Dead zones are low-oxygen, or hypoxic, areas in the world’s oceans and lakes. Because most organisms need oxygen to live, few can survive in hypoxic conditions. That is why these areas are called dead zones.

Dead zones occur because of a process called eutrophication, which is when a body of water has too many nutrients, such as phosphorus and nitrogen. At normal levels, these nutrients feed the growth of microscopic organisms called algae, including one type called cyanobacteria or blue-green algae. With too many nutrients cyanobacteria grow out of control, which can be harmful. Human activities are the main cause of excess nutrients washing into the ocean. These activities first wash the excess nutrients into rivers and streams, which then carry the nutrients to large bodies of water. As a result, dead zones are often located near coastlines rather than far offshore.

Understanding the eutrophication process provides the clearest picture of how and why dead zones develop.

Causes of Eutrophication

Eutrophic events have increased because of the rapid rise in intensive agricultural practices, industrial activities and improperly treated sewage, all of which are compounded by human population growth. These processes release large amounts of nitrogen and phosphorous, which then enter the air, soil and water. Human activities produce much more of these nutrients than natural processes.

Different activities vary in how much they contribute to eutrophication. In places where agriculture is a significant part of the local or regional economy, heavy use of animal manure and commercial fertilizer is often the main contributor to eutrophication. Rain and irrigation practices wash runoff from large agricultural fields into creeks and bays. The nutrients then settle near the coasts of large bodies of water.

Improperly treated wastewater from sewage is another source of eutrophication. Some water treatment facilities dump wastewater with harmful levels of nutrients into creeks, rivers, lakes or the ocean. However, improvements in wastewater treatment have reduced nutrient pollution where sewage is the dominant cause, such as in the Baltic Sea.

Atmospheric sources of nitrogen also contribute to eutrophication in some areas of the world. Fossil fuels and fertilizers release nitrogen into the atmosphere. This atmospheric nitrogen is then redeposited on land and in water either through precipitation or by gravity pulling nitrogen particles down.

In certain regions, all of these forms of nutrient pollution come together and compound the issue of eutrophication. This is the case in the coastal waters of China. Many of its coastlines have excess nitrogen, which has a variety of sources. One source of excess nitrogen is farms that use manure and fertilizer, while another is improperly treated human sewage. Burning fossil fuels to power industry or vehicles also creates a significant amount of nitrogen. No matter the source, the excess nitrogen eventually settles into the water system. The nutrient output from all these causes has increased in recent years following pressure to optimize agricultural output to feed China’s growing population. While this effort has largely been an agricultural and social success, it has also increasingly damaged the environment.

Another example of nutrient pollution is the Chesapeake Bay on the East Coast of the United States. Scientists have been monitoring this area since 1985. Development of the land for cities, towns and agriculture is responsible for the bay’s high levels of nitrogen. The watershed that feeds into the bay includes major metropolitan areas, such as Washington, D.C., which have large populations that contribute considerably to nutrient pollution. Poultry farming is also a major industry near part of the bay, and manure from those operations further pollutes the bay.

Since 1967, the Chesapeake Bay Foundation has led a number of programs that aim to improve the bay’s water quality and curb pollution runoff. However, the Chesapeake still has a dead zone. The continued pollution has harmed the waters and put the traditions and cultural practices of Indigenous people of Chesapeake Bay, such as the Rappahannock Nation, at risk. Indigenous communities are advocating for greater control over water management practices of the bay, given their history of success in environmental protection, like restoring fish populations.

Climate change is likely to worsen dead zones across the world. Scientists say that climate change has resulted in heavier rains and more severe storms. These heavy rains increase nutrient runoff, flooding low-oxygen areas at the coast with even more nutrients. Ocean animals also require more oxygen when temperatures are high, so as climate change makes the atmosphere and oceans warmer, marine animals’ demand for oxygen will increase as well.

Eutrophication and the Environment

The eutrophication process has severe environmental impacts. Dead zones result from these impacts, which include algal blooms and hypoxia.

Algal Blooms

Phosphorous, nitrogen and other nutrients increase the productivity, or fertility, of marine ecosystems, causing algae to grow quickly and excessively at the water’s surface. This rapid development of algae is called an algal bloom. Algae species, such as cyanobacteria, dinoflagellates and diatoms, make up these blooms. Because algal blooms can discolor the water in which they form, they are sometimes called “red tides” or “brown tides,” depending on the color of the algae. Algal blooms can last anywhere for a few days up to a year. They can also subside for a time and then come back later on. Under the right conditions, algal blooms can create dead zones beneath them.

Algal blooms prevent light from penetrating the water’s surface, which can harm underwater plants that need sunlight to survive. Algal blooms also prevent organisms beneath them from absorbing oxygen. Oxygen is necessary for almost all aquatic life, from sea grasses to fish, and thick blooms can even stop fish from breathing by sticking in their gills. By depriving organisms of sunlight and oxygen, algal blooms negatively impact a variety of species that live below the water’s surface. The number and diversity of benthic, or bottom-dwelling, species are especially reduced. Algal blooms can also lead to human illness. Shellfish, such as oysters, are filter feeders. As they filter water, they absorb toxins produced by algae. People can become sick or even die from shellfish poisoning. Even when algae isn’t toxic to humans, it can still cause irritation and make people sick. In 2025, South Australia experienced one of the most serious algal blooms in their waters. Surfers reported coughing, eye irritation and other issues after unknowingly swimming in the affected waters during the early days of the bloom.

