Dead zones are areas in the world's oceans and large lakes where oxygen levels are very low. These areas are called hypoxic, which means the amount of dissolved oxygen (DO) is 2 milligrams of oxygen per liter of water or less. When water becomes hypoxic, plants and animals in the water change their behavior to find areas with more oxygen. If oxygen levels drop below 0.5 milligrams of oxygen per liter, many aquatic animals die because they cannot survive in such conditions. These areas can no longer support life. In the past, some dead zones formed naturally. However, in the 1970s, scientists noticed more dead zones appearing, especially near coastlines where many aquatic animals live.

Coastal areas such as the Baltic Sea, the northern Gulf of Mexico, and the Chesapeake Bay, as well as large lakes like Lake Erie, have been affected by low oxygen levels caused by a process called eutrophication. Eutrophication happens when too many nutrients, such as nitrogen and phosphorus, enter water systems through rivers. These nutrients come from sources like urban and agricultural runoff and are made worse by deforestation. The extra nutrients increase the growth of plants and algae, which eventually sink to the bottom of the water. When these materials are broken down by organisms, oxygen is used up, leading to hypoxia or anoxia (no oxygen at all).

According to a 2004 report by the UN Environment Programme, there were 146 dead zones in the world's oceans where marine life could not survive because of low oxygen levels. Some of these areas were as small as one square kilometer (0.4 square miles), while the largest covered 70,000 square kilometers (27,000 square miles). A study from 2008 found 405 dead zones around the world.

Causes

Aquatic and marine dead zones can form when there is an increase in nutrients, especially nitrogen and phosphorus, in the water. This process is called eutrophication. These nutrients are essential for the growth of single-celled, plant-like organisms that live in water. Normally, the growth of these organisms is limited by the amount of nutrients available. When more nutrients are present, these organisms, such as algae and cyanobacteria, can grow rapidly. This rapid growth is called an algal bloom.

Limnologist David Schindler, who studied the Experimental Lakes Area and helped ban harmful phosphates in detergents, warned about the dangers of algal blooms and dead zones.

The main types of algae include cyanobacteria, green algae, dinoflagellates, coccolithophores, and diatom algae. An increase in nitrogen and phosphorus often leads to cyanobacteria blooms. Other types of algae are eaten by animals and do not build up as much. Cyanobacteria are not easily digested by zooplankton or fish. When they die, they sink to the bottom and decompose. The bacteria that break them down use oxygen from the water, creating low oxygen levels, or hypoxia.

Dead zones can form from both natural and human causes. Natural causes include coastal upwelling, changes in wind patterns, and water movement. Other factors that influence dead zones include long water residence times, high temperatures, and strong sunlight penetration through the water.

Natural ocean patterns can also reduce oxygen in parts of the water. For example, enclosed water bodies like fjords or the Black Sea have shallow entrances that trap water for long periods. The eastern tropical Pacific Ocean and northern Indian Ocean have areas with very low oxygen levels, called oxygen minimum zones (OMZ). These zones are often permanent or semi-permanent.

Sediment layers near the mouth of the Mississippi River show evidence of four hypoxic events before synthetic fertilizers were used. These layers contain remains of species that can survive without oxygen. These periods match historical records of high river flow in Vicksburg, Mississippi.

Changes in ocean circulation caused by climate change may worsen oxygen loss in the ocean.

Human causes of dead zones include the use of chemical fertilizers, which enter water through runoff and groundwater. Other causes include direct sewage discharge into rivers and lakes, and nutrients from large amounts of animal waste. Fertilizer use is the leading human cause of dead zones worldwide. Runoff from sewage, urban areas, and fertilizers can also contribute to eutrophication.

In August 2017, a report stated that the US meat industry and agricultural system are largely responsible for the largest dead zone in the Gulf of Mexico. Soil runoff and nitrate leaching, worsened by farming practices and manure and fertilizer use, polluted water from the Heartland to the Gulf. Much of the plant waste from crops in this region is used as animal feed for meat production companies, such as Tyson and Smithfield Foods. Over 86% of livestock feed is not edible for humans.

Notable dead zones in the US include the northern Gulf of Mexico near the Mississippi River, coastal areas of the Pacific Northwest, and the Elizabeth River in Virginia Beach. These areas have recurring dead zones. Globally, dead zones have formed in regions like the Baltic Sea, Kattegat, Black Sea, Gulf of Mexico, and East China Sea. These areas are important for fishing.

