THE FATE OF ANTHROPOGENIC NITRATE IN ORGANIC-RICH COASTAL SEDIMENTS

A modeling study by Van Meter et al (2018) “Legacy nitrogen may prevent achievement of water quality goals in the Gulf of Mexico” has demonstrated that the legacy of the accumulated nitrogen in the environment might have been underestimated in the current projections for improving water quality in coastal areas in the northern Gulf of Mexico. This alarming statement urged for a general revision of the nitrogen budget (anthropogenic and natural) and re-examining the mechanisms of pathways regulating the nitrogen transformations in coastal environments. In this study, we investigated a case of anthropogenic contaminations from agriculture in coastal areas in Alabama. We began this study motivated to evaluate the contribution of submarine groundwater discharge (SGD) to annual anoxic events, locally called Jubilees. Ultimately, we found that an organic-rich coastal sediment deposition, which is a result of sea-level oscillations in the Gulf of Mexico in the last 7-10k years, is the primary factor affecting the pathways and composition of the fluxes of nutrients delivered to the estuary via SGD. Multiple lines of evidence, including N- mass balance, DOM characterization, microbial community assessments, and stable isotopes strongly suggest that SGD- derived nitrogen fluxes outcompete anthropogenic fluxes contributing to regional coastal anoxia formation. Similar hydrogeological settings are widespread across continental margins and should be considered along with SGD in the nutrient budgets of coastal areas.