Declining bivalve species and functional diversity along a coastal eutrophication-deoxygenation gradient in the northern Gulf of Mexico

Coastal eutrophication and hypoxia are growing challenges globally, yet their impacts can be difficult to evaluate because of limited biomonitoring that typically postdates the onset of these stressors. We address this limitation by investigating how the taxonomic and functional diversity of marine bivalve communities vary with primary productivity, dissolved oxygen, temperature, and seafloor sediment properties across the northern Gulf of Mexico, a region that includes one of the world's largest dead zones. We hypothesized that taxonomic and functional richness would decline in eutrophic and hypoxic coastal environments. Live bivalve mollusks were sampled at 15 stations, spanning more than 600 km of continental shelf habitat. Individuals were identified to species and characterized based on feeding, mobility, fixation, life position relative to the sediment-water interface, and body size. Alpha and beta taxonomic and functional diversity were computed using Hill numbers and linear models used to assess their covariation with regional environmental conditions. Taxonomic and functional diversity were highest in less eutrophic environments characterized by normoxic conditions, and lowest in more eutrophic environments where oxygen was more limited. Community-level differences were underlain by functional shifts, with abundant shallow-infaunal, deposit and mixed feeders in more eutrophic settings, in contrast with less eutrophic settings where suspension feeders were more abundant. Median body size increased with eutrophication, possibly as a result of hypoxia-induced declines in predator and competitor populations. These results suggest that intensifying nutrient loading and deoxygenation in the coastal zone will cause declines in multiple dimensions of benthic biodiversity with implications for ecosystem function.