Types

Dead zones can be grouped into different types based on how long they last:

• Permanent dead zones are found in deep water and usually have less than 2 milligrams of oxygen per liter.
• Temporary dead zones are short-lived, lasting only hours or days.
• Seasonal dead zones occur each year, most often during the warm summer and autumn months.
• Diel cycling hypoxia is a type of seasonal dead zone that only becomes low in oxygen during nighttime hours.

The type of dead zone can also be grouped based on how long it takes for water to return to a healthy state. This time depends on the level of eutrophication and how much oxygen is missing from the water. A water body that becomes completely oxygen-free and loses many types of living organisms will need a longer time to recover. A water body that only has slightly low oxygen levels and still has a variety of living organisms will recover more quickly.

Effects

The most noticeable effects of eutrophication include algae blooms, which can be harmful, loss of different types of life, and very low oxygen levels (anoxia), which can cause large numbers of aquatic animals to die.

In areas with very low oxygen (dead zones), marine life is often limited. Many fish and mobile animals leave these areas as oxygen levels drop, and organisms living on the ocean floor (benthic populations) may suffer serious losses when oxygen drops below 0.5 mg per liter. In extreme cases with no oxygen (anoxia), bacteria communities change, with more anaerobic organisms growing and fewer aerobic microbes. These microbes may use other sources of energy, such as nitrate, sulfate, or iron. Sulfur reduction is especially concerning because it produces hydrogen sulfide, a toxic substance that harms most life in the area and increases the risk of death.

Low oxygen levels can make it hard for organisms to survive, especially when oxygen drops below lethal levels. Studies along the Gulf Coast of North America show that low oxygen reduces the reproduction and growth rates of many organisms, including fish and benthic invertebrates. When oxygen levels fall below 2 mg per liter, many organisms leave the area. Those that stay may show signs of stress and eventually die. Organisms that can survive in low-oxygen environments often have traits that help them, such as using oxygen more efficiently, growing more slowly, or relying on processes that do not require oxygen.

Periodic oxygen shortages, like those in seasonal dead zones or daily cycles, disrupt benthic communities. Over time, these conditions reduce species diversity through mass deaths. Recovery of benthic communities depends on nearby areas providing larvae for new growth. This often leads to communities dominated by species that grow quickly and have simple life strategies, changing the original makeup of the area.

The impact of dead zones on fishing and other ocean activities depends on how long they last and where they occur. Dead zones often reduce biodiversity and harm benthic populations, lowering the variety of fish available for fishing. However, in some cases, eutrophication can temporarily increase the number of certain fish, like anchovies, due to more nutrients. Studies suggest that the overall loss of productivity from dead zones outweighs any short-term gains. For example, dead zones in the Gulf of Mexico are estimated to have caused the loss of 17,000 metric tons of carbon, which is important for fish food. Hypoxia also worsens problems like the spread of invasive species and diseases in oysters, harming both the economy and ecosystems.

In the last 20 years, there has been a sharp rise in mass deaths caused by low oxygen levels, especially in coral reefs. Warmer water increases oxygen demand and speeds up ocean deoxygenation, creating large dead zones. Coral reefs respond to low oxygen based on how long and how severe the deoxygenation is. Effects may include slower growth, reduced photosynthesis, or coral bleaching. Low oxygen also increases algae growth and spreads coral diseases. Algae can survive in low-oxygen conditions better than coral, so more algae may grow where coral and algae live together, leading to more coral deaths. Coral diseases spread easily in hypoxic areas with high sulfide levels. This creates a cycle where coral reefs die, and fish and other marine life change their behavior, such as moving to areas with more oxygen or slowing their activity. Invertebrates may leave their homes and move to the surface of coral or to the tips of coral structures.

About six million people, mostly in developing countries, rely on coral reef fishing. Large-scale coral deaths from extreme low-oxygen events can severely harm reef fish populations. Coral reefs provide important services like protecting shorelines, fixing nitrogen, absorbing waste, and supporting tourism. The loss of oxygen in ocean areas near coral reefs is a serious issue because it takes many years for corals to recover.

Although most life forms die in low-oxygen areas, jellyfish can survive and often grow in large numbers in dead zones. Jellyfish blooms produce a lot of mucus, which changes ocean food webs because few animals eat them. Bacteria break down the mucus and release carbon dioxide into the atmosphere, a process called the "jelly carbon shunt." Research is studying how human activities may increase jellyfish populations in dead zones, as low oxygen can reduce competition and predators for jellyfish. More jellyfish could harm fisheries, damage fishing gear, and reduce tourism in coastal areas.

