Gulf of Mexico and Louisiana Inland Hypoxia

Brian LeBlanc

Hypoxia is a common environmental condition found in bodies of water. The term hypoxia is used to describe low dissolved oxygen concentrations that have the potential to interfere with normal ecosystem functions. An example often cited is oxygen concentrations in near-bottom waters that can negatively affect aquatic animal survival or reproduction. Hypoxia can be caused by a combination of water quality conditions that include excessive amounts of fertilizer in runoff, high water temperatures and excessive amount of salt. These and other factors combined can create extremely low levels of dissolved oxygen that can reduce or alter fish and aquatic wildlife populations.

Hypoxia in the Gulf of Mexico is one of the most publicized chronic environmental conditions in the United States. First noted in the 1970s and documented and measured since the mid-1980s, the size of the zone varies yearly depending on several factors, including the amount of rainfall in the Mississippi River watersheds and tropical storms in the Gulf. Scientists have documented variations in zone size from year to year, but it averages around 5,000 square miles per year.

Overenrichment of nutrients from runoff, including phosphorus and nitrogen, into the warm waters of the Gulf has a significant impact on the hypoxia zone, and production agriculture and people from all walks of life can play a part in reducing the potential for nutrient runoff. Nitrogen and phosphorus are nutrients essential to agricultural production and all life on earth. Although these elements are important and always present, excess or unused amounts, which run off the landscape via rainwater into streams, lakes and rivers, can promote the overgrowth of algae. Too much growth of algae often leads to an algae “crash,” or die-off, and as these micro plants begin to decompose, the process of decomposition uses the water’s dissolved oxygen and can lead to hypoxic conditions that rob fish and other aquatic organisms of needed oxygen. Excess nutrients come from a variety of watershed sources including animal and crop production, human waste water, natural atmospheric deposition, and suburban and urban activities like overfertilization of lawns and golf courses, to name a few.

Some research has indicated the Upper Mississippi River drainage basin, which includes the Missouri, Ohio and Upper Mississippi rivers and associated watersheds, delivers about 80 percent of the total nitrogen and about 74 percent of the total phosphorus load to the Gulf. The Lower Mississippi River drainage area, which includes the Arkansas, White, Red, Yazoo and Atchafalaya river watersheds, delivers only about a quarter of the nitrogen and phosphorus load. The hydrologically modified Mississippi River in Louisiana, which has been disconnected from its historic floodplain by levees, limits some nutrient contributions from Louisiana watersheds from being discharged directly into the Gulf of Mexico via the Mississippi River. According to the 2014 Louisiana Nutrient Management Strategy, and based on U.S. Geological Survey watershed modeling, Louisiana contributes only 1.7 percent and 2.4 percent, respectively, of the total nitrogen and phosphorus loads reaching the Gulf.

Although low by comparison to the upper and middle drainage basins of the Mississippi River floodplain, Louisiana’s contribution is not insignificant to the Gulf hypoxic zone, and further, hypoxia conditions are not limited to the Gulf of Mexico. Equally important to Louisiana is the contribution of excess nutrients from all sources including inland lakes, rivers, bayous and estuaries, which are often affected by land use practices surrounding them. According to a 2016 report from the Louisiana Department of Environmental Quality, Louisiana has 274 internal and coastal bodies of water listed as impaired for fish and wildlife propagation due to low dissolved oxygen or high concentrations of nutrients. A number of these impairments are suspected of being from natural sources, but a significant number are also attributed to previously described watershed sources such as agriculture, urban/suburban runoff, silviculture, and human sewage effluent.

The LSU AgCenter has long recognized both the economic and environmental benefit of reducing excessive, unnecessary and improper use of nutrients. Eleven commodity-specific BMP manuals are available online. These manuals are used in the Louisiana Master Farmer Program and other programs:

Sugarcane Best Management Practices

Environmental Best Management Practices for Sweet Potato Cultivation

Dairy Production Best Management Practices

Crawfish Environmental Best Management Practices

Aquaculture Environmental Best Management Practices

Poultry Production Best Management Practices

Swine Production Best Management Practices

Beef Cattle Production Best Management Practices

Protecting Louisiana’s Waters Using Best Management Practices

Agronomic Crops (Soybeans, Cotton, Wheat, Corn and Feed Grains) Best Management Practices

Rice Production Best Management Practices

The AgCenter and Louisiana Sea Grant at LSU are collaborating with Sea and Land Grant Universities in the upper Midwest and the National Weather Service to develop Runoff Risk Decision Support (RRDS) tools for Louisiana and other states in the south and central Mississippi River Drainage Basin. RRDS are state-led and will be specific for individual states and regions. More than just forecasting weather, these tools incorporate National Weather Service modeling, past and predicted weather patterns, humidity, temperature, soil conditions and type, topography, and several other factors to provide guidance to producers on application timing decisions and reduce the risk of valuable applied nutrients being lost to runoff. The hope is RRDS will eventually encourage voluntary behavioral change as farmers incorporate nutrient application timing concerns into their short-term planning.

Brian LeBlanc is Roy and Karen Pickren Professor in the School of Plant, Environmental and Soil Sciences and an agent with Louisiana Sea Grant at LSU.

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

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The Gulf of Mexico receives drainage from six large watersheds and comprises 41 percent of the drainage for the 48 contiguous states of the U.S., making it the third largest watershed drainage system in the world. Reproduced from the Louisiana Universities Marine Consortium website at

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Drainage of high nutrients loads into the Gulf of Mexico is similar to overfertilization and can promote overgrowth of algae, which eventually die. The resulting bacterial decomposition uses large amounts of oxygen and can cause marine life to be displaced or die. Reproduced from the Louisiana Universities Marine Consortium website at

1/10/2018 5:13:12 PM
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