Benthic biogeochemistry beneath the Mississippi River plume

Biogeochemical processes occurring near the sediment-water interface of shallow (similar to 20 m) water sediments lying beneath the Mississippi River plume on the Louisiana shelf were studied using benthic chambers and sediment cores. Three sites were chosen with distinctly different characteristics. One was overlain by oxic water where aerobic respiration dominated organic matter remineralization. The second site was overlain by oxic water but organic matter remineralization was dominated by sulfate reduction. The third site was overlain by hypoxic water and aerobic remineralization was of minor significance. Major differences were observed in the fluxes of CO2(17-56 mmol m(-2) d(-1)), O-2(2-56 mmol m(-2) d(-1)) and nutrients (e.g., NH4+, 2.6-4.2 mmol m(-2) d(-1)) across the sediment-water interface, and the relative importance of different electron accepters, even though the sites were in close proximity and at nearly the same water depth. Large variations in the efficiency of organic-C burial (3%-51%) were also calculated based on a simplified model of the relationships between the fraction of organic matter remineralized by sulfate reduction and the fraction of sulfide produced that is buried as pyrite. These observations demonstrate the high degree of spatial heterogeneity of benthic biogeochemistry in this important near-deltaic environment.