LIFE HISTORIES IN CHANGING SEAS: LIVE-DEAD MISMATCH REVEALS BIOTIC RESPONSE TO RECENT ENVIRONMENTAL CHANGE

Population growth and land use practices have resulted in excessive nutrient enrichment in many coastal environments. Climate change is expected to increase anthropogenic eutrophication due to changes in precipitation patterns. Our understanding of how marine organisms respond to these compound environmental factors is limited because biomonitoring often postdates the onset of anthropogenic eutrophication. Using live-dead samples of marine mollusks collected along a eutrophication gradient, I investigate life history response to recent environmental changes across the northern Gulf of Mexico. Enhanced primary production is expected to give individuals that produce many small eggs a fitness advantage over those that invest equivalent energy in fewer, larger eggs, leading to a reduction in egg size over time. Hypoxia associated with eutrophication can also cause predatory release which may lead to a reduction in egg size. Temperature also directly affects egg size, with smaller eggs expected under warmer conditions. The earliest bivalve larval shell (PI) is correlated with egg size and preserved through ontogeny, providing an opportunity to investigate the environmental drivers of life history variation over past decades and millennia. Comparison of live samples of the bivalve Nuculana acuta reveal statistically significant differences in mean PI size; populations in more nutrient-rich environments offshore Louisiana tend to have smaller PIs than those offshore Alabama. Geographic clines in PI size in associated death assemblages show a similar pattern, identifying historical geographic differences in productivity that predate the onset of anthropogenic eutrophication. Whereas comparison of death assemblages can provide a pre-anthropogenic baseline, geographic variation in live-dead agreement may serve as an indicator of recent life history shifts. Live-dead comparisons reveal statistically significant differences in PI size over time, however preliminary data are equivocal over whether live-dead mismatch is greater in regions where anthropogenic eutrophication is more pronounced. Live-dead comparisons along present-day environmental gradients can be used to address central questions in marine evolutionary ecology and predict ecological responses to modern climate change.