Pelagic Ecosystems

The sea surrounding Antarctica represents one of the largest and most dynamic environments on Earth. The unrestricted flow of currents circulating around the continent and the seasonal pulse of sea-ice freezing and breakup are the main factors controlling the Antarctic marine ecosystem. Views on the productivity of pelagic communities have changed greatly over the years.

High primary production near pack ice and the short food chain (diatoms-krill-whales) were taken as an indication of rich, potentially exploitable resources. It is now recognised that phytoplankton biomass and overall primary production in the oceanic part of the Southern Ocean are much lower than supposed. Besides the previously discussed lack of some micronutrients, other factors limit primary production, these being (1) the vertical instability of the water column, which reduces light availability and prevents algae from making full use of the rich supply of nutrients; (2) low temperatures, which depress the growth rate of algae; and (3) the biomass of herbivores such as krill, copepods and salps, which can further limit the growth of the phyto-plankton population.

In general, biogeographical subdivisions in the pelagic zone of the Southern Ocean reflect the different water masses, fronts and seasonal ice cover. In spite of within-zone and longitudinal variations, especially in areas of major oceanographic disturbance such as in the vicinity of the Antarctic Peninsula, three main circumpolar pelagic zones are usually identified (Hempel 1985): the zone of ice-free open waters, the seasonal pack-ice zone, and the permanent sea-ice zone. In general, the ice-free zone of the ACC is rather rich in nutrients but, apart from a few areas, relatively poor in primary production. The zooplankton is rather similar to that of the northern North Atlantic and is dominated by copepods, salps and small euphausiids. Squid, myctophids, small juveniles of benthic fish species and some petrels and whales also occur, but they play a minor role compared to the biomass in the boreal zones of the North Atlantic and North Pacific.

The seasonal pack-ice zone occupies most of the East Wind Drift; it is mainly ice-free in spring and summer, and seeding by ice algae contributes to a high primary production. Although large amounts of phytoplankton may sink to the bottom, the pelagic food web comprises large communities of salps, copepods, fish larvae, chaetognaths and, above all, krill (Euphausia superba). Shoals of krill provide the food base for baleen whales, crabeater seals (Lobodon carcinophagus), penguins and other seabirds.

In polynyas and the permanent sea-ice zone, primary production is very intense but limited to a short summer period. Krill is often replaced by the smaller E. crystallorophias, and the zooplankton biomass is low; therefore, most algae are not consumed and fall to the bottom to sustain a rich fauna of benthic feeders. Many species of benthic invertebrates exploit organic matter accumulating at the water-seabed interface during summer and survive long periods of starvation at negligible metabolic cost. These invertebrates are food sources for crustaceans and many fish species on which Emperor penguins (Aptenodytes forsteri) and Weddell seals (Leptonychotes weddellii) feed.

In general, mere survival in the Southern Ocean is a feat, and organisms in pelagic communities are often characterised by late and low reproduction, long lifespans, large body size and high total biomass with low net produc tion. Models of food web dynamics and the management of living resource exploitation have to take into account the low efficiency of the food chain and the possible effects of climate change on the extent of sea ice and on ocean circulation.

3.3.1 Primary Productivity

The study of Antarctic marine phytoplankton dates back to the James Clark Ross expedition (1839-1843), when the botanist and surgeon J.D. Hooker reported that diatoms "occurred in such countless myriads, as to stain the Berg and the Pack-ice" (Hooker 1847). He sent some samples to the German botanist C.G. Ehrenberg,who published the first book on Antarctic diatoms in 1844. Since then many papers have been written on the distribution and bio-geography of phytoplankton in the Southern Ocean (e.g. Hart 1934; El-Sayed 1968; El-Sayed et al. 1979; Sakshaug and Holm-Hansen 1984; Sullivan et al. 1988; Jacques and Fukuchi 1994; Priddle et al. 1994; Arrigo et al. 1998a; Saarhage 1998; Smith et al. 2000a). The seasonal cycle of phytoplankton biomass and productivity is characterised by large variations, which remain poorly resolved despite the importance of the Southern Ocean in the marine C cycle (Sarmiento et al. 1998). During the last decade, large-scale distribution and spatio-temporal variations in phytoplankton observed by satellites have been used to understand relationships between primary productivity and environmental forcing such as wind and sea ice (e.g. Banse 1996; Moore et al. 1999). However, satellites are often of relatively little use in the Southern Ocean, because they cannot estimate phytoplankton abundance for extended periods in areas of extensive cloud and/or ice cover. On the other hand, the large amount of data collected over the past decades through research vessels shows a rather scarce geographical and temporal coverage, because most studies are confined to short, often ice-free summer periods.

