| A three year (1992–1994) time-series of twice monthly vertical profiles of dimethylsulfide (DMS) and its precursor, particulate and dissolved dimethylsulfoniopropionate (DMSP), sampled in the Sargasso Sea was utilized to isolate relevant processes that contribute to upper ocean sulfur cycling. The DMS distribution displays a phenomenon known as the ‘summer paradox,’ peaking in the summer concomitant to seasonal minima in phytoplankton biomass, primary production, and its biological precursor, DMSP. Phytoplankton community succession as the source of this peak is inconsistent with the available data. The strongest relationship was observed between surface DMS concentrations and the mean mixed layer irradiance at 325 nm. Rate constants, rates, and the first apparent quantum yield of DMS photolysis were determined using samples collected from the surface mixed layer and the chlorophyll maximum during the summertime in the Sargasso Sea. Wavelength-resolved apparent quantum yields demonstrate that DMS photolysis is driven entirely by ultraviolet wavelengths in open ocean sites. Application to the seasonal mixed layer indicated that DMS photolysis peaks in the summer, exacerbating the summer paradox. In situ profiles of DMS photolysis and bacterial consumption were sampled throughout the Western Atlantic Ocean. Observed DMS photolysis rates decreased exponentially as bacterial consumption increased with depth in accordance with absorption by chromophoric dissolved organic matter. The incubations provided additional in situ estimates of the apparent quantum yield of photolysis and a parameterization for bacterial consumption in terms of biophysical forcings. The incubations also indicated the existence of a light-mediated DMS production mechanism supporting the hypothesized phytoplankton antioxidant role for DMSP and DMS. An experimental 1-D DMS biogeochemical cycling model was developed to synthesize the results of the field and laboratory studies and to test this hypothesis. The model was able to capture the salient features of the temporal and vertical distributions of DMS indicating the importance of the phytoplankton production flux pathway. Although DMS is produced and consumed in an exceedingly complex web of biological processes, it is the framework of the regular seasonal progression of the physical and optical environment that drives DMS stocks. |
