RECONSTRUCTING HISTORIES OF BENTHIC TURNOVER AND IDENTIFYING MITIGATION BENCHMARKS IN COASTAL LOUISIANA

One of the world’s largest oxygen-limited dead zones develops annually in the Gulf of Mexico, adjacent to the Mississippi River Delta. More than three decades of scientific surveys have documented variation in the dead zone’s extent. Longer-term sedimentary records have revealed correlated histories of nutrient loading and primary production that have resulted from changing land use practices in the Mississippi River watershed over the past 200 years. Benthic communities have changed in recent decades as conditions have gone from less to more hypoxic, however what continental shelf benthos were like before the onset of anthropogenic eutrophication is unknown. We used death assemblages of bivalve mollusks collected from surficial sediments at five stations along the -20 meters isobath to reconstruct historical conditions in coastal Louisiana. Live-dead comparisons revealed a functional shift from older communities dominated by suspension feeders, to present-day communities in which deposit feeders and mixed deposit/suspension feeders are most abundant. Radiocarbon analyses for two bivalve genera - a mixed feeder (Nuculana) that is abundant today and a suspension feeder (Lirophora) that has not been live-collected in the past three field seasons - reveal different temporal patterns; the Nuculana age frequency distribution (AFD) was characterized by a “L-shape,” with the modal value defined by Recent shell inputs, whereas the Lirophora AFD included an internal mode that predated the bomb period. Lirophora’s internal mode could reflect changes in the biological production of suspension feeders in response to anthropogenic eutrophication and/or changes in coastal hydrology following the cessation of freshwater input from the Lafourche subdelta of the Mississippi River approximately 300 years before present. Ongoing calibration of bomb period specimens in conjunction with analyses of d15N in dated Lirophora shells will help distinguish between these two potential drivers of live-dead discordance and thereby the extent to which these death assemblages can provide relevant benchmarks for assessing the efficacy of nutrient mitigation efforts in the Mississippi River watershed and adjacent northern Gulf.