What is hypoxia?

Hypoxia means "low oxygen." In aquatic ecosystems, low oxygen usually means a concentration of less than 2-3 milligrams of oxygen per liter of water (mg/l). A complete lack of oxygen (0 mg/L) is called anoxia. Since organisms that can live without oxygen (such as some microbes) are the only residents in these areas, they are sometimes called "dead zones". Hypoxia is primarily a problem in estuaries and coastal waters, although it can also be a problem in freshwater lakes.

Hypoxia Zone

The hypoxic zone in the northern Gulf of Mexico refers to an area along the Louisiana-Texas coast in which water near the bottom of the Gulf contains less than 2 parts per million of dissolved oxygen. Hypoxia can cause fish to leave the area and can cause stress or death to bottom dwelling organisms that can’t move out of the hypoxic zone. Hypoxia is caused primarily by excess nitrogen delivered from the Mississippi River in combination with seasonal stratification of Gulf waters. Nitrogen promotes algal and attendant zooplankton growth. The associated organic matter sinks to the bottom where it decomposes, consuming available oxygen. Stratification of fresh and saline waters prevents oxygen replenishment by mixing of oxygen-rich surface water with oxygen-depleted bottom water.