A vast low-oxygen "dead zone" has again formed in the Gulf of Mexico along the U.S. coast, according to new measurements released Wednesday by the National Oceanic and Atmospheric Administration (NOAA), though scientists found that the affected area was much smaller than expected after a tropical storm temporarily disrupted conditions.

The annual hypoxic zone, a region where dissolved oxygen levels are too low to support many forms of marine life, measured about 1,332 square miles—roughly the size of Rhode Island—during a July survey, making it the second-smallest dead zone recorded in four decades of monitoring. Hypoxic areas are often referred to as "dead zones" because fish, shrimp and other marine organisms either flee the area or struggle to survive when oxygen becomes scarce.

A NOAA map released alongside the findings shows the zone stretching across portions of the northern Gulf near Louisiana, where nutrient-rich waters carried by the Mississippi and Atchafalaya rivers fuel seasonal oxygen depletion. Areas marked in red indicate dissolved oxygen levels of 2 milligrams per liter or less, a threshold considered hypoxic and unsuitable for many bottom-dwelling species.

Newsweek reached out to NOAA by email for comment.

What Causes Hypoxic Zone?

Hypoxic zones typically form when excess nutrients, especially nitrogen and phosphorus from agricultural runoff, wastewater and stormwater, enter rivers and coastal waters and fuel large algal blooms. When those algae die and sink, bacteria break them down, consuming oxygen in the process. This can leave bottom waters with dangerously low oxygen levels, particularly during warm months when the water column becomes stratified and oxygen-rich surface water cannot easily mix downward.

Scientists have tracked the phenomenon for decades because of its ecological and economic consequences. Prolonged hypoxia can alter migration patterns, reduce habitat availability and affect commercially important fisheries. NOAA says understanding those impacts is critical for the seafood industry, recreational fishing and coastal tourism economies that depend on healthy Gulf ecosystems.

What Animals Are Most at Risk in Hypoxic Zones?

Marine species that live on or near the seafloor are often among the hardest hit when oxygen levels collapse. NOAA says hypoxic conditions can make critical habitat unavailable to bottom-dwelling organisms, forcing animals to relocate or suffer stress that affects growth and reproduction. Fish, shellfish, coral and aquatic plants are all vulnerable when oxygen levels fall below survivable thresholds.

Some mobile animals, including many fish species and shrimp, can escape the area if oxygen depletion develops gradually. However, their movement can crowd populations into smaller regions and disrupt feeding and breeding behavior. Less mobile organisms, particularly those attached to the seafloor or living within sediments, face greater risks because they have fewer options for avoiding hypoxic waters.

Ocean-dwelling mammals such as whales and dolphins are generally less vulnerable to hypoxia than fish or bottom-dwelling species because they breathe air and can swim away from low-oxygen waters. However, dead zones can still affect them indirectly by altering marine food webs and reducing the abundance or distribution of prey species such as fish and squid. When hypoxia forces prey to move or concentrates them in smaller areas, dolphins may have to travel farther or change their feeding behavior to find food.

Scientists also warn that repeated dead zones can have broader impacts throughout the food web. Habitat loss and reduced oxygen may affect species that support major commercial fisheries, while changes in where fish and shellfish can live may ripple through coastal ecosystems. Understanding these effects is one reason NOAA and partner institutions continue conducting annual surveys and improving predictive models.

Why Is This Year's Hypoxic Zone Smaller Than Normal?

Scientists said the unusually small measurement does not necessarily mean the Gulf escaped severe hypoxia this year. Instead, Tropical Storm Bertha passed through the northern Gulf shortly before researchers conducted their annual survey, stirring the water column and pushing oxygen-rich surface waters downward. That mixing temporarily reduced the extent of low-oxygen conditions visible during the measurement cruise.

Earlier this summer, NOAA had forecast a much larger dead zone covering roughly 7,027 square miles, larger than the long-term average and approaching the size of New Jersey. The discrepancy highlights how weather events can drastically influence the formation and breakup of hypoxic waters.

"Measuring the hypoxic zone provides more than just a score card of the Gulf's ocean oxygen levels—it helps us better understand the interconnectivity of our nation’s land, waterways and ocean,” NOAA Assistant Secretary of Commerce for Oceans and Atmosphere Timothy Petty said in a statement. “The data we've gathered from 40 years of measurements in the region will help inform the development of better management strategies upstream as part of our ongoing efforts to mitigate future impacts in the Gulf.”

Location of Other Hypoxic Dead Zones

Hypoxic "dead zones" are not unique to the Gulf of America. Scientists have documented recurring low-oxygen events in several major U.S. waterways, including Chesapeake Bay, Long Island Sound, portions of Lake Erie and coastal waters off Washington and Oregon. In many cases, excess nutrients fuel algal blooms that deplete oxygen as they decompose, although oceanographic conditions can also contribute to seasonal hypoxia in some regions. The resulting low-oxygen conditions can leave marine life with fewer places to feed, reproduce and survive.

Among the most closely watched examples are Chesapeake Bay, where researchers issue annual hypoxia forecasts because low-oxygen conditions can threaten fish, blue crabs and oysters, and Long Island Sound, which has historically experienced summertime dead zones linked to nitrogen pollution. Seasonal hypoxia has also been recorded along parts of the Pacific Northwest coast and in Lake Erie, underscoring that oxygen-depleted waters are a widespread environmental challenge affecting both coastal and inland ecosystems across the United States.

What Happens Next

Federal agencies and researchers are working toward a long-term goal of shrinking the Gulf's dead zone to 1,900 square miles by 2035 through efforts to reduce nutrient runoff across the Mississippi River watershed. While this year's measured zone fell below that target, scientists cautioned that the result was heavily influenced by Tropical Storm Bertha and does not necessarily indicate that the underlying nutrient problem has been solved.

Contact Newsweek editors on this story: Anna Skinner and Dave Siminoff.