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Tracking the Gulf Dead Zone

Photo of a CTD device in the Gulf of Mexico
Courtesy of NOAA Teacher at Sea Program

Like black holes in space, areas exist in the ocean with characteristics much different than the rest of the ocean. Rather than gobble up matter and light as in space, hypoxic zones rob the ocean of oxygen. The zones have such low concentrations of oxygen that marine life leaves, if possible, or dies. These dead zones, as they’ve come to be known, remain “dead” until the oxygen supply recovers enough to support life again.

Since 2001, NOAA’s Gulf of Mexico Hypoxia Watch program (Hypoxia Watch) has monitored the Gulf between June and July. NOAA develops near real-time, web-based contour maps and related data of regions with dissolved oxygen levels. The maps and data from Hypoxia Watch provide a baseline of the annual Gulf Dead Zone, which is later officially measured by a NOAA-supported monitoring survey in August.

To the multi-billion-dollar Gulf fishing industry and other sectors, the information provides invaluable insight. The scientific knowledge gives policymakers, commercial fishing interests, tourism offices, the general public, and others an understanding of Gulf conditions that can improve the economy and recreational pursuits.

What Causes Hypoxia?

Hypoxic zones can occur naturally, but scientists have found that they can be caused or worsened by human activity. Nutrient pollution is the primary cause of hypoxia as a result of human activities. Excess nutrients that run off land or are piped into rivers and coasts as wastewater can stimulate an overgrowth of algae, which then sink and decompose. This decomposition process consumes oxygen and depletes the supply available to marine life.

Dead zones occur all over the world. The largest dead zone covers the entire bottom of the Baltic Sea, about the same size as California. Besides the Gulf, U.S. dead zones occur in the Chesapeake Bay and off the coast of Oregon.

The Gulf Dead Zone is the largest in the United States and second largest in the world. The largest Gulf Dead Zone occurred in 2017, encompassing 8,776 square miles. The smallest measured 15 square miles in 1988. As of 2020, the five-year average size of the dead zone has been approximately 5,400 square miles.

The Annual Hypoxia Survey

The Gulf Dead Zone forms each summer off the coast of Louisiana and Texas, threatening the ecosystem that supports valuable commercial and recreational Gulf fisheries. The Mississippi River is the drainage basin for 41% of the contiguous United States, with the Gulf the final destination. The river carries nutrient pollution from agriculture, industry, and cities. Summer’s warmer temperatures and calm seas prevent freshwater from the Mississippi from mixing with Gulf water and circulating oxygen throughout the water column. Hypoxic waters form in the bottom waters as excess nutrient loading from the river promotes algal overgrowth that leads to eutrophication at the surface. When the algae die and decompose, oxygen is consumed, resulting in low levels of oxygen in the waters. The seasonal prevention of mixing causes oxygen-rich waters not to reach the seafloor, resulting in the formation of the hypoxic zone.

The Gulf of Mexico Hypoxia Watch evolved as a cooperative project of NCEI, NOAA Fisheries, and the NOAA CoastWatch–Caribbean/Gulf of Mexico–Regional NodeEach year from early June through mid-July, as part of the combined state/federal/university Southeast Area Monitoring and Assessment Program, scientists aboard the NOAA Research Vessel Oregon II measure dissolved oxygen and other water quality variables using CTDs (instruments that gauge conductivity [salinity], temperature, and depth) on the continental shelf from Texas to northwest Florida. Every one to three days, the ship transmits the data to NCEI scientists at Stennis Space Center in Mississippi. NCEI uses the data collected during the survey to create near-real-time bottom dissolved oxygen contour maps and environmental profiles that are publicly available online, usually the same business day as the data are received by NCEI.

How Hypoxia Watch Helps

The environmental data gathered during the annual cruises help scientists understand the effects of the physical environment on fish and other marine organisms. Hypoxia Watch offers the first look at the season’s hypoxia conditions and provides critical information for management of the hypoxic zone in the Gulf. Users access the data in two main ways—either directly from the Hypoxia Watch website through contour maps and the underlying dissolved oxygen data, or through forecast and management models, created with dissolved oxygen data from Hypoxia Watch.

 While knowledge gaps still exist about the impact of the hypoxic zone on individual species, observers have already seen habitat loss, migration shifts, and reproductive disruption. These impacts not only alter commercial and recreational fishing patterns but can also reduce the economic vitality of these fishing industries. NOAA and its partners are leading efforts to understand, predict, and ultimately reduce hypoxic events and their effects on ecosystems and coastal communities.