Long-term Nutrient Change in the Mermentau and Vermilion Rivers

Linda Benedict, Xu, Yi Jun  |  9/16/2015 10:31:04 PM

Songjie He and Y. Jun Xu

The Mermentau and Vermilion river basins are among the most intensive agriculture areas in Louisiana. Previous studies conducted in the region by LSU AgCenter researchers have shown that crop, dairy and aquaculture production can cause higher nitrogen and phosphorus release into the bayous and streams in river basins. To reduce nitrogen and phosphorus inputs to aquatic ecosystems, best management practices (BMPs) have been developed for agriculture in the state. BMPs include a set of guidelines and recommendations, such as nutrient management, pesticide management, irrigation management, grazing management, and erosion and sediment control.

In the past few decades, public education and technical assistance for agricultural BMPs have been provided by a group of agencies including the LSU AgCenter, the U.S. Department of Agriculture’s Natural Resource Conservation Service, the Louisiana Department of Agriculture and Forestry, the Louisiana Department of Environmental Quality and the USDA Farm Service Agency. Although BMPs are widely used today, it is not clear how effective they actually are in protecting or improving water quality in intensive-agriculture river basins.

To determine the effectiveness of BMPs in Louisiana, AgCenter scientists completed an assessment of changes in nitrogen and phosphorus in the Mermentau and Vermilion rivers from 1980 to 2010. Researchers collected daily discharge data from the U.S. Geological Survey and monthly water quality data, including total nitrogen and total phosphorus concentrations, from the Louisiana Department of Environmental Quality to quantify changes in nutrient levels. The selected river gauge stations and water quality sampling locations on the Mermentau and Vermilion rivers were close to the river mouths to represent the realistic nutrient loads from the two river basins.

The assessment of the long-term total nitrogen and total phosphorus was done for three decades – the 1980s, 1990s and 2000s. Overall, researchers found a steady decline of nitrogen and phosphorus concentrations in both rivers, mainly in the 1990s. The Vermilion River showed a higher concentration of both total nitrogen (1.83 milligrams per liter) and total phosphorus (0.41 milligrams per liter) than the Mermentau River (1.57 milligrams per liter of total nitrogen and 0.27 milligrams per liter of total phosphorus). Annually, on average, the Vermilion River delivered a total of 1,829 metric tons of nitrogen, while the Mermentau River delivered a total of 3,925 metric tons of nitrogen because of its higher discharge. Similar to the concentration trend, the rivers showed a decline in total nitrogen and total phosphorus loads to the Gulf of Mexico.

Although there is no report on actual implementation of BMPs in various land use practices in these two river basins, the findings from the long-term assessment on riverine nutrient change indicate their likely positive effect. A study in 2011 showed varying adoption of BMPs in the crawfish industry in southwestern Louisiana, from 3 percent for streambank and shoreline protection to 79 percent for irrigation water management. The Mermentau and Vermilion river basins drain much of this area, and it is likely that the BMPs for irrigation management and nutrient management may have made the greatest contribution to the nutrient reduction because of intensive rice cultivation. Despite this improvement, when compared with other coastal rivers in Louisiana, the two river basins still show much higher nutrient discharge rates (Table 1). Further studies of nitrogen and phosphorus in the Mermentau and Vermilion rivers are underway, with the aim of having a better understanding of the current nutrient conditions of these two rivers and helping to improve agricultural BMP effectiveness.

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

The summer 2015 issue of Louisiana Agriculture magazine includes articles on a variety of topics that affect Louisiana’s agriculture industry and the environment – water management at Catahoula Lake, 4-H youth wetland programs, artificial reefs for water conservation, corn nitrogen management in saturated soil conditions, and more. 36 pages

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