A light-driven, one-dimensional dimethylsulfide biogeochemical cycling model for the Sargasso Sea

We evaluate the extent to which dimethylsulfide (DMS) cycling in an open-ocean environment can be constrained and parameterized utilizing emerging evidence for the significant impacts of solar ultraviolet radiation (UVR) on the marine organic sulfur cycle. Using the Dacey et al. (1998) 1992-1994 Sargasso Sea DMS data set, in conjunction with an offline turbulent mixing model, we develop and optimize a light driven, one-dimensional DMS model for the upper 140 m. The DMS numerical model is primarily diagnostic in that it incorporates observations of bacterial, phytoplankton, physical, and optical quantities concurrently measured as part of the Bermuda Atlantic Time-series Study (BATS) and Bermuda Bio-Optical Project (BBOP) programs. With the exception of sea-to-air ventilation, each of the sulfur cycling terms is explicitly parameterized or altered by the radiation field. Overall, the model shows considerable skill in capturing the salient features of the DMS distribution, specifically the observed DMS summer paradox whereby peak summer DMS concentrations occur coincident with annual minima in phytoplankton pigment biomass and primary production. The dominant processes controlling the upper-ocean DMS concentrations are phytoplankton UVR-induced DMS release superimposed upon more surface oriented processes such as photolysis and sea-to-air ventilation. The results also demonstrate that mixing alone is not enough to parameterize DMS distributions in this environment. It is critical to directly parameterize the seasonal changes in the flux and attenuation of solar radiation in the upper water column to describe the DMS distribution with depth and allow for experimentation under a variety of climate change scenarios.