The Role of Fecal Pellets in the Flux of Carbon to the Sea Floor on a River-Influenced Continental Shelf Subject to Hypoxia.

The Louisiana continental shelf near the Mississippi and Atchfalaya River deltas is a stratified and highly productive coastal system characterized by the largest hypoxic (dissolved oxygen $\u3c$2 mg l$\sp{-1}$) zone in the western Atlantic Ocean. Carbon export from surface waters in the form of sedimenting zooplankton fecal pellets was examined to determine its importance in the formation and maintenance of oxygen deficiency in the bottom waters. Two sediment traps (5-6 and 15 m) were deployed in 1991 and 1992 in 20 m water depth within an area of chronic and seasonally severe hypoxia. I determined the fecal pellet number and carbon flux, and total carbon flux from the surface waters, the percent primary production exported as fecal pellets, and the potential for fluxed fecal pellet carbon to support bottom water hypoxia. I also quantitatively sampled the water column at discrete depths for fecal pellets and zooplankton to determine potential source organisms and their seasonal, diel and spatial variation. The highest densities of total organisms, copepods and copepod nauplii occurred during March and April (1992), when chlorophyll a concentrations in surface waters were highest, and decreased in summer and fall. The abundance of fecal pellets was positively correlated with total organisms, copepods and copepod nauplii, the likely source of fecal pellets. The fluxes of total particulate material, organic carbon, organic nitrogen, fecal pellet carbon and phytoplankton carbon varied similarly between seasons, and was lowest in summer and highest in spring. The fluxes were greater in 1991 than in 1992. Seasonal variations in fecal pellet number and carbon flux were positively correlated with indicators of high surface water productivity in 1991, but not in 1992. The flux of fecal pellets from surface to bottom waters accounted for 55% of the particulate material exported vertically, exceeded phytoplankton carbon fluxes, and was high enough to deplete the bottom water oxygen reserves in spring. The results support the hypothesis that the development of summer hypoxia is associated with the decomposition of organic matter accumulated in spring primarily by the sedimentation of a phytoplankton bloom via fecal pellets, and not as intact phytoplankton cells