16.2.3.4. Impacts on Biology of Southern Ocean

No single factor controls overall primary production in the Southern Ocean. The organisms in Antarctic marine communities are similar to the inhabitants of marine systems at lower latitudes, although there is substantial endemism in Antarctica (Knox, 1994). Ice cover and vertical mixing influence algae growth rates by modulating the flux of solar radiation (Priddle et al., 1992). Micronutrients, especially iron, are likely to limit phytoplankton growth in some areas. Experiments involving addition of iron to the ocean show dramatic increases in the biological activity of phytoplankton (de Baar et al., 1995; Coale et al., 1996; Sedwick et al., 1999). Findings by Boyd et al. (2000) demonstrate that iron supply controls phytoplankton growth and community composition during the summer, but the fate of algal carbon remains unknown and depends on the interplay between processes that control export, remineralization, and water-mass subduction. Grazing by zooplankton also may be important.

Several of the physical controls on phytoplankton production are sensitive to climate change. Although it is presently impossible to make numerical predictions, these controls have been outlined in a qualitative way by Priddle et al. (1992). They consider that projected changes in water temperature and wind-induced mixing of the Southern Ocean will be too small to exert much effect but that changes in sea ice are likely to be more important. Release of low-salinity water from sea ice in spring and summer is responsible for developing the shallow mixed layer in the sea-ice marginal zone—an area of the Southern Ocean that is nearly as productive as the coastal zone (Arrigo et al., 1998)—and plays a major role in supporting other marine life. Projected reductions in the amount of sea ice (Section 16.2.4.2) may limit the development of the sea-ice marginal zone, with a consequential impact on biota there. On the other hand, greater freshening of the mixed ocean layer from increased precipitation, ice-sheet runoff, and ice-shelf melting might have a compensating effect. It seems that the sea-ice marginal zone, under-ice biota, and subsequent spring bloom will continue, but shift to more southern latitudes, as a consequence of the retreat of the ice edge.

Research also demonstrates that the biological production of the Antarctic food web is linked closely to physical aspects of the ocean and ice ecosystem. Matear and Hirst (1999) point out that changes in ocean circulation will impact ocean biological production. They project a reduction in biological export from the upper ocean and an expansion of the ocean's oligotrophic regions. This will alter the structure and composition of the marine ecosystem. For example, interdecadal variations in sponge/predator population and in anchor/platelet ice at depths shallower than 30 m appear to be related to alterations in regional currents and ocean climate shifts (Jacobs and Giulivi, 1998). Changes in ocean currents could bathe new areas of the sea floor in near-freezing water, so that anchor ice and ice crystals will rise through the water column. This will be a liability for some benthic species. On the other hand, there could be a fresher and more stable layer, which could lead to changes in phytoplankton community structure (Arrigo et al., 1999), and stronger ocean fronts. Both of these physical changes would be beneficial to many parts of the marine ecosystem. A 20% decline in winter and summer sea ice since 1973 west of the Antarctic Peninsula region (Jacobs and Comiso, 1997) has led to a decline in Adelie penguins, which are obligate inhabitants of pack ice. By contrast, Chinstrap penguins in open water have increased in numbers (Fraser et al., 1992; Ainley et al., 1994). Krill recruitment around the Antarctic Peninsula seems to be dependent on the strength of the westerlies and sea-ice cover, with a 1-year lag (Naganobu et al., 2000). Both will decrease in the future, so there will be less krill.

The direct effect of a change in temperature is known for very few Antarctic organisms. Much of the investigation on ecophysiology has concentrated on adaptations to living at low temperature, with relatively little attention devoted to their response to increasing temperatures. Few data are available to assess quantitatively the direct and indirect impacts of climate change. Perhaps the best-studied example of temperature affecting the abundance of marine microorganisms is the increased rate of production of the cyanobacterium Synechococcus with increasing temperature, which approximately doubles for an increase in temperature of 2.5°C (Marchant et al., 1987).

The virtual absence of cyanobacteria represents a fundamental difference between the microbial loop in Antarctic waters compared to that in temperate and tropical waters. As discussed by Azam et al. (1991), metazoan herbivores apparently cannot directly graze Synechococcus; their production must be channelled through heterotrophic protists able to consume this procaryote. Adding another trophic step reduces the energy available to higher trophic levels. Coupled with the direct utilization of nanoplankton by grazers, this may account in part for high levels of tertiary production in the Southern Ocean, despite relatively low levels of primary production (but see Arrigo et al., 1998). Any increase in water temperature will increase the concentration of cyanobacteria and the heterographs that graze them. It is possible that the prey for krill and other grazers also will change, but the ultimate effects are unknown. Changes in the microbial loop may lessen carbon drawdown because of increased respiration by heterotrophs. Furthermore, there is an apparent uncoupling of bacterioplankton and phytoplankton assemblages that contrasts with temperate aquatic ecosystems (Bird and Kalff, 1984; Cole et al., 1988; Karl et al., 1996). The structure and efficiency of the Antarctic marine food web is temporally variable, and Karl (1993) has suggested that it is reasonable to expect several independent (possibly overlapping in space and time) food webs. There is no consensus with respect to the importance of bacteria and their consumers within this food web or their degree of interaction with photoautotrops. Bird and Karl (1999) have demonstrated, however, that uncoupling of the microbial loop in coastal waters during the spring bloom period was the direct result of protistan grazing. Although underlying mechanisms remain unclear, the distinct difference of the microbial loop in Antarctic waters, compared to more temperate waters, suggests that climate change will have profound effects on the structure and efficiency of the Southern Ocean food web.

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