Low or depleted oxygen in a water body often leads to 'dead zones'—regions where life cannot be sustained.

In ocean and freshwater environments, the term "hypoxia" refers to low or depleted oxygen in a water body. Hypoxia is often associated with the overgrowth of certain species of algae, which can lead to oxygen depletion when they die, sink to the bottom, and decompose. The above data visualization looks at the causes of hypoxia in the Gulf of Mexico.

Hypoxia Causes 'Dead Zones'

In some cases, vast stretches of open water become hypoxic. Unable to sustain life, these areas, called dead zones, may cause die-offs of fish, shellfish, corals, and aquatic plants. Since 1985, NOAA-sponsored research has monitored the largest dead zone in the United States, which forms every spring in the northern Gulf of Mexico. In 2014, it grew to cover more than 5,000 square miles of the sea floor.

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How Does Hypoxia Happen?

The amount of oxygen in any water body varies naturally, both seasonally and over time. This occurs due to a balance between oxygen input from the atmosphere and certain biological and chemical processes, some of which produce oxygen while others consume it.

Stratification in the water column, which occurs when less dense freshwater from an estuary mixes with heavier seawater, is one natural cause of hypoxia. Limited vertical mixing between the water "layers" restricts the supply of oxygen from surface waters to more saline bottom waters, leading to hypoxic conditions in bottom habitats.

Hypoxia occurs most often, however, as a consequence of human-induced factors, especially nutrient pollution (also known as eutrophication). The causes of nutrient pollution, specifically of nitrogen and phosphorus nutrients, include agricultural runoff, fossil-fuel burning, and wastewater treatment effluent.

In 2013, the U.S. Environmental Protection Agency estimated that high concentrations of nitrogen are present in 28 percent of the nation's stream length. For phosphorus, the estimate is 40 percent. The number of U.S. estuaries experiencing hypoxia has greatly increased in recent decades, and over half of these exhibit hypoxic conditions in any given year.

Hypoxia and Climate Change

Changes in both global and regional climates have the potential to make coastal and marine ecosystems even more vulnerable to hypoxic conditions. NOAA's National Centers for Coastal Ocean Science (NCCOS) Coastal Ecosystem Effects of Climate Change Program carries out interdisciplinary research to advance understanding of the relationship between ecosystem function and climate change. This type of research will ultimately assist decision makers and resource managers as they address the challenges of protecting ecosystems in a changing climate.

NOAA's Efforts to Combat Hypoxia

NOAA studies and funds research on the causes and impacts of hypoxia. NOAA also collaborates with local, state, and federal agencies, regional task forces, universities, conservation organizations, and industry partners to develop management strategies to reduce nutrient inputs into coastal waters.

Much of NOAA's hypoxia work is managed through NCCOS, which studies and monitors the effects of hypoxia and other sources of pollution nationwide. NCCOS has funded the development of hypoxia forecasts in the Gulf of Mexico since 1990 and in the Chesapeake Bay since 2005.

In the Gulf of Mexico, this research helped the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force set goals and prioritize actions to reduce the size of the annual Gulf of Mexico Dead Zone. In Chesapeake Bay, the research is providing important information to interagency management bodies like the Chesapeake Bay Program.

A new cross-NOAA initiative, the Ecological Forecasting Roadmap, is developing a national framework for hypoxia and other ecological forecasts (e.g., HABs, pathogens) so that their benefits to the public can be sustained and improved over time. NOAA also plays a major role in the U.S. Integrated Ocean Observing System (IOOS®). With its Coastal and Ocean Modeling Testbed, IOOS is evaluating a suite of hypoxia forecast models in the Gulf of Mexico and Chesapeake Bay.

NOAA-sponsored hypoxia studies are also ongoing in Lake Erie, Green Bay, and Narragansett Bay.

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Legislative Mandate

In June 2014, Congress reauthorized the Harmful Algal Bloom and Hypoxia Research and Control Act of 1998 (HABHRCA 1998) with the Harmful Algal Bloom and Hypoxia Amendments Act of 2004 (HABHRCA 1998, reauthorized in 2004, Public Law 113-124). The Act reaffirmed and expanded NOAA's mandate to advance scientific understanding of hypoxia, and support scientists' ability to detect, monitor, predict, and mitigate occurrences of hypoxia and harmful algal blooms.

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Did you know?

In June 2014, a team of NOAA-sponsored scientists from Texas A&M University deployed three autonomous underwater vehicles called gliders to monitor hypoxia in the northern Gulf of Mexico. Shaped like an airplane fuselage, a glider dives and climbs through ocean waters with buoyancy-based propulsion that requires very little power. Gliders are equipped with sensors that can collect vast amounts of data over large spatial and temporal scales.

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