Mississippi River Case Study

Dead Zone in the Gulf of Mexico

Agricultural runoff can contribute pollutants to natural waters, such as rivers, lakes, and the ocean, that can have serious ecological and economic impacts, such as the creation of areas with low levels of dissolved oxygen called dead zones caused by pollution from fertilizers. Nutrients, such as nitrogen and phosphorus, are elements that are essential for plant growth and are applied on farmland as fertilizers to increase the productivity of agricultural crops. The runoff of nutrients (nitrogen and phosphorus) from fertilizers and manure applied to farmland contributes to the development of hypoxic zones or dead zones in the receiving waters through the process of eutrophication (Figure 4.2.7).

Text description of the Dead Zone Formation diagram.

Cross-sectional diagram showing how nutrient runoff contributes to low-oxygen conditions in coastal waters.

On the left, river freshwater carries nutrients—nitrogen, phosphorus, and silicon—along with sediments and organic carbon into the coastal water. Sunlight and nutrients support the growth of phytoplankton near the surface. An enlarged circular inset shows phytoplankton, zooplankton, dead cells, and fecal pellets. Dead cells and fecal pellets sink, carrying organic material toward the bottom.

The water is separated into a lighter, fresher, warmer surface layer and a heavier, saltier, cooler lower layer. The boundary between them is labeled “pycnocline.” Yellow arrows show oxygen entering the surface water from the atmosphere. A downward arrow labeled “oxygen flux blocked” indicates that the layering restricts oxygen movement into the lower water.

Near the seafloor, sinking organic matter decomposes and oxygen is consumed. A crab moving toward shallower water is labeled “escape,” while dead bottom-dwelling animals in deeper water are labeled “mortality.” The shallow area supports a healthy bottom-dwelling community of worms, snails, bivalves, and crustaceans.

At the lower right, an arrow indicates upwelling nutrients and oxygen, with their effects labeled “unquantified.” Overall, the diagram shows how nutrient inputs, sinking organic matter, decomposition, and restricted oxygen replenishment can combine to produce low-oxygen bottom waters.

Credit: EPA: Mississippi River/Gulf of Mexico Hypoxia Task Force

Watch the following videos from NOAA’s National Ocean Service that show how dead zones are formed and explain the dead zone in the Gulf of Mexico:

• Video: Happening Now: Dead Zones in the Gulf 2017 (2:33) (Transcript)
• Video: Hypoxia (3:51) (Text description - video has no narration, just on screen text)

The nutrients that make our crops grow better also fertilize phytoplankton in lakes and the ocean. Phytoplankton are microscopic organisms that photosynthesize just like our food crops. With more nitrogen and phosphorus available to them, they grow and multiply. When the phytoplankton dies, decomposers eat them. The decomposers also grow and multiply. As they’re eating all of the abundant phytoplankton, they use up the available oxygen in the water. The lack of oxygen forces mobile organisms to leave the area and kills the organisms that can’t leave and need oxygen. The zone of low oxygen levels is called a hypoxic or dead zone. Streams flowing through watersheds where agriculture is the primary land use exhibit the highest concentrations of nitrogen (Figure 4.2.8).

Text description of the Nitrogen Concentrations graph.

Box-and-whisker graph titled “Total nitrogen” comparing nitrogen concentrations in streams draining watersheds with four predominant land uses: agricultural, urban, mixed, and undeveloped. The vertical axis shows concentration in milligrams per liter, ranging from 0 to 12.

For each category, the box spans the 25th to 75th percentiles, the line inside marks the median, and the whiskers extend to the 10th and 90th percentiles. Approximate values are:

• Agricultural: Median of 3.8 milligrams per liter; the box spans 2.3 to 6.9, and whiskers extend from 1.4 to 9.9.
• Urban: Median of 1.5 milligrams per liter; the box spans 1.1 to 3.4, and whiskers extend from 0.8 to 6.5.
• Mixed: Median of 1.4 milligrams per liter; the box spans 0.9 to 2.3, and whiskers extend from 0.6 to 4.4.
• Undeveloped: Median of approximately 0.5 milligrams per liter; the box spans 0.3 to 0.8, and whiskers extend from 0.2 to 1.3.

A horizontal reference line marks the background nitrogen concentration of 0.58 milligrams per liter. Agricultural watersheds have the highest median concentration and the greatest spread of concentrations. Undeveloped watersheds have the lowest concentrations, with a median near the background level.

Credit: Dubrovsky and Hamilton 2010, The Quality of Our Nation’s Waters, Nutrients in the Nation’s Streams and Groundwater: National Findings and Implications

The Mississippi River is the largest river basin in North America (Figure 4.2.9), the third largest in the world, and drains more than 40 percent of the land area of the conterminous U.S., 58 percent of which is very productive farmland (Goolsby and Battaglin, 2000). Nutrient concentrations in the lower Mississippi River have increased markedly since the 1950s along with increased use of nitrogen and phosphorus fertilizers (Figure 4.2.10). When the Mississippi River’s nutrient-laden water reaches the Gulf of Mexico, it fertilizes the marine phytoplankton. These microscopic photosynthesizing organisms reproduce and grow vigorously. When the phytoplankton die, they decompose. The organisms that eat the dead phytoplankton use up much of the oxygen in the Gulf’s water resulting in hypoxic conditions. The resulting region of low oxygen content is referred to as a dead zone or hypoxic zone. The dead zone in the Gulf of Mexico at the mouth of the Mississippi River has grown dramatically and in some years encompasses an area the size of the state of Connecticut (~5,500 square miles) or larger. Hypoxic waters can cause stress and even cause the death of marine organisms, which in turn can affect commercial fishery harvests and the health of ecosystems.

Credit: USGS Factbook - Nitrogen in the Mississippi Basin-Estimating Sources and Predicting Flux to the Gulf of Mexico

Credit: USGS: Trends in Nutrients and Pesticides in the Nation's Rivers and Streams