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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.
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Box 16-1. Climate Change and Fisheries The Southern Ocean has large and productive fisheries
that constitute part of the global food reserve. Currently, there are
concerns about sustainability, especially with regard to species such
as Patagonian toothfish. There are likely to be considerable changes
in such fisheries under the combined pressures of exploitation and climate
change. Spawning grounds of coldwater fish species are very sensitive
to temperature change. Warming and infusion of more freshwater is likely
to intensify biological activity and growth rates of fish (Everett and
Fitzharris, 1998). Ultimately, this is expected to lead to an increase
in the catch of marketable fish and the food reserve. This could be
offset in the long term by nutrient loss resulting from reduced deepwater
exchange. Fisheries on the margin of profitability could prosper because
the retreat of sea ice will provide easier access to southern waters.
Everett and Fitzharris (1998) discuss catch-per-unit-effort (CPUE) statistics
from the commercial krill fishery operating around South Georgia and
demonstrate that there is correlation with ice-edge position. The further
south the ice, the lower the CPUE in the following fishing season. Fedoulov
et al. (1996) report that CPUE also is related to water temperature
and atmospheric circulation patterns, and Loeb et al. (1997) document
the close relationships between seasonal sea-ice cover and dominance
of either krill or salps (pelagic tunicates). Ross et al. (2000) identify
that maximum krill growth rates are possible only during diatom blooms
and that production in Antarctic krill is limited by food quantity and
quality. Consequently, differences in the composition of the phytoplankton
community caused by changes in environmental conditions, including climate
change, will be reflected at higher trophic levels in the grazer community
and their levels of productivity. Arctic fisheries are among the most productive in the world. Changes in the velocity and direction of ocean currents affect the availability of nutrients and disposition of larval and juvenile organisms, thereby influencing recruitment, growth, and mortality. Many groundfish stocks also have shown a positive response to recent climate change (NRC, 1996), but Greenland turbot—a species that is more adapted to colder climates—and King crab stocks in the eastern Bering Sea and Kodiak have declined (Weller and Lange, 1999). Projected climate change could halve or double average harvests of any given species; some fisheries may disappear, and other new ones may develop. More warmer water species will migrate poleward and compete for existing niches, and some existing populations may take on a new dominance. These factors may change the population distribution and value of the catch. This could increase or decrease local economies by hundreds of millions of dollars annually. |
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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