Hypoxia 101

What is hypoxia and what causes it?

Hypoxia means a low level of oxygen and is primarily a problem for estuaries and coastal waters. Hypoxic waters have dissolved oxygen concentrations of less than 2-3 mg/L. Hypoxia can be caused by a variety of factors, including excess nutrients, primarily nitrogen and phosphorus, that leads to both eutrophication and waterbody stratification (layering) due to saline or temperature gradients. Eutrophication occurs when excessive nutrients cause a dense growth of algal blooms that can be seen. As algae ultimately die off and decompose, oxygen is consumed resulting in low levels of oxygen in the water.

Nutrients can come from many sources, including any of the following:

• Fertilizers from agriculture, golf courses, and suburban lawns
• Erosion of soil full of nutrients
• Discharges from sewage treatment plants
• Deposition of atmospheric nitrogen

The hypoxic zone in the Gulf of Mexico forms every summer and is a result of excess nutrients from the Mississippi/Atchafalaya River and seasonal stratification (layering) of waters in the Gulf. As nutrient-laden freshwater from the Mississippi/Atchafalaya River flows into the Gulf, it is less dense and remains above the heavier saline seawater. In addition to the saline gradient caused where the freshwater and saline water meet, the freshwater is warmer than the deeper ocean water, further contributing to the stratification. This stratification prevents the mixing of oxygen-rich surface water with oxygen-poor water on the bottom of the Gulf. Without mixing, oxygen in the bottom water is limited and the hypoxic condition remains.

Why is it important to reduce hypoxic zones?

One direct effect of hypoxia is fish kills, which deplete valuable fisheries and disrupt ecosystems. Mobile animals (e.g., adult fish) can typically survive a hypoxic event by moving to waters with more oxygen. Less mobile or immobile animals, such as mussels or crabs, cannot move to waters with more oxygen and are often killed during hypoxic events. Hypoxia also affects the ability of young fish or shellfish to find the food and habitat necessary to become adults. As a result, fish and shellfish stocks may be reduced or become less stable because less young reach adulthood. Hypoxia can also affect species that rely on fish for food. Such species might have to leave an area to find the necessary food to survive. Ultimately, hypoxia causes a severe decrease in the amount of life in hypoxia zones.

Where does hypoxia occur?

Hypoxia occurs naturally in many aquatic environments throughout the world, such as in deep basins in the ocean. Hypoxic waters have occurred throughout history, but they are occurring in shallow coastal and estuarine waters more frequently as anthropogenic (i.e. human) sources and inputs of nutrients increase.

The Gulf of Mexico hypoxic zone is located in the northern Gulf of Mexico off the coast of Louisiana and Texas. The zone was first documented in 1972. Learn more about the annual size and location of the Gulf of Mexico hypoxic zone since 2001. Hypoxia occurs in many places throughout the world. See the Hypoxia Research Team at the Louisiana Universities Marine Consortium website for more information. 

A study published in Science found that there are over 400 hypoxic zones in the world (Diaz and Rosenberg, 2008). The hypoxic zone in the Gulf of Mexico is the largest in the United States, and one of the largest globally.

Dead zones worldwide; Figure from Diaz and Rosenberg, 2008

In 2008, NASA mapped the locations of aquatic dead zones, showing relative dead zone size (areal extent in km²), along with population density (persons/km²) and particulate organic carbon (mg/m³). Darker blues in this image show higher concentrations of particulate organic matter, an indication of the overly fertile waters that can culminate in dead zones.

Sources

• Diaz and Rosenberg. 2008. "Spreading Dead Zones and Consequences for Marine Ecosystems."
• Ecological Society of America. No date. "Hypoxia."
• Jean-Philippe et. al. 2016. "Global Spread of Hypoxia in Freshwater Ecosystems During the Last Three Centuries is Caused by Rising Local Human Pressure."
• LUMCON. 2018. "Overview."
• LUMCON. 2018. "About Hypoxia."
• NOAA. 2001. "Hypoxia."
• Rabalais et. al. 2010. "Dynamics and Distribution of Natural and Human-Caused Hypoxia."
• Rao et. al. 2014. "On Hypoxia and Fish Kills Along the North Shore of Lake Erie."
• USGS. 2018. "Dissolved Oxygen and Water."
• USGS. 2017. "The Gulf of Mexico Hypoxic Zone."