What Is Eutrophication? Dangers of Algae Dead Zones

Certain detergents, agricultural runoff, and sewage are all partially to blame

Green water indicating eutrophication and excessive algae growth

Auscape / Getty Images

In This Article

Eutrophication is a tricky environmental problem worldwide, and even though we know the cause, there is not a lot being done to solve it. Get the facts on eutrophication and the algal blooms it causes.

What is Eutrophication?

Eutrophication refers to the process that results from the aging of lakes and streams due to high concentrations of nutrients. In this process, these nutrients—usually nitrogen and phosphorous—are food for aquatic organisms like algae, plankton, or other microorganisms, which leads to the creation of harmful algal blooms. Eutrophication can also occur outside of water. For example, soils can be eutrophic when they have high levels of nitrogen, phosphorous, or other nutrients.

What Causes Eutrophication?

Eutrophication often occurs when rainfall runs off of highly fertilized farmland, golf courses, playing fields, and lawns. The runoff then enters the bodies of water, including:

  • Streams
  • Lakes
  • Oceans


As the demand for more farmland productivity continues to increase, so will the increased use of phosphate- and nitrogen-rich fertilizers. These fertilizers are a major culprit for causing the growth of eutrophic dead zones. Avoid overfertilizing your lawn to do your part to reduce this pressing environmental concern.

It is also common when sewage, either treated or untreated, enters a body of water, and when the outflow from septic tanks enters a stream or pond. Some of the worst sources of nutrient-contaminating water are concentrated animal feeding operations.

When the nutrient-rich runoff enters the water, it fuels a population boom among algae and other organisms. The result is an algal bloom, which looks exactly like it sounds—streams, lakes, and oceans that used to be clear are suddenly green with algae.

This is often referred to as pond scum or duckweed when it is seen in lakes or creeks. When eutrophication occurs in the ocean, and the population of certain species of microscopic dinoflagellates explodes, the water can turn red, brown, or pink—this is commonly referred to as a red tide.

Though most of the worst cases of eutrophication are caused by human activity, it has sometimes occurred naturally. When a spring flood washes immense amounts of nutrients from the land into a lake, eutrophication can result, though it is usually short-lived.

Negative Effects of Eutrophication

Besides being ugly, when an algal bloom occurs, it has a devastating domino effect on aquatic animals and the environment. Some of the harmful effects of eutrophication range from hypoxia to water acidification.

Decomposing Organisms

As large populations of algae and other organisms reproduce, many also die off, and their bodies sink to the bottom of the lake or ocean. Over time, a substantial layer of dead and decomposing organisms fills the bottom.


Microbes that decompose these dead organisms use oxygen in the process and produce carbon dioxide. The result is the depletion of oxygen in the water, a condition known as hypoxia. Since most fish, crabs, mollusks, and other aquatic animals depend on oxygen as much as land-based animals, the result of eutrophication and algal blooms is the creation of an area where no aquatic animals can live—a dead zone.

Ocean Acidification

Additionally, the carbon dioxide lowers the pH of seawater, known as ocean acidification. This slows the growth of fish and shellfish and can prevent the shell formation of bivalve mollusks.

Dead Zones

Dead zones resulting from eutrophication are a growing problem worldwide. Sixty-five percent of estuaries and coastal waters studied in the contiguous United States are moderately to severely degraded by excessive nutrient input. According to some sources, 54 percent of the lakes in Asia are eutrophic. The numbers are similar for lakes in Europe, while in North America, almost half the lakes suffer from eutrophication.

Suffering Fisheries

This loss of aquatic life has a devastating effect on fisheries and the fishing industry. According to researchers at Carlton College who have studied the immense dead zone in the Gulf of Mexico, that body of water is a major source area for the seafood industry. The impact goes beyond the fishing industry. Recreational fishing, which is a significant driver of the tourism industry, also suffers from a loss of revenue.


Algal blooms can have a severe impact on our health. Humans can become seriously ill from eating oysters and other shellfish contaminated with the red tide toxin. The dinoflagellate that causes red tides can cause eye, skin, and respiratory irritation, as well as an allergic reaction to swimmers, boaters, and residents of those coastal areas.

Thinking Ahead: Eutrophication Solutions

Some steps that are slowly being taken can help to control the causes of eutrophic water, including the following:

  • Replacing older forms of detergents with high phosphate contents with low-phosphate detergents can help impede the flow of phosphate nutrients into streams and lakes.
  • Reducing the overuse of fertilizer on lawns and farmland can reduce its impact on waterways and algae bloom.
  • Increasing the size and diversity of wetlands, estuaries, and riverside natural areas can help manage the runoff of nutrient-rich water into streams and oceans.
  • Regulating sewage treatment facilities and septic tanks can greatly reduce nutrient flow, which can result in fewer algal blooms.
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Nutrients and Eutrophication. U.S. Geological Survey.

  2. Nutrient Pollution. The Sources and Solutions: In and Around Your Home. United States Environmental Protection Agency.

  3. What is Eutrophication? National Oceanic and Atmospheric Administration.

  4. The Effects: Dead Zones and Harmful Algal Blooms. United States Environmental Protection Agency.

  5. What is eutrophication? National Ocean Service. National Oceanic and Atmospheric Administration.

  6. Phytoplankton and Eutrophication Degree Assessment of Baiyangdian Lake Wetland, China. The Scientific World Journal.

  7. The Gulf of Mexico Dead Zone. Science Education Resource Center at Carleton College.

  8. Harmful Algae: Review of Florida Red Tide and Human Health Effects. Science Direct.