THE CYCLING OF SULFUR IN SURFACE SEAWATER OF THE NORTHEAST PACIFIC

Oceanic dimethylsulfide (DMS) emissions to the atmosphere are potentia lly important to the Earth's radiative balance. Since these emissions are driven by the surface seawater concentration of DMS, it is importa nt to understand the processes controlling the cycling of sulfur in su rface seawater. During the third Pacific Sulfur/Stratus Investigation (PSI-3, April 1991) we measured the major sulfur reservoirs (total org anic sulfur, total low molecular weight organic sulfur, ester sulfate, protein sulfur, dimethylsulfoniopropionate (DMSP), DMS, dimethylsulfo xide) and quantified many of the processes that cycle sulfur through t he upper water column (sulfate assimilation, DMSP consumption, DMS pro duction and consumption, air-sea exchange of DMS, loss of organic sulf ur by particulate sinking). Under conditions of low plankton biomass ( <0.4 mug/L chlorophyll a) and high nutrient concentrations (>8 muM nit rate), 250 km off the Washington State coast, DMSP and DMS were 22% an d 0.9%, respectively, of the total particulate organic sulfur pool. DM S production from the enzymatic cleavage of DMSP accounted for 29% of the total sulfate assimilation. However, only 0.3% of sulfate-S assimi lated was released to the atmosphere. From these data it is evident th at air-sea exchange is currently only a minor sink in the seawater sul fur cycle and thus there is the potential for much higher DMS emission s under different climatic conditions.