What Are Dead Zones in the Ocean?

How oxygen depletion in water works (and what we can do about it)

Underwater view of an algal bloom or red tide with grunt fish.
Underwater view of an algal bloom or red tide with grunt fish. James R.D. Scott / Getty Images

A dead zone is the common name for a region of reduced oxygen levels (hypoxia) in water. Because animals and plants need dissolved oxygen to live, entering a dead zone causes them to suffocate and die. However, dead zones aren't truly "dead," because bacteria thrive on the decaying matter.

Dead zones are found in rivers, lakes, oceans, ponds, and even aquaria. They can form naturally, but they can also form as a result of human activity. Dead zones kill fish and crustaceans, which immediately impacts the fishing industry. Surviving fish suffer reproductive problems, with low egg counts and spawn rates. Animals and plants that can't move have no escape. Dead zones are an important environmental issue.

Where Dead Zones Are Located

Red circles show the size and location of dead zones in 2010. Black dots indicate dead zones of unknown size. Dark blue regions indicate overly fertile water that may give rise to dead zones.
Red circles show the size and location of dead zones in 2010. Black dots indicate dead zones of unknown size. Dark blue regions indicate overly fertile water that may give rise to dead zones. NASA Earth Observatory

Any body of water has the potential to become a dead zone. Hypoxic regions occur in both fresh and saltwater worldwide. Dead zones mainly occur in coastal regions near watersheds, particularly in high population areas.

The largest dead zone in the world is located at the lower portion of the Black Sea. This is a natural dead zone, formed when the water of the Black Sea mixes with the Mediterranean Sea flowing through the Bosporus strait.

The Baltic Sea hosts the largest man-made dead zone. The northern Gulf of Mexico is the second-largest, covering over 8700 square miles (around the size of New Jersey). Lake Erie and the Chesapeake Bay have large dead zones. Almost the entire East Coast and Gulf Coast of the United States have dead zones. A 2008 study found over 400 dead zones worldwide.

Types of Dead Zones

Temperature changes and turbulence can cause natural eutrophication.
Temperature changes and turbulence can cause natural eutrophication. mattpaul / Getty Images

Scientists classify dead zones according to how long the hypoxia lasts:

  • Permanent dead zones occur in very deep water. Oxygen concentrations rarely exceed 2 milligrams per liter.
  • Temporary dead zones are hypoxic regions that last for hours or days.
  • Seasonal dead zones occur every year during the warm months.
  • Diel cycling hypoxia refers to dead zones that occur during warm months, but the water is only hypoxic at night.

Note that the classification system doesn't address whether dead zones form naturally or as a result of human activities. Where natural dead zones form, organisms can adapt to survive them. However, human activities may form new zones or else expand natural zones, throwing coastal ecosystems out of balance.

What Causes Dead Zones?

Red tide is a special form of eutrophication. Organisms in the red tide release toxins, plus they deoxygenate the water.
Red tide is a special form of eutrophication. Organisms in the red tide release toxins, plus they deoxygenate the water. y-studio / Getty Images

The underlying cause of any dead zone is eutrophication. Eutrophication is the enrichment of water with nitrogen, phosphorus, and other nutrients, causing algae to grow out of control or "bloom." Usually the bloom itself is non-toxic, but an exception is the red tide, which produces natural toxins that can kill wildlife and harm humans.

Sometimes, eutrophication occurs naturally. Heavy rains can wash nutrients from soil into the water, storms or heavy winds can dredge up nutrients from the bottom, turbulent water can stir up sediment, or seasonal temperature changes can invert water layers.

Water pollution is the primary human source of the nutrients that cause eutrophication and dead zones. Fertilizer, manure, industrial waste, and inadequately treated waste water overload aquatic ecosystems. In addition, air pollution contributes to eutrophication. Nitrogen compounds from automobiles and factories are returned to water bodies through precipitation.

