Oceanatmosphere coupling

The physical structure of the ocean is of obvious importance to all pelagic organisms. Less visible, but no less important in the long run, are feedback relationship between living organisms and atmospheric processes. In a controversial but provocative calculation. Takahashi {1989) proposed that the oceans account for more than twice the amount of C02 removed from the atmosphere than terrestrial systems. In fact, there are serious problems with many recent estimates of carbon flux, although most overlap in suggesting that the ocean is far more important in this equation than previously allowed (Siegcnthaler and Sarmiento 1993; Toggweiler 1993). The ocean is also a much greater reservoir for carbon than either the land or the atmosphere, with a total of about 4.0 x 10i9 g of carbon versus 0.2 x I0'9 g carbon for terrestrial organisms (Valiela 1984). Considering the fact that the oceans comprise more than 70% of the earth's surface, this should not be surprising. There is a strong bias among terrestrial biologists toward discounting the oceans as a carbon sink, however, due to the absence of large trees and other conspicuous concentrations of organic carbon.

Human impact on the diversity of life which influences atmospheric processes is difficult to ascertain because the relative contribution of different biogenic products on climate change, ozone depletion and C03 increase is still unknown. Dimethyl sulfide, carbonyl sulfate and bromoform are gases which can have an important impact on cloud formation or the greenhouse eiFect. Marine organisms, mainly non-calcifying coccolitho-phorids and diatoms, are known to produce all three gases (Charlson et al. 198?; Turner et at. 1988; Iverson et at. 1989; Kiene and Bates 1990). The relative abundance of these species and their distributions are poorly known, although anthropogenic activities will most likely alter their current status because different taxa respond differently to eutrophication, chemical pollution, UV-B radiation and species introduction. Possibly, only a narrow range of taxa play a disproportionately large role in controlling flux rates of these gases, such that even small changes in their relative abundances may lead to significant alteration of atmospheric processes. It has been predicted that changes in regional climate will occur to the extent that the relative abundances of carbonyl sulfate and dimethyl sulfide producing plankton are impacted by human activities in the open occan (Fuhrman and Capone 1991). Atmosphere ocean coupling may be one of the systems most susceptible to anthropogenic impacts on taxonomic diversity. A better understanding of the relative role of all these organisms in producing these gases is clearly an important area for future research.

The ocean is an important carbon sink, and changes to the phyto-plankton species composition may greatly affect globally increasing levels of carbon dioxide. These effects may be played out because of species-specific differences in carbon-fixing rates, or by changes to the size distribution of phytoplankton. Gradual changes in greenhouse gases may cross thresholds or "switches" in the ocean-atmosphere feedback system, causing rapid shifts to other stable states (Broecker 1987). These transitions could be accompanied by changes in ocean currents, and lead to a major reorganization of ocean biomes (Broecker and Denton 1989). This is a largely unexplored yet important field, with possibly wide-ranging ramifications given the potential impact on global weather predicted by small changes to climate models. The possibility that biology can drive the physical dynamics of the open ocean is an important hypothesis that needs further testing. For example, sea-surface heating through light absorption and heat release by the phytoplankton could play a significant role in regulating global climate.

16.5.6 Open-ocean continental-shelf coupling

The most commonly cited functional link between the open occan and coastal systems is probably the upwelling of nutrients along precipitous coastlines. These nutrients are brought the rest of the way to the surface by prevailing winds, and then entrained in surface waters, resulting in high primary productivity. As an adjunct, highly local process, the upwelling of hypoxic waters can generate organic carbon highs in benthic communities as a result of anoxic kills. Many of these processes arc driven by physical factors; however, the benthic biota control the breakdown and remineraliza-tion of nutrients (making them available to other ecosystems), and reduce oxygen availability. To this extent, this is an important ecosystem process with repercussions that extend beyond the open-ocean biome.

In addition, the open ocean and coastal biomes are interwoven functionally through the complex life histories of the organisms that live there. Anadromous fishes, born in the headwaters of the world's river systems, acquire the bulk of their mass during a period of several years spent at sea as part of the open-ocean food web. When they return to the rivers to spawn, the greater part of this mass is deposited, and utilized, within the watershed. For some catadromous species, net transport could be in the opposite direction. Newly metamorphosed young of species such as the Atlantic menhaden and American eel ascend rivers and estuaries to feed and grow for from one to five years, after which they return to sea. To the best of our knowledge, the energetic balance sheet for these biomass shifts has not been worked out.

Another crucial linkage lies in the reliance of both coastal and pelagic organisms on open-ocean currents for the early nurturing and dispersal of their young. The majority of marine organisms produce pelagic gametes or larvae which drift, usually within the portion of the water column occupied by the adults, over vast horizontal distances.

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