Algal blooms can also lead to the death of marine mammals and shore birds that rely on the marine ecosystem for food. Many birds depend on fish for survival. With fewer fish beneath algal blooms, these animals lose an important food source. Other species are at risk as well. When marine mammals, like sealions, eat fish that have ingested toxins from the algal blooms, they may suffer neurological injury, begin acting strangely and can die. During the 2025 South Australian bloom, thousands of marine animals from more than 450 different species died, including large animals like sharks. The bloom was particularly harmful to reef ecosystems, which are home to marine organisms that are less mobile, such as sponges and shellfish.

Algal blooms can also impact aquaculture, or the farming of marine life, and the fishing industry. One red tide event wiped out over 80% of the entire stock of Hong Kong’s fish farms in 1998. The 2025 South Australian bloom significantly decreased the output of the Australian fishing industry. People working in the fishing industry reported that the squid catch was particularly low, damaging businesses that revolved around calamari sales and putting livelihoods at risk. For example, third-generation Australian fisherman Nathan Eatts reported in summer of 2025 that he hadn’t caught a squid in three months because of the bloom.

Hypoxia

Hypoxia occurs when the oxygen levels in a body of water fall low enough to harm or kill aquatic life. Hypoxia events often follow algal blooms. As the algae from a bloom die off, they decompose. The decomposition process uses up much of the available oxygen. This lack of oxygen creates dead zones in which most aquatic species cannot survive.The Gulf of Mexico has a seasonal hypoxic zone that forms every year in summer. Its size varies each year, but the five-year average size is currently about 11,136 square kilometers (4,300 square miles). Concern over its large size led to the formation of the Mississippi River/Gulf of Mexico Task Force in 1997, which involves members from five U.S. federal agencies, 12 states and several tribes within the watershed. The task force’s mission today is to reduce the five-year running average of the Gulf of Mexico dead zone to less than 4,920 square kilometers (1,900 square miles) by 2035.

One of the world’s largest dead zones is in the Baltic Sea, spanning about 70,000 square kilometers (27,000 square miles). Runoff from agricultural fertilizers and sewage is the main cause of the eutrophication process in this region. Overfishing of Baltic cod has intensified the problem. Cod eat sprats, a small, herring-like species that eat microscopic zooplankton, which in turn eat algae. Fewer cod and more sprats mean more algae and less oxygen. The dead zone also reaches the cod’s deep-water breeding grounds, further endangering the species.The Baltic Sea has been targeted by legislation like the Baltic Sea Action Plan of the Helsinki Commission for the Protection of the Baltic Sea (HELCOM), which aims to combat pollution, reduce dead zones and promote sustainable use of marine resources. The initiative brings together regional and national partners, including Denmark, Estonia, the European Union, Finland, Germany, Latvia, Lithuania, Poland, Russia and Sweden.

History of Identifying Eutrophic Systems

Scientists began noting the existence of eutrophication in the 1970s. By 1985, there was a major study of the Gulf of Mexico that measured the dead zone there for the first time. In 1998, scientist Nancy N. Rabalais, who led the team that completed the 1985 study, testified about the dangers of hypoxia in front of the United States Congress. As a result, the Harmful Algal Bloom and Hypoxia Research and Control Act of 1998 was passed to fund more research into the phenomenon. Since then, scientists have identified at least 500 dead zones worldwide. Hypoxic areas have increased dramatically. Estimates show that there are 10 times more low-oxygenated water areas across the world today than there were in 1950.

Efforts to Reverse Hypoxia

Organizations and governments are working to reverse hypoxic systems. A “system in recovery” is one that once exhibited hypoxia but is now improving. One example of a system in recovery is the Black Sea. This body of water, bordered by Europe and Asia, has a natural dead zone at its lower depths, with oxygenated water near the surface. But the Black Sea has experienced artificial hypoxic events as well. Before the 1990s, runoff from farms flooded the sea with fertilizer.

After the collapse of the Soviet Union, fertilizer prices increased, forcing farmers to use less and improving the conditions of the Black Sea. While the Black Sea still faces problems from plastic pollution—and always has its naturally occurring dead zone—it is now in a state of recovery thanks to international conservation efforts to change agricultural practices.

Other systems in recovery, like the Boston Harbor in the United States, have also improved water quality. These are the results of better industrial and wastewater controls, such as improving and upgrading water treatment and discharging waste farther out to sea.

Local, regional and national actions are needed to put a eutrophic system into recovery. One such action is the Clean and Healthy Ocean Integrated Program, which Thailand developed as part of a global effort to make on-land changes to decrease hypoxia in marine waters. Thailand has taken this step to improve and provide long-term stability for its fishing industry as well as preserve important habitats.

More local actions combined with global partnerships are needed to stop hypoxic systems from worsening. Individuals can help decrease the amount of nutrients that go into the waterways too. Farmers and homeowners can plant grasses, trees and shrubs around fields or lawns where they use fertilizer. These additions help absorb some of the nutrients in the fertilizer before they leach into waterways. Even cleaning up pet waste instead of letting it flow into storm drains helps reduce nutrient pollution.