How Algae Reduces Oxygen

Eutrophication leads to an algal bloom. The algae block light from reaching deeper water. When they die, bacteria overgrowth deoxygenates the water, forming a dead zone.
Eutrophication leads to an algal bloom. The algae block light from reaching deeper water. When they die, bacteria overgrowth deoxygenates the water, forming a dead zone. Universal Images Group / Getty Images

You may be wondering how algae, a photosynthetic organism that releases oxygen, somehow reduces oxygen to cause a dead zone. There are a few ways this happens:

  1. Algae and plants only produce oxygen when there is light. They consume oxygen when it's dark. When the weather is clear and sunny, the oxygen production outperforms nighttime consumption. A string of cloudy days can reduce the ultraviolet levels enough to even the score or even tip the scales so more oxygen is used than produced.
  2. During an algal bloom, algae grows until it consumes the available nutrients. Then it dies back, releases the nutrients as it decays, and blooms again. When algae dies, microorganisms decompose it. The bacteria consume oxygen, quickly making water hypoxic. This occurs so rapidly sometimes even fish can't swim outside of a zone fast enough to escape death.
  3. Algae causes stratification. Sunlight reaches the algal layer, but it can't penetrate the growth, so photosynthetic organisms below the algae die.

Preventing and Reversing Dead Zones

Dead zones can be reversed if excessive nutrients aren't released into the water.
Dead zones can be reversed if excessive nutrients aren't released into the water. GOLFX / Getty Images

Dead zones in an aquarium or pond are preventable. Regulating the light/dark cycle, filtering water, and (most importantly) not over-feeding can help avoid hypoxic conditions.

In lakes and oceans, it's less a matter of preventing dead zones (since they exist globally) and more about reversing the damage. The key to remediation is water and air pollution reduction. Some dead zones have been remediated, although the species that went extinct cannot be recovered.

For example, a large dead zone in the Black Sea all but vanished in the 1990s when farmers couldn't afford chemical fertilizers. While the environmental effect wasn't entirely intentional, it did serve as proof that remediation is possible. Since then, policymakers and scientists have sought to reverse other dead zones. Reduction of industrial effluents and sewage along the Rhine River has reduced nitrogen levels by 35 percent in the dead zone in the North Sea. Clean-up along the San Francisco Bay and Hudson River have reduced dead zones in the United States.

Yet, cleanup isn't easy. Both mankind and nature can cause problems. Hurricanes, oil spills, increased industry, and nutrient-loading from increased corn production to make ethanol have all worsened the dead zone in the Gulf of Mexico. Fixing that dead zone will require dramatic changes by farmers, industries, and cities all along the coast, the Mississippi River, its delta, and its tributaries.

Taking Action

Do your part! Be mindful of the water you use and how your community might reduce its release of potentially harmful nutrients.
Do your part! Be mindful of the water you use and how your community might reduce its release of potentially harmful nutrients. ZenShui/Frederic Cirou / Getty Images

Today's environmental problems are so big they can seem overwhelming, but there are steps each individual can can take to help reverse dead zones.

  • Minimize water usage. Every bit of water you flush away eventually returns to the watershed, bringing man-made pollutants with it.
  • Avoid using fertilizers. Seed companies have developed strains of crops that require less nitrogen and phosphorus, and if you're uncomfortable with genetically modified plants, you can rotate garden crops to naturally replenish soil.
  • Be mindful of air pollution. Burning wood or using fossil fuels releases nitrogen into the air which will make its way into the water. The biggest steps most individuals can take are driving less and reducing power consumption in the home.
  • Be aware of legislation that can either worsen or improve the situation. Vote, and if you see a problem, raise your voice and become part of the solution.

Dead Zone Key Takeaways

  • Dead zones are places in the ocean or other bodies of water characterized by having a low oxygen concentration.
  • Dead zones occur naturally, but the number and severity of hypoxic zones is largely tied to human activities.
  • Nutrient pollution is the primary cause of dead zones. Nutrients from wastewater stimulate algae growth. When the algae die, decomposition depletes the oxygen, killing animals within the zone.
  • There are over 400 dead zones worldwide. The Baltic Sea has the largest dead zone. The northern Gulf of Mexico is the second largest.
  • Dead zones pose a significant economic threat to fishermen. However, the environment impact could signal a global disaster. If dead zones aren't addressed, they could lead to the collapse of the oceanic ecosystem.
  • In some cases, dead zones may be reversed by reducing water pollution. This is a major undertaking that requires cooperation between legislators, farmers, industries, and cities.

Sources

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Helmenstine, Anne Marie, Ph.D. "What Are Dead Zones in the Ocean?" ThoughtCo, May. 10, 2018, thoughtco.com/dead-zones-4164335. Helmenstine, Anne Marie, Ph.D. (2018, May 10). What Are Dead Zones in the Ocean? Retrieved from https://www.thoughtco.com/dead-zones-4164335 Helmenstine, Anne Marie, Ph.D. "What Are Dead Zones in the Ocean?" ThoughtCo. https://www.thoughtco.com/dead-zones-4164335 (accessed May 24, 2018).