EFFECTS OF NATURAL AND ANTHROPOGENIC EUTROPHICATION ON BIVALVE LIFE HISTORY IN THE NORTHERN GULF OF MEXICO

Regions of the northern Gulf of Mexico annually experience eutrophic and hypoxic conditions due to agricultural and urban runoff. Nutrient enrichment spurs phytoplankton production in coastal habitats, which can have cascading effects on resident marine species. If eutrophication has increased over time, the mean egg size is expected to decrease. Under lower nutrient conditions, mothers that produce larger, though fewer eggs, will leave behind more offspring. In contrast, under higher nutrient regimes mothers that produce more numerous, smaller eggs will leave behind more offspring. We measured the larval shells (Prodissoconch I, PI) of live and dead marine bivalves (Nuculana acuta) sampled on the continental shelf (-20 m water depth) offshore Alabama and Louisiana; larval shell size is correlated with egg size. We hypothesized that N. acuta PI size decreased over time as nutrient levels increased in the region. We also expected that differences in the mean PI size of live and dead samples would be more pronounced in coastal environments proximal to the Mississippi River. We assigned a taphonomic grade to all of the larval shells, and for this analysis, restricted our dataset to well-preserved specimens in order to account for potential preservational biases. The mean PI size of live populations in Alabama was smaller than the mean PI size of associated dead shells. In Louisiana, the mean PI size of live samples was also smaller than the mean PI size of associated dead shells. Furthermore, both live and dead larval shells were significantly smaller in Louisiana than in our samples from Alabama. Geographic variation in the mean PI size of death assemblages may reflect natural gradients in primary production across the northern Gulf. Live-dead disagreement in PI size in a given region may offer an index for the magnitude of anthropogenic eutrophication relative to a pre-Industrial baseline.