USING NITRATE ISOTOPES TO TRACK ANTHROPOGENIC NITROGEN IN THE NORTHERN GULF OF MEXICO

The Gulf of Mexico (GoM) is significantly impacted by anthropogenic nitrogen (N), with estimated total N inputs ranging from 1.1 to 1.3 x 10¹⁴ mmol N yr^-1 originating from the Mississippi-Atchafalaya River Basin (MARB). This excess N can cause detrimental environmental impacts such as eutrophication, coastal hypoxia, and subsurface ocean acidification. Despite the severe ecological consequences, the fate of this anthropogenic N remains poorly understood. To address this knowledge gap, we conducted isotopic analyses of NO₃^- in seawater collected from the northern GoM between 2021 and 2023. The surface NO₃^- concentrations are mostly zero at all sites, ranging from less than 25 km to over 200 km offshore, indicating the efficient conversion of anthropogenic N into organic nitrogen by phytoplankton. The nitrate δ¹⁵N depth profiles reveal a minimum between 150-200 m depth, similar to that of the open GoM. This similarity suggests that anthropogenic N from the MARB probably had a limited impact on the subsurface NO₃^-in the northern GoM during the study period, implying effective removal of anthropogenic N in nearshore regions through organic N burial and sedimentary denitrification/anammox.