Paper No. 93-7
Presentation Time: 9:00 AM-1:00 PM
SPATIOTEMPORAL VARIATION IN NUCULA LIFE HISTORY IN THE NORTHERN GULF OF MEXICO
Primary productivity in the northern Gulf of Mexico varies geographically and has changed over time. To what extent has spatiotemporal variation in food supply shaped the life histories of consumers in marine food webs? Large watersheds, like the Mississippi River, deliver tremendous nutrients to the coastal ocean, resulting in enhanced primary production. Enhanced primary production could mean more food for benthic mollusks, leading to shifts in their life histories. Whereas individuals born from smaller eggs may experience lower survival in food-limited environments, survivorship increases for all egg sizes when food is plentiful, and organisms that produce more numerous, smaller eggs will have greater fitness. Larval shell size is correlated with egg size in bivalves and thus can be used to assess life history variation along primary productivity gradients. Here we assess how the size of Nucula proxima larval shells varies geographically with proximity to the Mississippi River, and temporally in response to increased primary productivity over time. Live and dead specimens of N. proxima were collected from surface sediments at 15 sites on the -20m isobath offshore Louisiana, Alabama, and Florida. We hypothesized that the size of Nucula larval shells would be larger in coastal Florida than in areas adjacent to the Mississippi River. We also hypothesized that larval shell size would be smaller today compared to the past due to anthropogenic eutrophication. We observed a significant difference in larval shell size between regions, with smaller larval shells in Alabama and larger larval shells in Florida. In contrast, preliminary data shows no evidence that larval shell size has changed over time. Our results suggest that nutrient levels may be a primary driver of N. proxima life history variation in the northern Gulf, yet further work is needed to see if live-dead similarity reflects young death assemblages that postdate the onset of anthropogenic eutrophication.