Seagrass is declining globally. About 21% of the 71 known seagrass species are losing population, and 11% are listed as threatened. Hypoxia caused by eutrophication from ocean deoxygenation is a major reason for these declines. While eutrophication can increase nutrient availability and boost seagrass growth, too much nutrient enrichment can cause overgrowth of algae and other plants, leading to low oxygen levels.

Seagrass both produces and absorbs oxygen in water and sediment. At night, seagrass oxygen levels are closely linked to oxygen in the water. Low oxygen in the water can cause seagrass tissues to become hypoxic and eventually die. Normally, seagrass gets oxygen through photosynthesis or by absorbing it from water through leaves to roots. However, changes in oxygen balance can lead to hypoxic seagrass tissues. Seagrass exposed to low-oxygen water shows increased respiration and reduced growth.

Locations

In the 1970s, scientists first noticed marine dead zones in areas where human activity was heavy. These included the Chesapeake Bay on the U.S. East Coast, the Kattegat Strait in Scandinavia, which connects to the Baltic Sea, other important fishing areas in the Baltic Sea, the Black Sea, and the northern Adriatic.

Other dead zones later appeared in coastal waters of South America, China, Japan, and New Zealand. A study from 2008 found 405 dead zones around the world.

Researchers from the Baltic Nest Institute reported in a scientific journal that dead zones in the Baltic Sea have grown from about 5,000 square kilometers to more than 60,000 square kilometers in recent years.

Some causes of the increase in dead zones include the use of fertilizers, large farms with many animals, burning fossil fuels, and waste from municipal wastewater treatment plants.

The Baltic Sea is large, so it is easier to study it in smaller parts rather than as one whole area. In 2004, scientists divided the Baltic Sea into nine sub-areas: the Gulf of Bothnia, Archipelago region, Gulf of Finland, Gulf of Riga, Gulf of Gdansk, Swedish East Coast, Central Baltic, Belt Sea region, and Kattegat. Each sub-area has unique features and has responded differently to pollution and over-nutrition. However, some general patterns and solutions apply to the entire Baltic Sea. Researchers Rönnberg and Bonsdorff explained that while the effects of pollution vary in different areas, the main sources of pollution—such as agriculture, industry, sewage, and transportation—are similar throughout the region. Nitrogen from air pollution and local sources like farming and forestry also contribute.

In general, all parts of the Baltic Sea face similar human-caused problems. Rönnberg and Bonsdorff noted that eutrophication, or over-nutrition, is a serious issue in the Baltic Sea. However, efforts to improve water quality may need to be tailored to each area’s specific needs.

According to National Geographic, the Chesapeake Bay was one of the first areas where low oxygen levels were identified in the 1970s. The Bay experiences seasonal low oxygen due to high nitrogen levels. These nitrogen levels come from urban development, factories that release nitrogen into the air, and agriculture, including poultry farming on one side of the Bay, which produces manure that runs into the water.

From 1985 to 2019, people who care for the Chesapeake Bay worked to reduce the size of the low-oxygen area. There was progress in 2016–2017, but recent data shows more work is needed to address the effects of global warming.

The Elizabeth River estuary is used for commercial and military purposes and is a major port on the U.S. East Coast. Between 2015 and 2019, 11 conditions were measured in different parts of the river. High levels of nitrogen, phosphorus, and other pollutants were found throughout the river, harming bottom-dwelling animals. Pollution from military and industrial activities in the 1990s was the main cause. In 1993, the Elizabeth River Project began to restore the river. The group focused on the Mummichog fish, which had been heavily affected by pollution, and removed thousands of tons of contaminated sediment. In 2006, Maersk-APM, a shipping company, planned to build a new port and partnered with the Elizabeth River Project to restore Money Point, an area polluted by a substance called creosote. The company provided $5 million for the project. By 2012, they restored over 7 acres of tidal marsh, 3 acres of oyster reef, and created new shoreline. In 2019, the Money Point Project won the "Best Restored Shore" award.

A seasonal dead zone exists in the central part of Lake Erie, stretching from east of Point Pelee to Long Point and reaching shores in Canada and the United States. Between July and October, the dead zone can grow to cover 10,000 square kilometers. Excess phosphorus from agricultural runoff causes algae to grow rapidly, leading to low oxygen levels. Phosphorus comes from both nonpoint sources, like urban and agricultural runoff, and point sources, such as sewage and wastewater treatment plants. The dead zone was first noticed in the 1960s during a period of severe over-nutrition. In the 1970s, Canada and the U.S. worked to reduce pollution to reverse the growth of the dead zone. Scientists in 2018 said phosphorus runoff must decrease by 40% to prevent dead zones. The fishing industry has been greatly affected, and in 2021, a large number of freshwater drum fish died due to low oxygen levels. Water from the lake is used for drinking, but it sometimes smells bad and changes color during the summer when the dead zone is active.

