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Ocean Dead Zones: What Are They and Can Dead Zones Recover?

April 19, 2022
Time to read: 7 minutes
Credit: NOAA

The ocean might be the last thing on your mind when eating a hamburger or chicken nugget, but what we eat is more closely connected to the health of our oceans than many people realize. Factory farming—the predominant method of meat, milk, and egg production—is damaging habitats around the world, and marine ecosystems are no exception. 

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What Is a Dead Zone in the Ocean?

A dead zone, also known as an oxygen minimum zone,  is a large expanse of water in which there is not enough dissolved oxygen for animals and plants to survive. As oxygen levels in the water decrease, marine animals either escape or die, leaving a biodiverse ecosystem virtually lifeless. A well-known example of a dead zone is the vast stretch of hypoxic (low-oxygen) water found in the Gulf of Mexico, which covered 6,300 square miles according to the most recent measurement.

What Are the Types of Dead Zones?

An area of water may be hypoxic for hours, days, weeks, months, or even years. There are four main types of dead zones: permanent, temporary, seasonal, and diel-cycling. 

Permanent Dead Zones 

Permanent dead zones are found in deep water where oxygen levels remain low throughout the year and mostly stay below 2 milligrams of dissolved oxygen per liter of water. 

Temporary Dead Zones

Temporary dead zones, as the name suggests, are areas that only stay hypoxic for a short period of time, usually for hours or days. 

Seasonal Dead Zones 

Seasonal dead zones, the most common type of dead zone, form once each year, generally when temperatures are warmer and increased rainfall has washed more nutrients into waterways.

Diel-Cycling Hypoxia

Diel-cycling hypoxia is when an area is hypoxic at night and in the early hours of the morning, typically during the warmer months of the year. In shallow waters with a lot of plants, where these dead zones are commonly found, oxygen levels can change rapidly.  

What Causes Ocean Dead Zones?

Ocean dead zones are largely caused by a process known as “eutrophication,” which begins with nutrient pollution in the water. High concentrations of the nutrients nitrogen and phosphorus create algal blooms—the rapid and expansive growth of algae, cyanobacteria, or phytoplankton—which cover the surface of the water and stop sunlight from reaching the animals and plants below. The algal blooms eventually die off and sink to the bottom of the ocean, where they decompose using dissolved oxygen from the water, resulting in low-oxygen conditions that suffocate marine life. 

Agricultural Runoff

Agriculture is by far the biggest cause of nutrient pollution, responsible for 78 percent of freshwater and ocean eutrophication globally. Intensive farming practices, particularly in animal agriculture, lead to excessive quantities of nitrogen and phosphorus from chemical fertilizers and animal manure entering agricultural runoff. From there, the nutrients flow into waterways and can end up in coastal waters. Nitrogen and phosphorus fertilizers used to grow feed for farmed animals on factory farms, in particular corn and soy, are significant contributors to dead zones in the Gulf of Mexico.

Vehicular Emissions

After agriculture, road traffic is a key source of nutrient loading. Car and other vehicle exhausts release large amounts of nitrogen and phosphorus gasses. Significant quantities of these vehicular emissions eventually reach coastal waters through either urban runoff or atmospheric deposition. 

Industrial Emissions 

Major industrial sources of nitrogen emissions that lead to nutrient pollution include concentrated animal feeding operations (CAFOs), more commonly known as factory farms, and power plants where coal or other fossil fuels are burned to generate electricity. Like vehicular emissions, nutrient gasses from these sources can enter surface waters via atmospheric deposition.

Climate Change 

As global temperatures rise, dead zones are set to become an even bigger problem than they are now. Climate change driven by greenhouse gasses—which are produced in large quantities by animal agriculture—contributes to eutrophication in numerous ways. 

Not only does more intense rainfall increase agricultural runoff, but increased levels of carbon dioxide in the air and water fuel the growth of algae. Warmer temperatures also promote the formation of harmful algal blooms. 

In addition, climate change is causing global sea temperatures to rise, and warmer water holds lower amounts of dissolved oxygen. Overall, the open ocean has lost at least 77 billion tons of oxygen since 1950, much of it due to global warming.  

The ocean also absorbs 30% of the CO2 that is released into earth’s atmosphere, which is leading to a phenomenon known as ocean acidification. Increasing ocean acidification is also linked to harmful algal blooms. 

Natural Factors

While dead zones are mostly driven by human activity, they can form naturally as a result of physical, chemical, and biological processes. One example of a natural factor that can cause water hypoxia is the seasonal current upwelling of deep ocean water that is low in oxygen and contains high levels of nutrients. 

What Are the Effects of Dead Zones?

When dead zones or any other environmental issues are discussed, the focus is most often on how humans will be affected. But while the impacts we face are significant, the more immediate threat is often to the wild animals we share our planet with. 

