Nutrient Management Strategy

Although nutrients are required to sustain life, too many nutrients in surface waters can lead to less than desirable consequences. Excess nutrients (such as nitrogen or phosphorus) are a nationwide concern in surface waters as they may promote algal growth, result in contamination of drinking waters supply, or even be toxic to aquatic life. Further, excess nutrients draining to surface waters play a role in the development of seasonal low dissolved oxygen levels, potentially causing areas of hypoixa or summer hypoxic zones such as found in the northern Gulf of America.

Hypoxia (or low dissolved oxygen) is when oxygen levels fall below the level necessary to sustain most aquatic life. Waters that have dissolved oxygen concentrations less than 2 parts per million (ppm) for extended periods of time are generally considered hypoxic. Hypoxia can be a problem for estuaries, coastal waters, and some inland waters and can be caused by a variety of factors, including excess nutrients (primarily nitrogen and phosphorus),water body stratification due to salinity or temperature gradients in still waters, and low flows in streams and bayous. These excess nutrients (or eutrophication) can promote algal growth and as the algae die, they sink to the bottom, and decompose; this process of algal growth and decomposition consumes oxygen, resulting in low dissolved oxygen levels in the water.

The direct effects of extended hypoxia include fish kills, which not only deplete valuable fish stocks and damage the ecosystem, but are unpleasant for local residents and can harm local tourism. Oxygenated water is necessary for aquatic animals to breathe. Mobile animals, such as adult fish, can often survive extended times of hypoxia by moving into oxygenated waters. When they cannot move, such as when young fish need to spend time in the habitat that has become hypoxic, the result is a fish kill. Non-mobile animals, such as clams, cannot move into healthier waters and are often killed by hypoxic episodes. This causes a severe reduction of the amount or in extreme cases the complete loss, of animal life in hypoxic zones.

Hypoxia is a complex ecological phenomenon that arises from the convergence of several factors, some of which are altered by human activities. The shape of the water body and the strength and direction of water flow determine how well-mixed (and aerated) the water is. Wind strength and direction and the influence of major storms also affect the rate of mixing and aeration. Human activities, such as changing the amount of freshwater entering an estuary or altering flows of rivers and streams, can reduce the amount of mixing and aeration. Poorly-mixed water increases the chance hypoxia will develop.

Nutrients (primarily nitrogen and phosphorus, but sometimes also including silica, iron, zinc, and magnesium) are necessary for plant growth. An overabundance of these nutrients, combined with enough light and warm, slow-moving, or poorly-mixed water, can result in an algae bloom. This is the first step in the chain of reactions that can lead to hypoxia. These overabundant nutrients may come from point sources such as waste water treatment plant discharges and nonpoint sources such as agricultural activities, urban runoff, groundwater, and atmospheric deposition. The amount of pollutants generated and the rate at which they reach rivers, estuaries and coastal waters are increased by human activities such as the destruction of wetlands, grasslands, and forests in favor of urban, suburban or agricultural landscapes.

Source: World Research Institute

The term "point source" means any discernible, confined and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, concentrated animal feeding operation, or vessel or other floating craft, from which pollutants are or may be discharged. This term does not include agricultural storm water discharges and return flows from irrigated agriculture. Nonpoint source (NPS) pollution, unlike pollution from industrial and sewage treatment plants, comes from many diffuse sources. NPS pollution is caused by rainfall or snowmelt moving over and through the ground. As the runoff moves, it picks up and carries away natural and human-made pollutants, finally depositing them into our local watersheds including lakes, rivers, wetlands, coastal waters, and even our underground sources of drinking water. These pollutants include: excess fertilizers, herbicides, and insecticides from agricultural lands and residential areas; oil, grease, and toxic chemicals from urban runoff and energy production; sediment from improperly managed construction sites, crop and forest lands, and eroding stream banks; salt from irrigation practices and acid drainage from abandoned mines; bacteria and nutrients from livestock, pet wastes, and faulty septic systems; atmospheric deposition and hydro modification are also sources of nonpoint source pollution source.

 

Source: Mississippi River Basin Watershed Nutrient Task Force, 2010

A watershed is an area that drains or contributes water to a particular point, stream, river, lake, or ocean. Larger watersheds are also referred to as basins. Watersheds range in size from a few acres for a small stream to large areas of the country such as the Mississippi River Basin. In Louisiana, water quality monitoring by the Department of Environmental Quality (DEQ) recognizes 12 River Basins, but within those basins, DEQ has delineated over 485 smaller watersheds, called subsegments, that are also recognized and monitored, as illustrated by the Plaquemine-Brule Watershed.

