Three Decadal Inputs of Nitrogen and Phosphorus from Four Major Coastal Rivers to the Summer Hypoxic Zone of the Northern Gulf of Mexico

Nutrient enrichment is considered one of the most important causes for summer hypoxic conditions in the northern Gulf of Mexico (NGOM) off the Louisiana coast. While many studies on nutrient inputs from the large Mississippi-Atchafalaya River System have been conducted, little is known about nutrient inputs from other coastal rivers in Louisiana. In this study, we utilized long-term (1980–2009) records on river discharge and nutrient concentrations of four major Louisiana coastal rivers—the Sabine, Calcasieu, Mermentau, and Vermilion—to estimate daily, monthly, and annual inflows of nitrate and nitrite nitrogen (NO₃ + NO₂), total Kjeldahl nitrogen (TKN), and total phosphorus (TP) into the NGOM. The three-decade-long nutrient inflows from these rivers were analyzed for their seasonal fluctuations, interannual variabilities, and decadal trends. Fluxes of NO₃ + NO₂, TKN, and TP for these river basins were estimated to assess land use effects on riverine nutrients. Our study found that the four coastal rivers discharged each year a considerably large amount of NO₃ + NO₂ (total of 1755 t), TKN (12,208 t), and TP (1833 t) into the NGOM, with a peak input of nitrogen during the spring. The Mermentau and Vermilion Rivers, which drain intensive agriculture areas, had significantly higher NO₃ + NO₂, TKN, and TP concentrations when compared with the Sabine and Calcasieu Rivers, which drain forest-pasture-dominated lands. The fluxes of NO₃ + NO₂, TKN, and TP from the Mermentau River Basin (156 kg km⁻² year⁻¹ NO₃ + NO₂, 942 kg km⁻² year⁻¹ TKN, and 206 kg km⁻² year⁻¹ TP) and the Vermilion River Basin (374, 1078, and 360) were much higher than those combined from the Sabine and Calcasieu River Basins (66, 710, and 62). These findings fill a major knowledge gap concerning the quantity and characteristics of nitrogen and phosphorus transport from coastal watersheds to North America’s largest hypoxic zone.