To fish, crabs, and other aquatic animals, hypoxic waters can be deadly. Although we tend to think of these beings in terms of numbers, the loss of individual animal lives because of human action is not just a conservation issue but a moral one. 

Environmental Impacts of Dead Zones

Dead zones have severe impacts on underwater biodiversity and precious marine ecosystems such as coral reefs. Very low oxygen levels can kill or displace fish of most species, resulting in the decline of ecologically important fish populations, as well as disrupting important marine food webs. Meanwhile, with a lack of predators, the few aquatic animals who can tolerate low-oxygen conditions— such as jellyfish and some species of squid—can become overabundant, as can bacteria. 

Dead zones not only harm the ocean, which is one of nature’s best defenses against climate change, but also contribute to climate change more directly by releasing substantial quantities of heat-trapping gasses. Research suggests that when oxygen levels are low, seafloor sediments can emit nitrous oxide (N20), a strong greenhouse gas that has almost 300 times the global warming potential (GWP) of carbon dioxide. 

Economic Impacts of Dead Zones

Coastal dead zones can have a wide range of economic impacts. Low oxygen levels in the water can reduce the availability of commercially desirable species such as large shrimp, and result in lower fish catch

The harmful algal blooms that cause dead zones can make water unsafe for swimming and pollute coastal air, impacting tourism in those areas. The blooms can also release unpleasant odors, and cause fish kills that can wash hundreds of dead fish onto beaches. 

Harmful algal blooms also cause toxicity in shellfish, leading to paralytic shellfish poisoning for humans who ingest them. Shellfish harvest closures can cause profound social and cultural impacts to tribes and other coastal communities.

By damaging popular underwater tourist attractions such as coral reefs, oxygen minimum zones can have knock-on economic impacts on numerous businesses in coastal areas including hotels, taxi drivers, and restaurants. 

Dead zones have the biggest economic impact on people in lower-income countries in the Global South such as the Philippines, whose livelihoods depend on the ocean. Unlike large industrial trawlers, small fisheries that are hit by water hypoxia may not have the resources to move to areas where there are more fish. 

How Many Dead Zones Are There in the Ocean? 

Recent decades have seen a sharp increase in the number of dead zones worldwide. In 1950, the Earth’s coastal areas were home to fewer than 50 hypoxic sites. Today, scientists know of at least 500 and there may be many more dead zones that have not yet been discovered. As of 2018, the total area of open ocean water devoid of oxygen had increased fourfold, or grown by more than 4.5 million square kilometers (1.7 million square miles), in just 50 years. 

Ocean Dead Zones Map

Dead zones can be found in the open ocean and along the coastlines of numerous countries around the world. Areas that are particularly affected include the coast of Peru, the Baltic Sea, and the Gulf of Mexico. 

Can Dead Zones Recover?

The good news in all of this is that ocean dead zones can recover. Hypoxic waters in the Black Sea, the Hudson River, and San Francisco Bay are among a small number of dead zones that have started to bounce back. Restoring oxygen-depleted areas to the thriving ecosystems they once were by tackling root causes such as agricultural runoff is not only possible but also imperative. 

In some areas of water, however, agricultural chemicals have built up over time and caused so much damage that the recovery process may take decades. For example, even in the highly unlikely scenario that no more excess nitrogen enters the Mississippi River Basin, the nitrogen pollution that is already there means that eutrophication in the Gulf of Mexico would persist for another 30 years, according to a 2018 study. Even more worryingly, scientists who have studied fossil records from rapid climate changes in Earth’s past have concluded that ecosystem recovery took 1,000 years. This makes the need to reform land use practices all the more urgent.

How To Fix Dead Zones in the Ocean 

According to researchers, the solution to dead zones has three key parts: tackling the nutrient pollution and climate change that cause dead zones, protecting threatened marine animals, and improving scientific monitoring of potentially hypoxic areas. The fact that animal agriculture is a major contributor to nutrient pollution and climate change suggests that a plant-based food system could play an important role in addressing this issue. 

Growing plants directly for human consumption instead of growing them to feed farmed animals would significantly reduce the amount of nitrogen and phosphorus entering waterways. Legumes, in particular, have nitrogen-fixing properties that mean they don’t need to be fertilized. Producing a kilogram of protein from kidney beans, for example, needs twelve times less fertilizer than producing a kilogram of protein from beef, according to one study. Other data shows that whether measured per 100 grams of protein or per kilogram of food, the eutrophying emissions of plant proteins such as nuts, peas, and tofu are much lower than those of meat.

Conclusion

Human activity—particularly the factory farming of animals for their flesh, milk, and eggs—is threatening the health of our oceans. Dead zones are a growing environmental problem that will only get worse if current farming practices continue unabated. In the face of a climate crisis, the need to protect marine ecosystems has never been more urgent.

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