Healthy water contains oxygen; usually in concentrations above 6.5-8 mg/L. Aquatic organisms need this oxygen to breathe. The oxygen dissolved in water is necessary for fish, invertebrates, bacteria, and aquatic plants to survive. It is also required for the decomposition of all organic matter. If the oxygen content in water is depleted to a level lower than 2 mg/L there is not enough oxygen to support most marine life. This is a dead zone.
Dead zones are areas in the ocean (or a large lake or river) that do not have enough oxygen to support marine life. Dead zones can be found in the open ocean and in coastal areas; and there are several ways in which they form.
Dead zones can develop naturally based on water conditions: upwellings, changes in wind or circulation, topography, etc. In fact, the open ocean has several low oxygen areas. Wherever there is an aquatic area in which the water isn’t able to mix, it is unable to reoxygenate, and a dead zone will form. For example, the deepest part of the Black Sea has been depleted of oxygen for millennia.
In coastal areas, where mixing is rarely a problem, dead zones develop differently. When there is an increase in nutrients (usually nitrogen and phosphorus) in the water, massive amounts of algae or phytoplankton will grow creating an algal bloom. This can completely cover the surface of the water blocking sunlight from the plants below. When this additional surface plant life eventually dies off, it decomposes on the sea floor and bacterial decomposition further depletes the oxygen in the water to the point that it can no longer sustain most marine life.
Today, dead zones can be found all over the world, in North and South America, northern and southern Europe, Africa, China, Japan, Australia, New Zealand, and elsewhere.
This map shows known ocean dead zones as of 2008; red circles show location and size, black dots are dead zones of unknown size.
When a dead zone is formed, anything that can move away (fish, crabs, etc.), will do so. Immobile, or slow-moving, organisms (coral, sponges, etc.) that are unable to relocate will die without enough oxygen. Open ocean sharks, tuna, and swordfish have high metabolic rates (they require a large number of calories to keep their body functioning) which means higher oxygen needs, these species are particularly vulnerable to oxygen depletion.
Even a small decline in oxygen levels can have disastrous effects on marine life causing stunted growth, reproductive difficulties, increased disease rates and death. In addition to the direct reduction of marine biodiversity in these areas, when an ecosystem, like a coral reef, dies, that further reduces biodiversity by removing important habitat for a number of species. Overall, whether through disease, death, or relocation, a dead zone means a loss of biodiversity in the ecosystem.
To make matters worse, microbes that thrive in low oxygen environments can move in; thus producing and emitting nitrous oxide (a greenhouse gas 300x more powerful than CO2) at higher levels adding to global warming. In warmer waters, marine organisms breathe faster and use up oxygen more quickly; and so the cycle continues.
Sadly, the number of dead zones worldwide is growing. Between 1970 and 2010 the open ocean lost between 0.5 to 3.3% of oxygen overall. In 1950 there were 42 dead zones globally, in 2004 scientists counted 146, by 2008 there were 405, and today there are at least 500 near coastal areas worldwide. Dead zones now cover an area, if combined, of over 4.5 million square kilometers (1.7 million square miles), roughly the size of the European Union.
Although ocean dead zones can form naturally, human activity has certainly increased the size of naturally occurring ones and added many more oxygen depleted areas to the ocean. A number of factors can contribute to dead zone formation and the combination of these factors in some areas (especially coastal areas), means that oxygen levels decrease even faster.
Global fertilizer use in the last 70 years has increased tenfold. This has resulted in the near doubling of nitrogen and tripling of phosphorus in the water. Agriculture is the leading source of nutrient pollution (and, subsequently, ocean dead zones) in the United States and the European Union. Rains wash the nutrients used in fertilizers into rivers and streams which eventually make their way into the ocean. On average, 20% of nitrogen fertilizer is lost through surface run-off or into groundwater.
In South America, Asia, and Africa, urban wastewater, and not agricultural runoff, is often the primary contributor to ocean dead zones. This can include untreated wastewater flowing directly into the ocean or other water sources as well as animal waste (cow manure is rich in nutrients including nitrogen and phosphorus). Even in countries with strict waste treatment regulations, treated wastewater can still contain high levels of nutrients.
Vehicles, factories, and power plants are responsible for high levels of air pollution. Once nitrogen and other nutrients are in the atmosphere, they are absorbed by the ocean. On average, 60% of nitrogen fertilizer, used in agriculture, can vaporize into the atmosphere. In the Baltic Sea, the Chesapeake Bay in the United States, and the Yellow Sea in China, between 25-30% of nitrogen deposits come from air pollution.
Warmer water contains less oxygen than cold water; oxygen is less soluble in warm water. As ocean temperatures rise, oxygen levels decrease. In the open ocean, global warming, resulting from greenhouse gas emissions, is the prime cause of deoxygenation and dead zones increasing in size.
The nutrient pollution that causes ocean dead zones is a problem we can solve; they are reversible if the causes are reduced or eliminated. Of course, global action to address climate change and the current system of industrial agriculture are huge tasks that will help with dead zones. But even individual and local actions can help reduce nutrients reaching the ocean.
All water on Earth is connected; remember that no matter where you live, what you put in your water and into the air and ground around you affects the ocean. You can help to keep the ocean healthy by reducing your carbon footprint and by reducing your use of chemicals, both inside and outside your house.
Make your voice heard.
Support farms, businesses and organizations that are making positive changes for a healthy ocean.
There have been real successes in reversing ocean dead zones already; and conservationists are developing and practicing nature-based solutions to the problem as well.
River Thames, London, England
In 1957, the Natural History Museum declared the Thames River to be “biologically dead” as it could no longer support life. Since that time, with changes in water treatment and regulations on industrial dumping, water quality has improved. It is now the cleanest major city river in the world with over 400 species of invertebrate, 125 species of fish as well as thousands of seals, hundreds of porpoise, and even an occasional whale.
Chesapeake Bay, United States
For twenty years, recovery efforts have been underway to reduce dead zones in the Chesapeake Bay. Pollution reduction efforts and the addition of natural filters such as trees, grasses, and wetlands have started to pay off. In 2020, the dead zone was the second smallest it has been since monitoring began.
Black Sea Dead Zone Reduction
The Black Sea was once the world’s largest marine oxygen depleted area (15,000 square miles/nearly 39,000 square kilometers). In the 1990’s, after the dissolution of the Soviet Union, agricultural subsidies ended and chemical fertilizers spiked in price causing a 50% reduction in fertilizer runoff. Within three years, this dead zone largely disappeared. While this was an unintentional benefit, it taught valuable lessons to scientists and policymakers and gave hope to conservationists.
Oysters are filter feeders; one adult oyster can clean 50 gallons of water per day, removing nutrients, such as nitrogen, and returning cleaner water. Oyster gardening enlists people who live on coastal waters to grow oysters in mesh cages along their docks. Once grown, these oysters are transplanted on coastal reefs in protected areas. Not only does this help to create cleaner water, but it prevents erosion and naturally balances the ecosystem. Oyster gardens have been successfully running in coastal states across the United States and in Australia.
Seaweed not only absorbs nitrogen and phosphorus in the water, but also produces oxygen. This combination makes seaweed a powerful ally in the fight against ocean dead zones. Research into seaweed farms (aquaculture) has shown that seaweed could be strategically cultivated near dead zone regions and help to combat the problem.