Physical And Chemical Characteristics

The discussion of plankton in this chapter will be prefaced by a brief examination of the unique physical and chemical characteristics of the Antarctic seas, since these have a strong bearing on the biology, distribution, and abundance of the planktonic organisms. A more detailed account of the hydrology of the Pacific Sector of the Southern Ocean is given by Whitworth and Patterson (this volume).

Unique features of the waters surrounding Antarctica include: (a) the presence of pack-ice around the Antarctic continent and its seasonal waxing and waning; (b) variability of the light regime, which, south of the Antarctic Circle, alternates between continuous darkness in the winter and continuous daylight during the summer; (c) extensive cloud cover; (d) the circumpolarity of Antarctic waters, and (e) high concentrations of inorganic nutrients. It is in response to the physico-chemical environments that the Antarctic planktonic organisms have developed their characteristic features.

The Southern Ocean is a region of relatively simple hydrographic conditions. These conditions have been discussed in detail by Sverdrup (1933) and Deacon (1933, 1963, 1964, 1984) and are only briefly summarized here. The waters between 40°S and the Antarctic continent can be divided into several well-defined circumpolar zones (Nowlin, 1985). Of the several fronts, the Polar Front appears most stationary (Fig. 8.1), and it forms one of the major boundary zones of the World Ocean. The principal physical property by which its location can be mapped is the steep temperature gradient at the sea surface. At the northern limit of the Polar Front, Antarctic Surface Water sinks beneath the less dense, south-flowing sub-Antarctic Water to form the Antarctic Intermediate Water which flows northward (Fig. 8.2). The Antarctic Bottom Water is formed close to the Antarctic continent. Immediately above this Bottom Water, there is an exception-

Polar Frontal Zone

Fig. 8.1. Surface regimes of the Southern Ocean. Position of the Subtropical Front separating the subtropics from the Subantarctic Zone is after Deacon (1982). Locations of the Subantarctic Front and Polar Front bounding the polar frontal zone are modified from a figure by Clifford (1983). The antarctic zone is south of the Polar Front. Summer ice extent is shown near Antarctica as are locations where a water mass transition near the continental slope has been observed (Clifford, 1983) (after Nowlin, 1985, and Nowlin and Klinck, 1986).

ally thick relatively warm water mass, the Circumpolar Deep Water, characterized by high salinity and high concentrations of nutrients.

The distributions of the three inorganic nutrient salts best studied in the Southern Ocean (i.e., phosphate, nitrate and silicate) show marked geographic variations. The effect of the Antarctic Converge on the distribution of these nutrient salts is quite significant. In general, the nutrient distributions show higher concentrations south of the convergence than north of it. However, it is interesting to note that even their lowest levels of concentrations are higher, in

Convergence Front
Fig. 8.2. Schematic representation of water masses and their movements in the Southern Ocean sector south of Africa (after Lutjeharms et al., 1985).

general, than the winter maximum of temperate regions. At a deeper level, between the northward-moving Antarctic Intermediate Water and the southward-moving Deep Water, phosphate and nitrate appear to be regenerated, possibly because of the large mortality of sinking phytoplankton as indicated by the abundant deposits of diatom ooze below this region. The decomposition enriches the south-going deep water and, in the Antarctic zone, the highest phosphate and nitrate concentrations are found in the Circumpolar Deep Water. Silicates are most abundant in the Antarctic Bottom Water, possibly because of regeneration from the dissolution of the diatom frustules near the bottom.

According to Foster (1984), there does not seem to be a clear correlation between the surface distribution of nutrients and upwelling, although the Circumpolar Deep Water is probably the source of the nutrients in the surface waters south of the convergence.

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