Examining Oxygen Depleted Zones in the Gulf
Dr. Pete Eldridge | EPA Ecologist

The Mississippi River Basin is the largest watershed in North America, draining 41% of the continental United States and discharging into the Northern Gulf of Mexico. Aquatic ecosystems, and national and local economies, depend on healthy, oxygen-rich Gulf waters, which can be negatively impacted by excess nutrients (eutrophication). In 1997, a coalition of federal, state, and tribal agencies established the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force to understand the causes and effects of eutrophication in the Gulf of Mexico and to coordinate activities to help reduce the area of oxygen-depleted water in the Gulf, also known as the hypoxic zone.
Hypoxia means an absence of oxygen reaching living tissues. In coastal waters, it is characterized by low levels of dissolved oxygen, so that not enough oxygen is available to support fish and other aquatic species. Each year, an area of the Gulf of Mexico becomes devoid of fish and other aquatic life.  The phenomenon is a result of oxygen-depleted water. Now commonly referred as the “dead zone,” it is one of the largest in the world.  It occurs off the Louisiana shore westward from the mouth of the Mississippi River, and in some years, can extend as far as the Texas coast.
Since 2002, EPA’s Gulf Ecology Division has been conducting seasonal cruises with intensive sampling throughout the hypoxic zone. Core samples are studied to determine how rapidly oxygen is depleted in the overlying water, and how oxygen depletion is associated with sediment and water chemistry. 

Balancing the demands of navigation, agriculture, recreation, industry, and wildlife is not easy. However, a number of collaborative partnerships are fostering a better understanding of this dynamic ecosystem, and coordinated actions are under way to address some of its most pressing challenges.

Dr. Pete Eldridge is an ecologist within EPA’s Western Ecology Division at the National Health and Environmental Effects Research Laboratory.

Learn more about the Mississippi River Basin