A dead zone exists in the Lower St. Lawrence River, from east of the Saguenay River to east of Baie Comeau, and is most severe at depths over 275 meters. Scientists in Canada are concerned about the impact on fish in the area.

A hypoxic zone covers the coasts of Oregon and Washington and reached its largest size in 2006, covering over 1,158 square miles. Strong winds from April to September cause upwelling, which brings nutrients to the surface and increases algae growth, making the low oxygen levels seasonal. The zone has lower temperatures, and some sea creatures move away, while others die from lack of oxygen. In 2009, scientists found thousands of dead crabs, worms, and sea stars on the seafloor. In 2021, $1.9 million was spent to monitor and study the hypoxic conditions in the area.

The largest recurring dead zone in the United States occurs off the coast of Louisiana in the Gulf of Mexico each summer. It forms due to summer warming, water movement, wind, and high freshwater flow from the Mississippi River, which drains 41% of the U.S. The river carries high levels of nutrients like nitrates and phosphorus into the Gulf, causing the dead zone.

Energy Independence and Security Act of 2007

The Energy Independence and Security Act of 2007 requires the production of 36 billion gallons (140,000,000 cubic meters) of renewable fuels by 2022, including 15 billion gallons (57,000,000 cubic meters) of corn-based ethanol. This would triple the current production of ethanol and require more corn to be grown. However, this plan creates a new problem: increased corn production leads to more nitrogen runoff. Nitrogen, which makes up 78% of Earth’s atmosphere, is an inert gas, but it can change into reactive forms, such as nitrate and ammonia, which are used to make fertilizer.

Fred Below, a professor of crop physiology at the University of Illinois at Urbana-Champaign, explains that corn needs more nitrogen-based fertilizer because it produces more grain per unit area than other crops and depends completely on nitrogen already in the soil. A study published on March 18, 2008, in Proceedings of the National Academy of Sciences found that increasing corn production to meet the 15-billion-gallon ethanol goal would raise nitrogen levels in the Dead Zone by 10–18%. This would double the nitrogen levels currently recommended by the Mississippi Basin/Gulf of Mexico Water Nutrient Task Force, a group of federal, state, and tribal agencies that has monitored the Dead Zone since 1997. The task force states that reducing nitrogen runoff by 30% is needed to shrink the Dead Zone.

Reversal

The recovery of benthic communities depends on how long and how severe the low-oxygen conditions are in the area. If the oxygen loss is mild and short-term, benthic communities can recover quickly because larvae from nearby areas can move in and regrow the population. However, if low-oxygen conditions last a long time and are very severe, it takes much longer for the communities to return. Recovery also depends on how much the water layers are separated, called stratification. In warmer waters with strong stratification, recovery is harder because these areas are more likely to experience long-term low-oxygen conditions and are more vulnerable to problems caused by too many nutrients. As ocean temperatures rise, these challenges may make it harder to restore dead zones in the future.

Smaller low-oxygen areas that are surrounded by healthy ecosystems are more likely to recover after the flow of nutrients that caused the problem stops. However, large low-oxygen areas may also recover over time, sometimes taking up to 10 years. For example, the Black Sea dead zone, once the largest in the world, shrank significantly between 1991 and 2001 after the cost of using fertilizers increased following the end of the Soviet Union and changes in economic systems in Eastern and Central Europe. Fishing has since become a major activity in the region again.

The Black Sea’s improvement happened by accident, as fertilizer use dropped. However, the United Nations has supported other efforts to reduce low-oxygen zones by cutting pollution from large industries. Between 1985 and 2000, the North Sea dead zone saw a 37% drop in nitrogen pollution after countries along the Rhine River reduced sewage and industrial waste. Similar efforts have also helped clean up areas along the Hudson River and San Francisco Bay.

Modelling

Mathematical and computational models are an important part of studying dead zones. These models help leaders understand how different factors affect dead zones. Scientists use models to track how nutrients like nitrogen and phosphorus move through water bodies and predict how these nutrients influence algal blooms and oxygen levels. Models also help predict how applying nitrogen and phosphorus to fields affects the amount of runoff and nutrients that enter nearby water bodies.