FLUVIAL INFLUENCE ON TIME AVERAGING AND NUTRIENT LOADS ON THE CONTINENTAL SHELF, NORTHERN GULF OF MEXICO

Rivers transport freshwater, sediments, and nutrients to coastal ecosystems. Marine primary productivity is nutrient-limited, consequently agricultural and urban runoff have increased primary production in many regions. Anthropogenic eutrophication is a primary correlate of live-dead disagreement, and can be associated with changes in species traits, abundance, and community composition. Live-Dead comparisons can also be affected by sedimentation rates. Seafloors that experience greater sedimentation may be dominated by relatively recent dead that postdate the onset of eutrophic conditions, thereby recording live-dead similarity despite histories of nutrient enrichment. Here we assess the influence of river systems on continental shelf environments across the northern Gulf of Mexico using a multi-proxy approach. We collected live and dead bivalve mollusks along the -20m isobath at 15 sites distributed between Louisiana and Florida. Shells of the suspension-feeding venerid bivalve, Lirophora clenchi, were picked from death assemblages for geochemical analyses. Oxygen and nitrogen isotopes were used to assess variation in salinity and nutrients, respectively, and shells were dated using radiocarbon. d18O values vary across the Gulf with generally lighter values closer to the Mississippi Delta. The variance in d18O values in Florida and Alabama is considerable, however, indicating complex interactions between coastal currents and fluvial inputs in the northeastern Gulf. Geographic variation in the ages of Lirophora shells are consistent with the d18O data. Median shell age increases from Louisiana to Alabama, which may reflect reduced sedimentation further from the Mississippi Delta. The median age of shells in Florida is younger than in the other regions, which may reflect faster rates of shell destruction due to even slower sedimentation. d15N data are currently being gathered to assess spatiotemporal variation in nutrient inputs. Developing a clearer understanding for how river systems affect continental shelf environments across the northern Gulf is critical for assessing the environmental drivers of live-dead dissimilarity, and for interpreting seemingly paradoxical cases of live-dead similarity in areas with known histories of anthropogenic eutrophication.