What Are Ocean Dead Zones? Definition, Causes, and Impact

A dead zone is an area of ocean with very low oxygen levels. Throughout the world’s oceans, there are many dead zones where the majority of marine life cannot survive. These are the oceanic equivalent of a hot desert, with reduced biodiversity due to the extreme conditions.

While these dead zones can form naturally, the vast majority are linked to either agricultural practices on land or the effects of climate change.

Dead zones are bad news for marine biodiversity as they effectively destroy the ecosystem within an affected area. They also have the potential to destroy economies by impacting the availability of seafood as an income and food source. Around the world, it’s estimated that three billion people rely on seafood as their primary source of protein. 

How Does a Dead Zone Form in the Ocean? 

There are two main ways that a dead zone forms in the ocean:

Pollution

Our waterways are at risk of pollution from a wide range of sources, including fertilizers and pesticides from on-land agriculture. Other pollutants make their way into the ocean from stormwater and sewage. 

The National Oceanic and Atmospheric Administration (NOAA) estimates that 65% of coastal waters and estuaries around the contiguous U.S. are affected by excessive nutrients from land-based activities. The input of these nutrients starts a process known as eutrophication. 

When excess nutrients from pollution, including nitrogen and phosphorous, enter waterways, they stimulate the growth of algae. As a large amount of algae grows at the same time, an algal bloom is created. This then creates a drop in oxygen levels, which may create the conditions that lead to the formation of a dead zone.

Some algal blooms, including those containing cyanobacteria or blue-green algae, can also contain dangerous levels of toxins, at which point they’re classified as harmful algal blooms (HAB). As well as affecting the ocean, these blooms can wash up on shore and pose a danger to people and animals exposed to them.   

As the algal bloom dies, it starts to sink into deeper waters, where the decomposition of algae increases biological oxygen demand. In turn, this removes large amounts of oxygen from the water. It also increases the levels of carbon dioxide, which lowers the pH of seawater. 

Any mobile animal life within this oxygen-depleted, or hypoxic water, will swim away if they can. Immobile animal life dies, and as they decompose and are consumed by bacteria, the levels of oxygen in the water fall further. 

As the concentration of dissolved oxygen falls below 2ml per liter, the water is classed as hypoxic. Areas of the ocean that have undergone hypoxia are classified as dead zones. 

Climate Change

Scientists suggest that there are many different climate change variables that also have the ability to affect the formation of dead zones. These include changes to temperature, ocean acidification, storm patterns, wind, rain, and rising sea levels. It's thought that these variables act together to contribute to the increase seen in the number of dead zones globally. 

Warmer waters hold less oxygen, so dead zones can form more easily. These higher temperatures also reduce oceanic mixing, which can help bring additional oxygen into depleted areas.  

Dead zones can form seasonally, as factors like mixing of the water column change. For example, the Gulf of Mexico dead zone tends to start forming in early spring and dissipate in the fall as the water column undergoes increased mixing during the stormy season.  

The Impact of Dead Zones

While dead zones have been a feature of our oceans for millions of years, they are getting worse.

Researchers have found that over the last 50 years, there’s been a 2% decrease in the levels of dissolved oxygen in the open ocean. This is expected to become a 3% to 4% decrease by 2100 if action isn’t taken to reduce oceanic pollution as well as the impacts of climate change like increased atmospheric greenhouse gases.

As dead zones form in the ocean, they have the potential to impact the overall health of these waters, as well as the animals and people who rely on them. 

Environmental Impacts 

Fish and other mobile species will usually swim out of a dead zone, leaving behind immobile species including sponges, corals, and mollusks like mussels and oysters. As these immobile species also need oxygen to survive, they will slowly die. Their decomposition adds to the low oxygen levels already present.

Hypoxia—insufficient levels of oxygen—acts as an endocrine disruptor in fish, affecting their reproductive abilities. Low oxygen levels have been linked to reduced gonadal development as well as reduced sperm and egg quality, fertilization rates, hatching success, and the survival of fish larvae. Mollusks, crustaceans, and echinoderms are less sensitive to low oxygen levels than fish, but dead zones have been linked to reduced growth in brown shrimp.

Loss of oxygen in the deep ocean can lead to the increased production of the greenhouse gases nitrous oxide, methane, and carbon dioxide. During oceanic mixing events, these may reach the surface and be released.  

Researchers also suspect that the presence of dead zones may be linked to the mass death of coral reefs in affected areas. The majority of reef monitoring projects don’t currently measure oxygen levels, so the effect of dead zones on coral reef health is likely to be underestimated currently. 

Economic Impacts  

For fishermen who rely on the ocean to provide a livelihood, dead zones cause problems because they have to travel further from shore to try and find areas where fish congregate. For some small boats, this additional mileage is impossible. The extra costs for fuel and staffing also make traveling greater distances impractical for some boats.

Larger fish like marlin and tuna are extremely sensitive to the effects of low oxygen, so may leave their traditional fishing grounds, or be forced into smaller surface layers of more oxygen-rich water. 

Scientists at NOAA estimate that dead zones cost the U.S. seafood and tourism industries around $82 million each year. For example, the dead zone in the Gulf of Mexico has an economic impact on the fishing industry by increasing the price of larger brown shrimp, as these are less commonly caught in the dead zone compared to smaller shrimp.  

Can Dead Zones Recover?

The overall number of oceanic dead zones has been increasing steadily and there are now four times the number of dead zones compared to the 1950s. The number of coastal dead zones with nutrient runoff, organic matter, and sewage as the main cause has increased tenfold.

The good news is that certain dead zones can recover if actions are taken to control the impacts of pollution. Dead zones formed through the effects of climate change may be harder to resolve, but their size and impact can be slowed down. 

One well-known example of dead zone recovery is the Black Sea dead zone, which was once the largest in the world but recovered as the use of expensive fertilizers was drastically reduced after the collapse of the Soviet Union in 1991. 

When countries surrounding the River Rhine in Europe agreed to take action, the levels of nitrogen entering the North Sea were reduced by 37%. 

As countries start to realize the vast negative impact that dead zones can have, a variety of measures are being implemented to reduce their occurrence.

Shellfish Aquaculture and Nutrient Removal

Bivalve mollusks like oysters, clams, and mussels can play an important role in the removal of excess nutrients, as they filter these out of the water in a process known as bioextraction.

Research carried out by NOAA and EPA found that cultivating these mollusks through aquaculture can offer not only improved water quality but also provides a sustainable source of seafood.   

Best Management Practices

The EPA publishes nutrient reduction strategies designed to promote best practices when it comes to reducing the levels of nitrogen and phosphorus. These vary by state but include actions like limiting the levels of specific ingredients in fertilizers, implementing appropriate stormwater management practices, and using agricultural best practices to reduce the pollution of waterways with nitrogen and phosphorus.  

Efforts to conserve wetlands and floodplains are also important. These habitats help to absorb and filter excess nutrients before they reach the oceans.