Songjie He and Y. Jun Xu

The near-shore coastal waters of Louisiana in the northern Gulf of Mexico have experienced severe oxygen depletion each summer at least since the mid-1980s. While information gaps remain regarding the specific factors contributing to this hypoxia, the evidence suggests enrichment by nitrogen and phosphorus found in the Mississippi-Atchafalaya River System contribute to the hypoxic zone.

As the largest river system in North America, the Mississippi-Atchafalaya River System has been investigated intensively. The flow of nutrients from that system into the Gulf have been estimated for the past several decades. Little is known, however, about the flow of nitrogen and phosphorus from the major coastal rivers of southwest Louisiana that discharge a considerable amount of freshwater and sediment into the Gulf.

In 2015, AgCenter researchers estimated that four coastal rivers in southwest Louisiana — the Sabine, Calcasieu, Mermentau and Vermilion — each year discharge into the Gulf: nitrate nitrogen and nitrite nitrogen (1,783 tons); ammonia, ammonium and organic nitrogen (12,404 tons); and phosphorus (1,862 tons), with a peak input of nitrogen during the spring. However, these estimates are based on discharge and nutrient concentrations at sites that are many miles upstream of the river mouths. The time nitrogen may be present in any given pool can range from a few days to several hundred years. The global movement of phosphorus from rivers to the sea is about 21 million tons per year with approximately 10 percent of this being available to marine life, while the remaining 90 percent is deposited in sediment particles. It is not clear how much of the nitrogen or phosphorus loads from these four rivers are delivered into the Gulf of Mexico and ultimately available to marine life. To have a better understanding of this situation, it is important to study the chemical composition and space-time distribution of nitrogen and phosphorus within a salinity layer.

LSU AgCenter scientists evaluated mixing dynamics and time variations of nitrogen and phosphorus in the Calcasieu River with salinity ranging from 0.02 to 29.50 parts per thousand. The Calcasieu River flows 200 miles through a low-lying Chenier Plain in southwest Louisiana before entering the Gulf of Mexico. This study focused on the river’s last 55-mile reach (Figure 1) that has a clear salinity gradient and tidal water influence. From April 2014 to February 2016, monthly water samples were collected from six locations along the 55 miles and were analysed for nitrate/nitrite, organic nitrogen, ammonia nitrogen, phosphate, total phosphorus and dissolved carbon. Measurements were conducted at each sampling location to record ambient water conditions.

Phosphate concentrations consistently showed a positive deviation from those expected by a conservative mixing model. A similar trend of dissolved organic carbon with salinity was also found, suggesting a possible presence of photosynthesis by plant sources. However, all nitrogen levels along the river were inconsistent. Most nitrogen discharged to the estuary was organic, while a significant portion of phosphorus discharged to the estuary was inorganic. The phosphate-to-total-phosphorus ratios varied from 0.06 to 0.93, with an average of 0.32, and the nitrate/nitrite nitrogen-to-total-nitrogen ratios ranged from near zero to 0.52, with an average of 0.18, suggesting a large variation in chemical composition. Annually, the river discharged a total of 2,174 tons of nitrogen and 272 tons of phosphorus into the northern Gulf of Mexico. Although the nutrient flows are comparably marginal to those from the nearby Mississippi-Atchafalaya River System, the coastal rivers may have strong seasonal effects on near-shore water quality and nutrient availability.

Songjie He is a graduate student, and Y. Jun Xu is a professor in the School of Renewable Natural Resources.

(This article appears in the fall 2017 issue of Louisiana Agriculture.)

Songjie He takes measurements in the Calcasieu River. Photo by Zhen Xu

Figure 1. Geographic location of the Calcasieu River entering the Gulf of Mexico and the locations of six sampling sites and U.S. Geological Survey discharge and tidal gaging sites.