MOLLUSCAN FUNCTIONAL RESPONSES TO ANTHROPOGENIC EUTROPHICATION IN COASTAL ALABAMA

Nutrient delivery to the northern Gulf of Mexico has increased markedly over recent centuries as a result of waste management and land-use practices. Nutrient enrichment increases rates of primary productivity, resulting in greater organic accumulation on the seafloor, which can lead to the development of hypoxic conditions. Whereas the response of marine benthos to severe hypoxia is increasingly well-documented, continental shelf environments that are eutrophic, yet experience only intermittent hypoxia, have been historically understudied. Consequently, our understanding of benthic response in nutrient-rich, oxygenated environments is limited. Using death assemblages of bivalve mollusks collected from surficial sediments at five stations along the -20 meters isobath, we reconstructed the functional composition of benthic communities in coastal Alabama. Bivalves were categorized into functional groups using information about their fixation, mobility, feeding type, and substrate preference. The composition and relative abundance of taxonomic and functional groups were compared between live and dead samples at each station. Results indicate a live-dead discordance in species abundance across sites and a shift in functional composition, specifically from historical communities in which epifaunal and infaunal suspension feeders were most abundant, to present-day communities characterized predominantly by surficial deposit and mixed deposit-suspension feeders; chemosymbiotic feeders were similar in their presence in both living and death assemblages. The increasing abundance of organic-tolerant mollusks (i.e., Abra, Nuculana, Ameritella) signals a biotic shift in response to anthropogenic eutrophication in the northern Gulf of Mexico, including in those settings in which hypoxia is uncommon. Furthermore, our results indicate that suspension-feeding mollusks may be particularly sensitive to these environmental changes. Ongoing isotopic analyses will help distinguish the driver of observed functional live-dead discordance, thereby providing insight into which groups are most vulnerable to eutrophic conditions, and can potentially determine baseline conditions in which mitigation and restoration targets can be applied.