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The Gulf of Mexico Hypoxic Zone

Measured Area of the Hypoxic Zone (km2) Source: Dr. Nancy Rabalais Universities Marine Consortium

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, causing a condition referred to as hypoxia (USEPA: What is Hypoxia?). 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 believed to be caused primarily by excess nutrients delivered from the Mississippi River in combination with seasonal stratification of Gulf waters. Excess nutrients promote 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.

The size of the hypoxic zone and other information about dissolved oxygen levels in the northern Gulf of Mexico are measured each summer.

• The Louisiana Universities Marine Consortium, supported by NOAA's NGOMEX
• NOAA's Gulf Of Mexico Hypoxia Watch