The major cause of hypoxia in the Gulf of Mexico is the annual influx of excess nutrients, primarily nitrogen and phosphorus, delivered by the Mississippi River system. This nutrient overload stimulates biological and physical events that strip the deeper water of dissolved oxygen. Hypoxia refers to a condition where dissolved oxygen concentration falls below two milligrams per liter, a level too low to sustain most marine life. This phenomenon drives the Gulf of Mexico’s recurring “Dead Zone,” which forms every summer off the coast of Louisiana and Texas.
This low-oxygen area is the largest recurring hypoxic zone in the United States and has appeared annually since the early 1970s. The oxygen-depleted water stresses or kills bottom-dwelling invertebrates, while mobile creatures like fish and shrimp are forced to emigrate. The size of the Dead Zone varies each year, sometimes reaching an area comparable to the size of Massachusetts.
The Primary Driver: Nutrient Pollution
The nutrients causing Gulf hypoxia are carried by the Mississippi and Atchafalaya Rivers, which drain about 41% of the continental United States. The flow from this expansive watershed carries nutrient loads that have increased significantly since the mid-20th century, coinciding with changes in land use and agricultural practices.
Agricultural activities are the predominant source of this pollution, mainly through the runoff of commercial fertilizers and animal manure used throughout the Mississippi River Basin. Nitrogen and phosphorus are applied to crops, particularly in the midwestern “Corn Belt,” and excess amounts wash into tributaries during rainfall and snowmelt. The extensive use of tile drainage systems in some agricultural areas also accelerates the transport of nitrates directly into waterways, bypassing natural soil filtration.
Other Sources of Nutrients
While agriculture is the largest contributor, other sources also contribute to the nutrient load. Municipal wastewater treatment plants and discharges from urban areas introduce additional nutrients into the river system. Atmospheric deposition of nitrogen, often from the burning of fossil fuels, also contributes to the nutrients that eventually reach the Gulf waters.
The Biological Mechanism of Oxygen Depletion
When nutrient-laden freshwater reaches the Gulf, it triggers eutrophication, the biological process that causes oxygen loss. The abundance of nitrogen stimulates the rapid growth of microscopic marine plants known as phytoplankton or algae. These algal blooms occur primarily in the surface waters where sunlight is plentiful.
The algae eventually die, and their remains sink toward the bottom of the Gulf. This organic matter settles on the seafloor, becoming food for bottom-dwelling microorganisms, mainly bacteria. The decomposition process carried out by these bacteria is a form of respiration that consumes dissolved oxygen from the surrounding water. This biological oxygen consumption often happens faster than the oxygen can be replenished, leading to a sustained drop in dissolved oxygen levels in the bottom layer.
Physical Factors and Geographical Scale
The persistence and extent of the hypoxic zone are influenced by the region’s physical geography. The large volume of freshwater from the Mississippi River is less dense than the Gulf’s saltier, colder water. This density difference creates stratification, a layering effect where warm, less dense freshwater remains on the surface above the cooler, denser saltwater below.
Stratification acts as a physical barrier, preventing the vertical mixing of the water column. Surface winds and waves would normally help mix the oxygen-rich surface layer with the deeper water, but the density difference inhibits this natural aeration. As bacteria consume oxygen in the bottom layer, the depleted water remains trapped beneath the surface layer, unable to be refreshed by contact with the atmosphere.
The hypoxic zone typically forms on the shallow continental shelf off the coast of Louisiana and Texas, where the Mississippi River outflow concentrates. Seasonal timing is also tied to physical conditions, as stratification is strongest during the warmer summer months when river discharge is high and winds are calmer. This combination of nutrient loading and physical stratification allows the Dead Zone to form and persist annually until strong weather events, such as hurricanes, break up the water layers and restore oxygen to the bottom.