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Concentrations of organic carbon and rates of dissimilative sulfate reduction in surface sediments of marine mesocosms were examined along an experimental eutrophication gradient. Phytoplankton biomass increased due to addition of inorganic nutrients (N. P, Si). This increase was especially pronounced during the winter spring diatom blooms, which increased in magnitude and duration along the nutrient gradient. Net system production in winter and spring resulted in carbon deposition and accumulation in surface sediments (maximum net accumulation 17 mol C m-2). Benthic remineralization of carbon exceeded depositional supply during summer and fall. Sediment carbon concentrations approached background levels in December and February, suggesting very little annual accumulation of sediment carbon Sediment oxygen consumption and sulfate reduction rates both increased as a result of carbon sedimentation. Sulfate reduction rates in organic enriched sediments were an order of magnitude higher than control and were correlated with temperature and carbon concentrations (r2 = 0.85). Anaerobic respiration rates in unenriched sediments were related only to seasonal patterns of temperature (r2 = 0.70). Anaerobic metabolism was the dominant metabolic pathway in control and treated sediments, with 50 to 70% of annual carbon remineralization due to sulfate reduction.