Sources: Louisiana River Basins Map and Bayou Plaquemine-Brule Watershed Map

 

The Mississippi-Atchafalaya River Basin (MARB) drains ~41% of the continental United States, bringing with it increased loads of nutrients to Louisiana state and federal waters in the Gulf of America.

The US Geological Survey (USGS) has developed and consistently updated SPARROW models to estimate the amount nitrogen and phosphorus released to the Gulf.  Two major USGS releases have occurred based on data sourced:

2002 (published 2013):  Sixty-six percent (66%) of nitrogen in the basin was found to originate from cultivated crops, mostly corn and soybean; animal grazing and manure contributed about 5 percent.  Additionally, atmospheric contributions were important, accounting for 16% of nitrogen.  Phosphorus levels were found to be led by animal manure—both on pasture/rangeland and for crop fertilizer—accounting for 37% and 43%  of total loads, respectively.  Findings suggested that phosphorus associated with the wastes of unconfined animals may have been a larger source than previously recognized.  In total, agricultural sources contributed more than 70 percent of the nitrogen and phosphorus delivered to the Gulf, versus only 9 to 12% from urban sources.

2012 (published 2019):  Agriculture activities remained the most important contributors to nitrogen and phosphorus loadings from the MARB.

Both releases indicate that the major source of nitrogen and phosphorus originates from Corn Belt states, with Louisiana contributing ≤ 2.5% of the total nutrient contribution to the Gulf (2019).  SPARROW mapping applications are available online to assist with understanding loadings.

Sources: Differences in Phosphorus and Nitrogen Delivery to the Gulf of Mexico from the Mississippi River Basin 2014

Sparrow Nutrient Modeling: Mississippi/Atchafalaya River Basin (MARB) 2019

Sparrow Mapping Application 2021

The Mississippi River Basin carries heavy loads of nutrients to the Gulf of America, routinely resulting in seasonal hypoxic zones. This area of bottom-water hypoxia in the Gulf has been monitored and mapped in summer from 1985 to present, except for 1989 and 2016. A grid of stations was laid out in 1985, transects (lines of stations) were added over the years, and transects expanded to the west as the size of the hypoxic zone increased. The main purpose of the summer monitoring cruise is to map the extent of hypoxia and the physical, chemical and biological parameters associated with it and to link the size with conditions in the Mississippi River watershed. The monitoring and mapping generally begins at Southwest Pass of the Mississippi River (except in 2010) and progresses westward. Data and field measurements are collected for different depths including depth, temperature, salinity, dissolved oxygen, turbidity, percent photosynthetically available radiation and often phytoplankton biomass. Each line of stations is transited so that the stations continue in an offshore or inshore (within the draft of the ship) direction until the area of hypoxia is exited. Some transects are extended past the offshore limits of hypoxia for comparison with prior years. If there is insufficient time to map the entire area, the estimated size may be less than actual size. There has been a consistent mapping protocol over a similar grid since 1985. Instrumentation is calibrated before, during and after the cruise to ensure quality data. The annual summer hypoxia cruise represents one of the longest ecological data sets on record.

Responsibility for the survey fully transitioned from the Louisiana University Marine Consortium to the National Oceanic Atmospheric Administration (NOAA) National Centers for Coastal Ocean Science (NCCOS) in ~2016.  Ship problems forced cancelation of that cruise, but resumed in 2017 with NCCOS sponsorship.  These continued efforts are in support of the federal Harmful Algal Bloom (HAB) and Hypoxia Research and Control Amendments Act (HABHRCA), initiated in 2014, to advance scientific efforts concerning HABs and hypoxia events in marine and freshwaters of the U.S.  Reauthorization of this legislation is pending in 2026 (passed Senate in 2025).

Sources: Gulf Hypoxia, Gulf Hypoxia Mapping, HABHRCA and NCCOS Gulf Hypoxia Monitoring

The Louisiana Nutrient Management Strategy was released in 2014 after extensive outreach and development efforts. According to provisions laid out within the initial strategy document, 5-year updates and annual reporting have occurred since. The most recent Louisiana Nutrient Reduction and Management Strategy was developed 2024, and released in early 2025. Five state agencies, the Governor’s Office of Coastal Activities, and multiple stakeholder partners (including conservation districts and nonprofits) work towards reducing nutrients delivered to state and federal waters throughout Louisiana. The primary website for the Strategy is hosted by LDEQ and includes all relevant information concerning nutrient activities within the state.

Source: LA Nutrient Reduction and Management Strategy