The most well-known dead zone sits in the northern Gulf of Mexico, stretching along the Louisiana and Texas coastline. But it’s far from the only one. Dead zones, areas where dissolved oxygen drops so low that marine life suffocates or flees, exist in hundreds of locations worldwide, from the Baltic Sea to the coast of China. They form wherever excess nutrients fuel massive algal blooms that consume the water’s oxygen as they decompose.
The Gulf of Mexico Dead Zone
The largest dead zone in U.S. waters forms every summer in the northern Gulf of Mexico, hugging the coasts of Louisiana and Texas. In summer 2025, NOAA-supported scientists measured it at approximately 4,402 square miles, the 15th smallest zone recorded in 39 years of monitoring. That still translates to more than 2.8 million acres of seafloor habitat potentially unavailable to fish and bottom-dwelling species like shrimp and crabs.
The zone’s size fluctuates year to year, driven largely by how much rain washes fertilizer and other nutrients down the Mississippi River basin in the spring. In its worst years, it has ballooned to over 8,000 square miles, roughly the size of New Jersey. NOAA estimates the dead zone costs U.S. seafood and tourism industries $82 million annually, hitting Gulf shrimpers and fishing operations hardest.
Other Major Dead Zones Around the World
The Baltic Sea contains one of the largest and longest-running dead zones on the planet. Over the past century, its oxygen-depleted waters expanded from about 5,000 square kilometers to over 60,000 square kilometers, an area larger than Denmark. That expansion has reduced fish populations, particularly cod, and fueled toxic algal blooms across the region. Unlike the Gulf of Mexico’s seasonal dead zone, the Baltic’s persists year-round in its deepest basins because the sea’s limited connection to the open ocean restricts the flow of fresh, oxygenated water.
China’s Changjiang (Yangtze River) Estuary hosts another significant dead zone in the East China Sea. Off the coasts of Oregon and Washington, a dead zone appears seasonally along the Pacific shelf. The Chesapeake Bay, the Black Sea, and waters off the coasts of Japan, South Korea, and parts of South America all experience recurring hypoxia. Scientists have documented more than 400 dead zones globally, and the number has roughly doubled every decade since the 1960s.
How Dead Zones Form
A dead zone develops when dissolved oxygen in the water drops below 2 to 3 milligrams per liter. At that concentration, most fish, shrimp, and crabs either flee the area or die. The process starts with nutrients, primarily nitrogen and phosphorus, entering waterways. The biggest source is agricultural runoff: fertilizers applied to cropland wash into streams and rivers during rainstorms. Wastewater discharge, automobile exhaust, and animal waste also contribute.
Those nutrients feed explosive algal growth at the water’s surface, turning the water green and murky. When the algae die, they sink to the bottom, where bacteria break them down. That decomposition process consumes enormous amounts of oxygen. Meanwhile, warm summer temperatures create a layered water column where the warm surface water sits on top of cooler, denser bottom water. This layering, called stratification, prevents oxygen from the atmosphere from mixing down to the depths where it’s being consumed fastest. The result is a suffocating layer of water along the seafloor.
When Dead Zones Peak and Fade
Most dead zones follow a seasonal cycle tied to temperature and nutrient loading. In the Gulf of Mexico, the zone typically begins forming in late spring as warm weather stratifies the water column and spring rains deliver a pulse of nutrients from the Mississippi River watershed. It reaches its peak size in July and August, then weakens through the fall as storms and cooler temperatures mix oxygen back into deeper water. By winter, it largely disappears.
The pattern is similar in other nutrient-driven dead zones. Research on China’s Yangtze Estuary dead zone found that hypoxia first develops in June, expands through July and August as it spreads outward from the river mouth, then recedes through September and disappears by winter. This summer peak is consistent across most coastal dead zones in the Northern Hemisphere, though the exact timing shifts depending on local climate and river flow patterns.
Natural Low-Oxygen Zones in the Open Ocean
Not all low-oxygen water is caused by pollution. Oxygen minimum zones occur naturally in the open ocean, typically at depths between about 200 and 1,000 meters. These persistent layers form where biological oxygen consumption is high but mixing with oxygen-rich surface water or cold deep water is limited. They’re essentially sandwiched between two better-oxygenated layers: the surface, where atmospheric gases dissolve into the water, and the deep ocean floor, where extremely cold water holds dissolved oxygen well.
These natural zones have existed for geological timescales, and deep-sea organisms have adapted to survive in them. However, climate change is causing these zones to expand. Warmer ocean temperatures hold less dissolved oxygen and increase stratification, making it harder for oxygen to reach mid-depth waters. The distinction matters: nutrient-driven coastal dead zones can potentially be reversed by reducing pollution, while the expansion of natural oxygen minimum zones is tied to broader climate trends that are harder to address quickly.
What Drives the Size Year to Year
The single biggest predictor of how large the Gulf of Mexico dead zone will be in any given summer is how much nitrogen the Mississippi River carries in May. The river drains about 41% of the continental United States, collecting runoff from farms across the Midwest. A wet spring with heavy rains across Iowa, Illinois, Indiana, and neighboring states flushes more fertilizer into the river system, feeding larger algal blooms downstream. A dry spring produces a smaller zone.
Algal growth is usually limited by whichever nutrient is in shorter supply relative to the other. In coastal marine waters, nitrogen is typically the limiting factor, meaning additional nitrogen from fertilizer runoff has an outsized effect on algal blooms. In freshwater systems like lakes, phosphorus more often controls algal growth. This distinction shapes where cleanup efforts need to focus: reducing nitrogen runoff is the priority for the Gulf dead zone, while phosphorus reduction matters more for freshwater bodies like Lake Erie, which has its own recurring dead zone.
What Lives (and Dies) in a Dead Zone
Mobile species like adult fish and shrimp can detect dropping oxygen levels and swim away, but their displacement concentrates them into smaller areas of usable habitat, making them more vulnerable to predators and commercial fishing gear. Bottom-dwelling organisms that can’t move quickly, such as worms, clams, starfish, and young crabs, often die in place. The seafloor in a severe dead zone can become a biological desert, stripped of the small organisms that form the base of the food web.
Even after oxygen levels recover in the fall, the ecological damage lingers. Reproductive cycles get disrupted, juvenile fish lose critical nursery habitat, and the loss of bottom-dwelling organisms ripples up through the food chain. In the Baltic Sea, the expansion of dead zones has been directly linked to declining cod stocks, since cod eggs need oxygenated bottom water to develop. The fish don’t just need water they can survive in today; they need conditions that support reproduction season after season.