Low-oxygen conditions, or hypoxia, can have devastating impacts on marine organisms ranging from microscopic zooplankton to large fish, such as grouper and snapper. The U.S. Geological Survey has been monitoring regional to global trends in these conditions for over a decade. Hypoxia is typically caused by an accumulation of organic matter and nutrients in the water column, leading to enhanced phytoplankton growth and consumption of dissolved oxygen. It is estimated that over the past 50 years, hypoxic waters have expanded in nearshore coastal areas of the United States, particularly in the northern Gulf of Mexico and along the California coast.
Long-term monitoring in the western Gulf of Mexico has shown the bottom waters are becoming less and less oxygenated. Scientists measure the amount of oxygen dissolved in the water using oxygen-saturation readings. An area is considered hypoxic when oxygen levels drop below 2 milligrams per liter of seawater. When oxygen levels drop below 1 milligram per liter, the area is considered severely hypoxic.
Monitoring programs also measure the extent and intensity of hypoxic areas. The geographic location of the hypoxic zone can range from several kilometers offshore to near the coast. In some cases, hypoxic areas may expand over hundreds of square kilometers and reach depths of more than 40 meters.
This increase in hypoxia has been linked with increased water temperatures and the associated decrease in water density. Warmer temperatures reduce the amount of oxygen that can dissolve in the water column, causing hypoxic events to become more frequent and intense. In addition, climate change has been implicated in the expansion of hypoxia in the western Gulf of Mexico. Research has found that increasing temperatures are significantly correlated with increasing hypoxia. Climate change models predict that in the coming decades, hypoxia will continue to expand in both intensity and geographical extent, leading to an increased risk of mortality for organisms living within these hypoxic zones.
Having a way to monitor current and future hypoxic zones is critical for understanding how these conditions are likely to impact ocean ecosystems. The USGS Hypoxia Program helps provide this information by mapping areas of hypoxia, examining trends in oxygen saturation, and assessing the impacts of low oxygen on aquatic organisms. To do this, researchers use a variety of tools, including physical oceanography monitoring platforms, autonomous submersibles, and satellite imagery to measure the location, extent, and intensity of hypoxic areas.
Through ongoing monitoring and assessment, the Hypoxia program provides essential information to resource managers and coastal decision makers. This knowledge can be used to inform policy and help alleviate the effects of low oxygen conditions on marine organisms. The program also serves as an early warning system, alerting researchers and managers when changes in hypoxic conditions occur so they can take steps to prevent widespread harm to marine life.
