Three Decadal Inputs of Nitrogen and Phosphorus from Four Major Coastal Rivers to the Summer Hypoxic Zone of the Northern Gulf of Mexico Songjie He & Y. Jun Xu Received: 19 May 2015 /Accepted: 11 August 2015 # Springer International Publishing Switzerland 2015 Abstract 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 conduct- ed, 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 con- centrations of four major Louisiana coastal rivers—the Sabine, Calcasieu, Mermentau, and Vermilion—to esti- mate daily, monthly, and annual inflows of nitrate and nitrite nitrogen (NO 3 + NO 2 ), 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 vari- abilities, and decadal trends. Fluxes of NO 3 + NO 2 , 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 NO 3 + NO 2 (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 NO 3 + NO 2 , TKN, and TP con- centrations when compared with the Sabine and Calcasieu Rivers, which drain forest-pasture-dominated lands. The fluxes of NO 3 + NO 2 , TKN, and TP from the Mermentau River Basin (156 kg km -2 year -1 NO 3 + NO 2 , 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. Keywords Eutrophication . Riverine nitrogen and phosphorus . Coastal water quality . Hypoxic zone . Southwest Louisiana . Gulf of Mexico 1 Introduction Eutrophication has become the primary water quality issue in the world. Eutrophication in estuarine and coastal waters of the USA often causes hypoxic and anoxic conditions and large-scale harmful algal blooms (Scavia and Bricker 2006), leading to fish kill and lost food processing income (O’Connor and Whitall 2007; Diaz and Rosenberg 2011). The near-shore coastal waters of Louisiana in the northern Gulf of Mexico (NGOM) have experienced severe oxygen depletion each summer since the mid-1980s. A recent study (Obenour et al. 2013) shows an increasing trend in hypoxic layer thickness in the NGOM. Researchers have attributed this hypoxic zone, a.k.a. “dead zone, ” primarily to the enrichment of nitrogen and phosphorus (Turner and Rabalais 1991, 1994) as high annual inputs of these nutri- ents have been found in the Mississippi-Atchafalaya River Water Air Soil Pollut (2015) 226:311 DOI 10.1007/s11270-015-2580-6 S. He : Y. J.Xu (*) School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA e-mail: yjxu@lsu.edu