LIVE-DEAD ASSESSMENT OF THE IMPACTS OF ANTHROPOGENIC EUTROPHICATION ON BIVALVE COMMUNITIES IN THE NORTHERN GULF OF MEXICO

Anthropogenic eutrophication in aquatic environments enhances primary production which can result in hypoxic conditions. Understanding how these combined biotic and abiotic factors affect marine communities is hindered by limited baseline data. Focusing on shelf benthos in the northern Gulf of Mexico, we used live-dead analysis to assess changes in bivalve communities in response to anthropogenic nutrient enrichment. Anthropogenic eutrophication is well documented in the northern Gulf, as are links between offshore hypoxia and industrial farming and runoff in the Mississippi River basin.

We compared the taxonomic composition, relative abundance, and percentage of “organic-loving” species (sensu Kidwell 2008) in living bivalve communities and associated historical death assemblages on the inner shelf offshore Cocodrie, Louisiana, and Dauphin Island, Alabama. These two locations are distributed along a nutrient gradient that extends from within the hypoxic zone on the Louisiana shelf to outside that zone seaward of the Mississippi-Alabama barrier islands; the Alabama shelf has historically experienced much lower rates of nutrient input relative to coastal Louisiana.

Live and dead assemblages at both locations exhibited high statistical agreement between species relative abundance, and mixed agreement with respect to taxonomic similarity. In Louisiana, the proportion of “organic-loving” species in both live and dead samples was ≥ 99%. In Alabama, the live sample was composed of 80% “organic-loving” species compared with 30% in the dead sample. Radiocarbon dates for a preliminary sample of dead Nuculana acuta from Louisiana were all post-bomb, indicating that our live and dead Louisiana samples postdate the onset of anthropogenic eutrophication. In contrast, the median age of dead N. acuta in Alabama was 590 years before present. The Alabama death assemblage predominantly records conditions that existed prior to the onset of anthropogenic eutrophication and the live-dead difference in the abundance of “organic-lovers” may reflect a community shift in response to anthropogenic eutrophication. Our results broadly agree with those of previous studies that have indicated anthropogenic eutrophication can lead to a decline in live-dead agreement between historical and current benthic communities.