Isotopic evidence for preferential transport of fertilizer nitrogen into the northern Gulf of Mexico during high water discharge

Anthropogenic nitrogen inputs from the Mississippi-Atchafalaya River Basin have caused substantial environmental challenges in the northern Gulf of Mexico, such as coastal eutrophication, harmful algal blooms, and seasonal hypoxia. Addressing these issues requires a better understanding of the complex sources of nitrogen, which include fertilizers, groundwater, manure, and sewage. In this study, we analyzed the nitrogen isotopic composition of dissolved nitrate and particulate nitrogen from the Wax Lake Delta, a major distributary of the Mississippi-Atchafalaya River Basin that flows into the Gulf of Mexico. Our findings revealed that during the wet season, δ15N values of both nitrate and particulate nitrogen were consistently 2-3‰ lower compared to the dry season. This suggests that fertilizer-derived nitrogen, which has lower δ15N, is predominantly exported to the Gulf of Mexico during periods of high water discharge. These findings imply that adjusting fertilizer application timing could help reduce nitrogen loading and mitigate its environmental impact on the Gulf of Mexico. Fertilizer-derived nitrogen with low isotopic nitrate and particular matter values is mainly exported to the Gulf of Mexico during wet season, according to an analysis combining hydrographic and nutrient data, and isotope samples.