Studies on net phytoplankton or microplankton (i.e. plankton 20-200 |m in size) show that algae mainly consist of colonial or chain-forming diatoms. More than 100 diatom species, often belonging to the genera Chaetoceros, Odontella, Thalassiosira, Rhizosolenia and Nitzschia, have been reported from Southern Ocean waters. Dinoflagellates (about 60 species; Knox 1994), chiefly the genera Ptotoperidium and Dinophysis, silicoflagellates, and especially unicellular motile and colonial prymnesiophytes of the genus Phaeocystis are other important algal groups (e.g. Holm-Hansen and Huntley 1984; Estep et al. 1990; Caron et al. 2000). If the phytoplankton sampled with nets appears dominated by quite large diatoms, since the 1970s the use of other sampling approaches revealed that ultraplankton (microorganisms<20 |m long) may account for a large proportion (up to 80-90 %) of total estimated phytoplank-ton carbon in many regions of the Southern Ocean. These microorganism communities include nanoplankton (size 2-20 | m, mostly small diatoms belonging to the genera Chaetoceros, Fragilariopsis and Nitzschia, and unicellular green flagellates with calcareous or siliceous skeletal elements) and picoplankton (size 0.2-2 |m, including unicellular eukaryotes such as coccoid green flagellates and, above all, cyanobacteria and prochlorophytes). Viruses, bacteria, phagotrophic flagellates and ciliates provide heterotrophic activity which balances the phototrophic activity of ultraphytoplankton. The average density of bacterioplankton in the Southern Ocean (about 106 cells ml-1) usually corresponds to that in temperate seas and correlates with phytoplankton biomass. Archaebacteria, once thought to be restricted to hypersaline, extremely hot or anoxic habitats, may be particularly abundant in Antarctic waters (more than 30 % of prokaryotic biomass; de Long et al. 1994).

Although most species of Antarctic marine phytoplankton are circumpolar, there are large spatio-temporal variations in species composition. The algal bloom in coastal Antarctic waters is often dominated by a single opportunistic species (e.g. Thalassiosira tumida in the south-western Weddell Sea or the prymnesiophyte Phaeocystis in the southern Ross Sea; El-Sayed and Fryxell 1994; Sweeney et al. 2000). Diatoms are undoubtedly the most important phytoplankton class in the Southern Ocean, but there is evidence (di Tullio et al. 2000) that huge blooms of Phaeocystis antarctica determine an early and rapid C export to deep water and sediments in the Ross Sea. Sequence data from 18S small subunit ribosomal DNA have shown that Phaeocystis antarctica is genetically distinct from P. pouchetii, the northern cold water form, and P. globosa, the warm water species, from which the former two seem to have evolved (Medlin et al. 1994). In general, the most important taxa of Antarctic phytoplankton are cosmopolitan but, in addition to the monospecific genera (Charcotia and Micropodiscus), about 80 % of Antarctic dinoflagellate species and 37% of diatom species seem to be endemic (Fogg 1998).

A low standing crop of phytoplankton (average chlorophyll concentrations of about 0.5 mg m-3, with maximum values at 50-70 m depths) is usually reported in oceanic waters, the Drake Passage, and the Bellingshausen Sea. Average productivity in these areas is low (about 0.1-0.2 g C m-2 day-1) and corresponds to that of oligotrophic seas such as the eastern Mediterranean or the North Pacific gyre (El-Sayed and Fryxell 1994). In contrast, high standing crop and primary production are usually reported in coastal waters and in the vicinity of Antarctic and sub-Antarctic islands. In the southern Ross Sea, for instance, concentrations of chlorophyll and particulate organic carbon in the surface layer may reach values exceeding 15 |g l-1 and 0.85 |mol l-1 respectively. These values are twice the maximum concentrations reported in the Peruvian upwelling system or in the shelf of the Bering Sea (Smith et al. 2000b).

In general, phytoplankton growth follows the seasonal cycle of radiation fairly closely - biomass increases rapidly during the austral spring, and primary productivity reaches a maximum in December/January (average values about 2.6 g C m-2 day-1) and declines in February (Smith et al. 2000a). Quite similar trends, with productivity peaks above 3 g C m-2 day-1, have been reported in many other coastal regions of Antarctica (El-Sayed and Fryxell 1994).

Ultraplankton in Antarctic waters has a seasonal periodicity similar to that of microplankton, although the peak occurs later in summer and the curve of the standing crop is flatter than the bell-shaped curve of microplankton. In winter, the standing crop of ultraplankton is about five times that of microplankton and may play a very important role for planktonic and benthic consumers (Clarke and Leakey 1996).

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