NOAA forecasts an average summer 'dead zone' in Gulf of America

NOAA is forecasting this summer’s Gulf of America (formerly Gulf of Mexico) “dead zone” to be average-sized, covering approximately 5,574 square miles — an area roughly three times the size of Delaware. The dead zone, or hypoxic zone, is an area of low oxygen that can kill fish and other marine life. It occurs every summer and is primarily a result of excess nutrient pollution from human activities in cities and farm areas throughout the Mississippi-Atchafalaya watershed.

When excess nutrients reach the Gulf via the Mississippi-Atchafalaya River Basin, they stimulate an overgrowth of algae. When these algae die and decompose, they deplete oxygen in the water as they sink to the bottom. Large dead zones lead to extensive habitat loss for several ecologically and economically important species in the Gulf, proving detrimental to U.S. seafood and tourism industries.

The U.S. Geological Survey (USGS) provides Mississippi and Atchafalaya river discharge and nutrient loading data for the month of May, which are key factors used by NOAA forecast models to estimate the size of the Gulf’s dead zone during the summer. In May 2025, discharge in the Mississippi and Atchafalaya rivers was about 29% above the long-term average between 1980 and 2024, and the nitrate and phosphorus loads were about 24% below and 31% above the long-term averages, respectively.

USGS measures river discharge and nutrient levels using approximately 3,007 real-time streamgages, 97 real-time nitrate sensors and 37 long-term monitoring sites in rivers throughout the Mississippi-Atchafalaya watershed. These data are used to track long-term changes in nutrient inputs to the Gulf and to build models of nutrient sources and hotspots within the watershed. 

USGS continues to develop cutting-edge science tools and assessments to help better understand the sources and impacts of water quality problems in the Mississippi-Atchafalaya watershed. This past winter, USGS released the National Integrated Water Availability Assessment, a broader long-term assessment that combines water supply, water quality, and water use to form a more comprehensive perspective on water availability in the nation.

The annual forecast is a key metric that informs the collective efforts of the Interagency Mississippi River and Gulf of America Hypoxia Task Force, which has set a long-term goal of reducing the dead zone to 1,900 square miles by 2035. NOAA’s hypoxia measurements and USGS monitoring of nutrients and water discharge in rivers help to improve understanding of how hypoxia in the Gulf is linked to nutrients coming from throughout the Mississippi-Atchafalaya watershed. The Task Force members in turn use this information to inform their state’s nutrient reduction strategies across the Mississippi watershed.

This is the eighth year NOAA has produced a dead zone forecast using a suite of models jointly developed with partners at the University of Michigan, Louisiana State University, William & Mary’s Virginia Institute of Marine Science, North Carolina State University, and Dalhousie University. NOAA integrates the results of these models into an aggregate “ensemble” model forecast.

To confirm the size of the hypoxic zone and refine the forecast models, a NOAA-supported monitoring survey is conducted each summer, with results released in early August. NOAA and its partners continue to develop additional hypoxia forecasting capabilities and tools to understand impacts on living marine resources under various nutrient reduction actions.