Gulf of Mexico Dead Zone Is Second-smallest in 38 Years

The annual hypoxic zone measured about 1,332 square miles during a July survey, making it the second-smallest dead zone in four decades of monitoring. A NOAA map shows the zone stretching across portions of the northern Gulf near Louisiana, where nutrient-rich waters from the Mississippi and Atchafalaya rivers fuel seasonal oxygen depletion. Areas in red indicate dissolved oxygen levels of 2 milligrams per liter or less, a threshold considered hypoxic.

Hypoxic zones form when excess nutrients from agricultural runoff, wastewater, and stormwater fuel algal blooms; when algae die and sink, bacteria consume oxygen. Prolonged hypoxia can alter migration patterns, reduce habitat, and affect commercial fisheries. Bottom-dwelling species are hardest hit, while mobile animals like fish and shrimp may escape but face crowding.

Whales and dolphins are less vulnerable but can be affected indirectly through food web changes. Scientists said the small measurement does not mean the Gulf escaped severe hypoxia; Tropical Storm Bertha passed through shortly before the survey, stirring the water and pushing oxygen-rich surface waters downward.