SHIFTS IN NITROGEN BIOGEOCHEMISTRY AND BENTHIC BIODIVERSITY OVER THE LAST TWO MILLENNIA IN THE GULF OF MEXICO

Hypoxic waters develop annually in the northern Gulf of Mexico west of the Mississippi River delta and extend over a region of the continental shelf roughly ten times the size of present-day Lake Pontchartrain. Monitoring of this dead zone began in the 1970s after the onset of eutrophic and hypoxic conditions. Consequently, the relative roles of physical (e.g., thermal stratification) and biogeochemical processes (e.g., oxidation of algal blooms) in determining dead zone extent remain unclear. Understanding these interacting drivers is critical for effectively managing land use in adjacent watersheds (e.g., fertilizer use), the health of coastal resources (e.g., fisheries), and anticipating biotic response to projected 21^st century increases in nutrients and decreases in dissolved oxygen. Here we investigate geographic gradients in nitrogen biogeochemistry in the northern gulf over past millennia through analysis of the isotopic composition of shell-bound organic matter in radiocarbon-dated mollusk shells. We collected ~180 shells of the suspension-feeding marine bivalve Lirophora obliterata from nine stations along the -20 m isobath offshore Louisiana, Alabama, and Florida. Using a bomb-period radiocarbon curve that we generated using recent and historical live-collected specimens from the northern gulf, we calibrated the post-mortem ages of L. obliterata shells at a subdecadal resolution. δ¹⁵N values in Louisiana, Alabama, and Florida increased up to 10‰ since 1800 CE, coincident with the expansion of human populations, agricultural and urban development, and the associated growth of hypoxic waters adjacent to the Mississippi River. Finer scale variation in δ¹⁵N may reflect times of more severe hypoxia which resulted in incomplete denitrification (high values), and times of reduced fluvial input of nitrate (low values). Over the past 200 years, relatively greater nitrogen increases proximal to the Mississippi River led to a steepening of the coastal nitrogen gradient that existed before the Industrial Revolution. Spatiotemporal variation in nitrogen biogeochemistry across the gulf is associated with pronounced live-dead differences in the taxonomic and functional diversity of molluscan communities and in species life histories.