A team of scientists, including a University of Michigan aquatic ecologist, is forecasting an above-average summer “dead zone” in the Gulf of Mexico covering about 5,827 square miles—an area roughly the size of Connecticut.

This summer’s dead zone is about 12% higher than the average dead zone measurement over the 37-year period of record.

The dead zone, or hypoxic area, 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.

“The action plan to reduce the size of the Gulf of Mexico dead zone has been in place
for over two decades, but each year the size of the dead zone varies around the
long-term average—and this year it’s expected to be even bigger. But that long-term
average is almost three times the goal set in 2001,” said Don Scavia, professor
emeritus at the University of Michigan School for Environment and Sustainability, who
leads one of several research teams partnering with the federal government on the
annual forecast.

“Year-to-year fluctuations in the dead zone are driven primarily by weather in the Corn
Belt, but the long-term trends are driven by nutrient loads from agriculture. Lack of a
downward trend in the dead zone illustrates that current efforts to reduce those loads
have not been effective. Clearly, the federal and state agencies and Congress continue
to prioritize industrial agriculture over water quality.”

The U.S. Geological Survey provides Mississippi River discharge and nutrient loading
data for the month of May, which are key factors used by National Oceanic and
Atmospheric Administration forecast models to estimate the size of the gulf’s dead zone
during the summer. In May 2024, discharge in the Mississippi and Atchafalaya rivers
was about 5% above the long-term average between 1980 and 2023, and the nitrate
and phosphorus loads were about 7% below and 22% above the long-term averages,
respectively.

“Reducing the impact of hypoxic events and lessening the occurrence and intensity of
future dead zones continues to be a NOAA priority,” said Nicole LeBoeuf, National
Ocean Service assistant administrator. “These forecasts are designed to provide
crucial data to scientists, coastal managers and communities, and are used as
guideposts in the development of planning actions.”

When excess nutrients reach the Gulf of Mexico 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. The resulting low oxygen
levels cause motile animals, like fish and shrimp, to leave the area. Exposure to
hypoxic waters has been found to alter fish diets, growth rates, reproduction, habitat
use and the availability of commercially harvested species like shrimp.

This is the seventh year NOAA has produced a dead zone forecast using a suite of
models jointly developed by the agency and its partners—teams of researchers 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.

The U.S. Geological Survey measures river discharge and nutrient levels using more
than 3,000 real-time streamgages, 73 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 has monitored streams and groundwater in the Mississippi-Atchafalaya
watershed for decades to help better understand the sources and impacts of water
quality problems,” said Joshua Joseph, acting USGS associate director for water
resources. “A recent USGS study found that in the Illinois River—a major tributary of
the Mississippi River—algae initially grow in the upper river and are then transported
downstream, potentially causing algal toxins and decreased dissolved oxygen in the
lower river.”

This annual forecast is a key metric that informs the collective efforts of the
Interagency Mississippi River and Gulf of Mexico 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 forecast models and USGS monitoring of nutrients and water discharge in
rivers help to predict how hypoxia in the Gulf of Mexico is linked to nutrients coming
from throughout the Mississippi River watershed. The hypoxia task force in turn uses
this information to inform nutrient reduction targets across the Mississippi watershed
states.

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, tools and models to understand impacts on living marine
resources under various nutrient reduction actions.