Differences in Phosphorus and Nitrogen Delivery to The Gulf of Mexico from the Mississippi River Basin

TLDR

Seasonal hypoxia in the northern Gulf of Mexico has been linked to increased nitrogen fluxes from the Mississippi and Atchafalaya River Basins, though recent evidence shows that phosphorus also influences productivity in the Gulf. The study developed a spatially explicit SPARROW water‑quality model to reveal differences in the sources and transport processes that control nitrogen and phosphorus delivery to the Gulf. The model is structurally detailed and captures spatially explicit transport processes of nutrients from the Mississippi River Basin to the Gulf. Model simulations show that agricultural activities supply over 70 % of Gulf‑delivered nitrogen and phosphorus, with corn and soybean accounting for 52 % of nitrogen and 25 % of phosphorus, while animal manure on pasture and rangeland contributes 37 % of phosphorus; the proportion of nutrients reaching the Gulf rises with stream size, reservoir trapping of phosphorus creates regional delivery disparities, and effective management must address distinct N and P sources, atmospheric nitrogen, downstream phosphorus, and control near large rivers.

Abstract

Seasonal hypoxia in the northern Gulf of Mexico has been linked to increased nitrogen fluxes from the Mississippi and Atchafalaya River Basins, though recent evidence shows that phosphorus also influences productivity in the Gulf. We developed a spatially explicit and structurally detailed SPARROW water-quality model that reveals important differences in the sources and transport processes that control nitrogen (N) and phosphorus (P) delivery to the Gulf. Our model simulations indicate that agricultural sources in the watersheds contribute more than 70% of the delivered N and P. However, corn and soybean cultivation is the largest contributor of N (52%), followed by atmospheric deposition sources (16%); whereas P originates primarily from animal manure on pasture and rangelands (37%), followed by corn and soybeans (25%), other crops (18%), and urban sources (12%). The fraction of in-stream P and N load delivered to the Gulf increases with stream size, but reservoir trapping of P causes large local- and regional-scale differences in delivery. Our results indicate the diversity of management approaches required to achieve efficient control of nutrient loads to the Gulf. These include recognition of important differences in the agricultural sources of N and P, the role of atmospheric N, attention to P sources downstream from reservoirs, and better control of both N and P in close proximity to large rivers.