In this paper, we review the state of the art and major challenges in current efforts to incorporate biogeochemical functional groups into models that can be applied on basin-wide and global scales, with an emphasis on models that might ultimately be used to predict how biogeochemical cycles in the ocean will respond to global warming. We define the term ''biogeochemical functional group'' to refer to groups of organisms that mediate specific chemical reactions in the ocean. Thus, according to this definition, ''functional groups'' have no phylogenetic meaning-these are composed of many different species with common biogeochemical functions. Substantial progress has been made in the last decade toward quantifying the rates of these various functions and understanding the factors that control them. For some of these groups, we have developed fairly sophisticated models that incorporate this understanding, e.g. for diazotrophs (e.g. Trichodesmium), silica producers (diatoms) and calcifiers (e.g. coccolithophorids and specifically Emiliania huxleyi). However, current representations of nitrogen fixation and calcification are incomplete, i.e., based primarily upon models of Trichodesmium and E. huxleyi, respectively, and many important functional groups have not yet been considered in open-ocean biogeochemical models. Progress has been made over the last decade in efforts to simulate dimethylsulfide (DMS) production and cycling (i.e., by dinoflagellates and prymnesiophytes) and denitrification, but these efforts are still in their infancy, and many significant problems remain. ARTICLE IN PRESSOne obvious gap is that virtually all functional group modeling efforts have focused on autotrophic microbes, while higher trophic levels have been completely ignored. It appears that in some cases (e.g., calcification), incorporating higher trophic levels may be essential not only for representing a particular biogeochemical reaction, but also for modeling export. Another serious problem is our tendency to model the organisms for which we have the most validation data (e.g., E. huxleyi and Trichodesmium) even when they may represent only a fraction of the biogeochemical functional group we are trying to represent.When we step back and look at the paleo-oceanographic record, it suggests that oxygen concentrations have played a central role in the evolution and emergence of many of the key functional groups that influence biogeochemical cycles in the present-day ocean. However, more subtle effects are likely to be important over the next century like changes in silicate supply or turbulence that can influence the relative success of diatoms versus dinoflagellates, coccolithophorids and diazotrophs. In general, inferences drawn from the paleo-oceanographic record and theoretical work suggest that global warming will tend to favor the latter because it will give rise to increased stratification. However, decreases in pH and Fe supply could adversely impact coccolithophorids and diazotrophs in the future.It may be necessary...

[1] The factors driving dimethylsulfide (DMS) cycling in oligotrophic environments are isolated using a time-series of DMS sampled in the Sargasso Sea. The observed distribution of DMS is inconsistent with bottom-up processes related to phytoplankton production, biomass, or community structure changes. DMS concentrations and estimates of net biological community production are most highly correlated with physical and optical properties, with the dose of ultraviolet radiation (UVR) accounting for 77% of the variability in mixed layer DMS concentrations. Physiological stresses associated with shallow mixed layers and high UVR are the first order determinant of biological production of DMS, indicating that DMS cycling in open-ocean regions is fundamentally different than in eutrophic regions where phytoplankton blooms provide the conditions for elevated DMS concentrations. The stress regime presented here effectively closes the DMS-climate feedback loop for open-ocean environments. This response may also provide a climatic role for phytoplanktonic processes in response to anthropogenic forcing.

Apparent quantum yields and rates of dimethylsulfide (DMS) photolysis were determined from Sargasso Sea seawater with the goal of assessing the extent to which photoreactions affect the unusually elevated upper ocean concentrations of DMS during the summer, the so-called DMS summer paradox. Apparent quantum yields determined with monochromatic radiation decrease exponentially with increasing wavelength and indicate that DMS photolysis is driven by ultraviolet (UV) radiation. The relative spectral partitioning differs between samples collected from the surface mixed layer (15 m) and from the chlorophyll a maximum (80 m), presumably because of differences in chromophoric dissolved organic matter (CDOM) quality (e.g., apparent degree of bleaching). Quantum yields are also temperature dependent, and an approximate doubling of photolysis rates occurs for a 20ЊC increase in temperature. The significance of DMS photolysis to upper ocean sulfur budgets is explored using a multiyear (1992-1994) DMS time series, concurrent irradiance determinations and temperature profiles, and estimates of CDOM absorption. Depth-integrated, mixed-layer DMS photolysis rates peak in the summer (15-25 mol m Ϫ2 d Ϫ1 ) and decline to Ͻ1 mol m Ϫ2 d Ϫ1 in the winter. These rates correspond to specific turnover rates of ϳ0.29 d Ϫ1 in the summer and Ͻ0.02 d Ϫ1 in the winter. Seasonal changes in solar radiation, temperature, and DMS concentrations drive the 30-fold differences in photolysis rates, overshadowing differences caused by photosensitizer (CDOM) quantity or quality (21-35%). These results demonstrate that although photolysis is not the primary driver of the summer paradox, it makes an important contribution to the time-depth pattern of DMS concentrations observed in the Sargasso Sea.The biogeochemical cycling of sulfur between the upper water column of the ocean and the atmospheric marine boundary layer has received a great deal of attention over the last several decades because of its implication in a cloud albedo feedback loop (e.g., Shaw 1983;Bates et al. 1987;Charlson et al. 1987). The biogenic production of dimethylsulfide (DMS) in the marine environment is an important source of atmospheric sulfur. Although DMS concentrations exhibit considerable spatial and temporal variability, DMS AcknowledgmentsThis work was supported by NASA under an Earth System Science Fellowship and the SIMBIOS Program. We are extremely grateful to John Dacey for the use of his DMS and DMSP time series datasets. The authors acknowledge George Westby and Todd Medovich (SUNY-ESF) for their assistance with laboratory photolysis studies; Rod Johnson, Paul Lethaby, Rachel Parsons, Chrissy Van Hilst, Karen Paterson, and the many BATS technicians for their assistance at sea; and Andrew Hall, Toby Westberry and two anonymous reviewers for their assistance with the development of this manuscript.