Do biological processes matter quantitatively in atmosphere/ocean CO2 exchange in the Southern Ocean?

The mean primary productivity of open-water areas south of the Antarctic Polar Frontal Zone (APFZ) is only 30 - 40 gC m-2 y-1 (e.g., Smith et al., 1988), four times lower than that in the Sargasso Sea near Bermuda (Lohrenz et al., 1992; Michaels et al., 1994), and lower than that in any large oceanic area except the permanently ice-covered central Arctic (Smith and Sakshaug, 1990). Productivity is significantly higher in continental-shelf areas near Antarctica (e.g., DeMaster et al., 1992) and may be higher within the APFZ (e.g., Bathmann et al., 1995; Turner and Owens, 1995), but those areas are restricted enough spatially that the average productivity for the region as a whole probably cannot exceed 50 gC m-2y-1. The APFZ and all waters to the south comprise an area of about 4 x 107 km2 (e.g., Comiso et al., 1993), a little more than 10% of the surface area of the ocean. The maximum estimate of primary productivity in the Southern Ocean is thus approximately 50 gC m-2 y-1 x 4.0 x 1013 m-2 = 2.0 x 1015 gC y-1 (2.0 GT C y-1). Exportable "new" production has been estimated in several subsystems of the Southern Ocean to be approximately 50% of total primary productivity, over a wide range of low- to high- productivity conditions (numerous studies, summarized by Smith and Sakshaug, 1990; Nelson, 1992). Thus the estimated maximum carbon flux from the surface layer of the Southern Ocean is on the order of 1 GT C y-1, meaning that the magnitude of the biologically driven flux is considerably less than the present uncertainty in the net gas exchange.

If the net absorption or release of CO2 by the Southern Ocean is at present very nearly zero, or if photosynthetic carbon uptake is of major importance in maintaining oceanic undersaturation in those areas of net absorption, then biological pumping mechanisms are quantitatively significant in the regional CO2 balance. Otherwise they are not. Recent efforts by J. Sarmiento (pers. comm.) combines projected climate changes in the deep circulation of the ocean with models of ocean biogeochemistry. This work suggests that there will be a significant impact on the atmospheric CO2 content as result of biological activity, especially in the Southern Ocean. This impact is magnified if there is a concurrent change in the structure of the marine ecosystem that impacts biological activity.

Implications for field measurements: Measurements of primary productivity, nutrient uptake rates and biogenic particle flux should be coordinated very closely with those of delta-pCO2 to evaluate the role of phytoplankton photosynthesis and organic-matter export in the uptake of CO2 by the ocean. All biological process studies should be conducted in a way that addresses the dominant biological time scale in the Southern Ocean, which is seasonal. Seasonal cycles of primary productivity and biogenic particle flux are more pronounced in the Southern Ocean than in other regions because of the strong seasonality in solar irradiance and, in the southern portion of the system, ice cover (e.g., Smith and Sakshaug, 1990; Fischer et al., 1988). Moreover, the specific growth rates of phytoplankton are lower in the Southern Ocean than in lower-latitude systems (e.g., Wilson et al., 1986), primarily because of low surface-layer temperatures and light limitation. This means that highly transient bloom events lasting one day to several days, which have been observed in low-latitude systems (e.g., Marra et al., 1990), cannot be quantitatively significant in comparison with the seasonal signal and may not develop at all.

Beyond these purely practical considerations, all biological studies should be approached with the understanding that the processes they are evaluating may or may not be of major importance in the effort to understand carbon fluxes. A comprehensive biological program should be part of the JGOFS AESOPS, but its central purpose should be to understand the pelagic ecosystem and biogeochemical cycles of the region. We should seek to understand processes that are significant ecologically (e.g., photosynthesis, nutrient uptake and regeneration, grazing, biogenic particle flux) regardless of whether or not they are major carbon-flux terms. As noted earlier, the work by Sarmiento indicates that the structure of the marine ecosystem will have significant impacts on air/sea exchange of carbon.

Implications for modeling: If biological terms can be neglected, carbon-flux models can be immensely simpler and still have predictive value. Such models would have to consider wind forcing, circulation patterns, gas exchange, inorganic chemical equilibria and very little else. The models would still have to select appropriate temporal and spatial scales of resolution - a non-trivial problem - but they would be inherently simpler and more testable than those that require an ecosystem submodel.

If biologically driven carbon fluxes are important quantitatively, pelagic ecosystem models will have to be incorporated into the carbon-flux models. Ecosystem models should be constructed, whether or not they are incorporated into ocean/atmosphere CO2 exchange models, and we recommend that such models consider on the order of 20 biotic and abiotic variables including pCO2, alkalinity, NO 3+, NH4+, Si(OH)4, Fe, O2, POC (considering suspended and sinking POC separately), DOC, phytoplankton (considering 3-5 functional groups within the phytoplankton) and grazers (considering 3-5 functional groups of grazers). The recommendation for this level of complexity, rather than greater or less complexity, is explained in section III.C of this report.

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