NUCULANA BODY SIZE CORRELATES WITH SPATIOTEMPORAL CHANGES IN PRIMARY PRODUCTIVITY IN THE NORTHERN GULF OF MEXICO

Human activities have increased the flow of nutrients into coastal ecosystems, which has led to enhanced primary productivity and the development and expansion of oxygen-limited dead zones. How has variation in food and oxygen availability affected the ecology and physiology of benthic taxa? Here we investigate how the body sizes of marine bivalves vary in response to spatial and temporal changes in primary productivity across the northern Gulf of Mexico. The Mississippi River watershed drains 40% of the continental United States and the tremendous primary productivity in the adjacent northcentral Gulf of Mexico has led to one of the most extensive dead zones on Earth. We focus on the genus, Nuculana, an infaunal deposit feeder that is abundant in benthic communities throughout the northern Gulf. Live and dead Nuculana were picked from samples of seafloor sediment collected at -20 meter depth at fifteen sites spanning coastal Louisiana, Alabama, and Florida. The body size of each specimen was measured, and these sizes were compared among localities and over time. Data on present-day primary productivity and dissolved oxygen were compiled from the Bio-ORACLE database. The median body size of live and dead Nuculana varies across the Gulf, with larger individuals in Louisiana and smaller individuals in north Florida. Quantile regression results indicate that both live and dead body size significantly correlate with longitude, chlorophyll A, and dissolved oxygen. In each region, live specimens were also significantly larger than dead specimens, and these live-dead differences were robust to potential methodological biases. The greater size of Nuculana in more productive coastal environments could be due to greater food availability and/or oxygen limitation; enhanced food supply could lead to elevated growth rates, whereas lower dissolved oxygen levels could decrease predation pressure and result in greater longevity of prey. Larger basal consumers could potentially support larger higher order consumers, such as snails, crabs and fish, yet hypoxia may also limit the extent of these predator populations. Whether the environmental correlates of body size observed in Nuculana are generalizable to other basal consumers in the northern Gulf of Mexico is the subject of ongoing work.