The Ecological Role of Sea

The presence of algae in Antarctic sea ice was discovered by Hooker (1847), and several reviews (e.g. Horner 1985; Vincent 1988; Garrison 1991; Palmisano and Garrison 1993; Ackley and Sullivan 1994; Lizotte and Arrigo 1998; Fogg 1998) address the biology and ecophysiology of rich microbial assemblages in Antarctic sea ice. Owing to the extension of the ice in the Southern Ocean in October (up to 19x106 km2), organism assemblages growing on the surface, interior and bottom of the ice constitute one of the largest ecosystems on Earth. Sea-ice biota interacts with and contributes to the high productivity and dynamics of the food web in the ice-edge zone (Smith and Garrison 1990).

Sea-ice surfaces and microhabitats such as brine inclusions or interstices, which develop when sea ice forms and ages, may be colonised by airborne propagules as well as planktonic organisms. The bulk of biota in the ice (microalgae, bacteria, protozoans and small metazoans) stems from the sea, and the composition of interior biotic assemblages of recently formed ice usually reflects that of the plankton beneath. Marine organisms are collected and concentrated by ice crystals floating to the sea surface or by water drawn through aggregations of frazil ice (Fogg 1998). As a rule, this composition changes with time, and differences exist between pack ice and land-fast ice. In the latter, the lower margin of the columnar ice (skeletal layer) is frequently inhabited by abundant ice microalgae; these communities are absent or much less developed in pack ice (Legendre et al. 1992). In land-fast ice the growth of sea-ice microalgae in the skeletal layer is often limited by salinity and light attenuation due to the thickness of the overlaying snow cover. On the contrary, pack-ice algae usually grow at or near the sea-ice surface and have about an order of magnitude higher photosynthetic capacity than land-fast ice algae (Arrigo et al. 1998b). The availability of nutrients is the main limiting factor for the development of surface or near-surface pack-ice assemblages. Although snow cover reduces the amount of light available for algal growth, it also directly and indirectly provides nutrients through surface flooding caused by snow loading and submersion of the pack ice. Seawater flooding on depressed ice floes can form a layer of infiltration ice along the snow-ice interface. Physical processes, such as ice rafting, pressure-ridge formation, the break-up of ice floes by waves and sea swell, and the formation of brine cells, contribute to the heterogeneous structure of sea ice. The presence of algae during seasonal warming may increase the absorption of solar radiation, thus contributing to localised melting within floes. Consequently, the volume of brine increases and adjacent cells tend to fuse and migrate downwards, forming brine channels in late-season ice or multiyear ice. Figure 26 illustrates the main habitats in Antarctic sea ice and the usually identified, three broad categories of surface, interior and bottom assemblages. In general, the most common algae in sea ice are small diatoms (over 100 species), nano- and micro-phytoflagellates, and the prymnesiophyte Phaeocystis. The majority of studies have concentrated on these organisms, which account for most of the biomass. However, bacteria (free-living, or attached to algae or detritus), fungi, autotrophic and heterotrophic protozoans (mostly ciliates, flagellates, amoebae and foraminifers), and a few metazoans co-inhabit the three kinds of assemblages. A detailed list of organisms from Antarctic sea ice is reported by

Fig. 26. Schematic representation of habitats and algal assemblages in Antarctic sea ice

Fig. 26. Schematic representation of habitats and algal assemblages in Antarctic sea ice

Palmisano and Garrison (1993). Algae photosynthesis drives the ecosystems and provides organic matter for heterotrophic organisms. Bacteria usually reach higher densities in sea ice than in seawater, and are consumed by het-erotrophs which regenerate mineral nutrients for autotrophs. These close and self-sustaining systems can reach primary production rates greater than 1.0 g C m-2 day-1 in pack-ice surface-layer assemblages (Garrison and Buck 1991), and as high as 2.1 g C m-2 day-1 in land-fast ice bottom-layer assemblages (Grossi et al. 1987). A cryopelagic community of metazoans such as adult and larval zooplankton (amphipods, copepods, larval euphausiids) and fish grazes off the algae on the undersurface of the sea ice. In shallow regions some benthic species may seasonally colonise and graze on ice-associated biota; in any case,benthic organisms on the continental shelf will receive considerable amounts of living and/or detrital material released from the melting sea ice.

The seasonal melting of pack ice is characterised by the release of ice biota into the waters of the progressively receding ice-edge zone. There is evidence (e.g. Sullivan et al. 1990; Garrison and Buck 1991) that algae, bacteria and protozoans in the sea-ice assemblages could be a source of "seed" populations to developing ice-edge phytoplankton blooms.

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