmercury) is transferred from benthic invertebrates to demersal fish and higher vertebrates. Concentrations in bird feathers increase in the order snow petrel (zooplankton feeder)<Adelie penguin (zooplankton and fish feeder)<Emperor penguin (fish feeder)<Antarctic skua. The skua has an omnivorous diet consisting of marine organisms, eggs and chicks of penguin and skuas, adult snow petrels, and human refuse from the scientific stations (Court et al. 1997).

If seabirds are now recognised worldwide as important components of marine ecosystems, it is above all thanks to research in polar regions. Antarc tic seabirds are considered among the most reliable indicators of environmental change in the Southern Ocean, and several international research programmes are promoted by CCAMLR to monitor changes in the size and distribution of populations breeding in Antarctica.

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