Effects of eutrophication on the biogeochemical cycling of carbon, oxygen, sulfur and energy in coastal marine ecosystems

The effects of eutrophication on the biogeochemical cycling of the major elemental cycles, carbon, oxygen, and sulfur and on energy flow, were investigated in experimental marine ecosytems (mesocosms). Eutrophic environmental conditions were created within mesocosms by daily addition of varying amounts and sources of nutrients and sewage sludge. Inorganic nutrient loading resulted in an increase in the magnitude and duration of winter-spring phytoplankton populations and production with a correlated increase in sedimentation and accumulation of carbon in surface sediments. Net ecosystem production during the winter and spring was followed by net system heterotrophy during the summer and fall with a concomitant decrease in sedimentary carbon concentrations. Carbon sedimentation and accumulation (up to 19 mol C m;Oxygen dynamics in eutrophic marine systems were regulated by phytoplankton cycles and sulfur cycling in the sediments. Initiation of hypoxia (oxygen less than 2 mg l;Whole system carbon and oxygen budgets showed the sediments to be an important and influential component in eutrophic marine ecosystems. A significant fraction of allochtonous sewage carbon was remineralized via sulfate reduction in the sediments. This led to the dissociation of carbon cycling from oxygen and energy flow in the system due to an increased role of sulfur biogeochemistry. Accumulation of solid sulfides in the sediments from sulfate reduction during the experiment explained the discrepancy between carbon dioxide and oxygen consumption at the sediment water interface (respiratory quotients...