What Is Eutrophication and How Does It Cause Algal Blooms?

Green water indicating eutrophication and excessive algae growth

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

What Is It?

In simplest terms, eutrophication is a high concentration of nutrients in a body of water. These nutrients—usually nitrogen and phosphorous—are food for aquatic organisms like algae, plankton or other microorganisms. Eutrophication can also occur outside of water; for example, soils can be eutrophic when they have high levels of nitrogen, phosphorous or other nutrients.

Eutrophication often occurs when rainfall that runs off of highly fertilized farmland, golf courses, playing fields and lawns enters a stream, lake, ocean or another body of water. It's 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 nutrients are CAFOs, or concentrated animal feeding operations.

All of these sources of nutrient-rich runoff are great fertilizer for plants, but when these nutrients enter the water, they fuel 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's 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's usually short-lived.


Besides being ugly, when an algal bloom occurs, it has a devastating effect on aquatic animals. 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. 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 end result of eutrophication and algal blooms is the creation of an area where no aquatic animals can live—a dead zone.

Dead zones resulting from eutrophication are a growing problem worldwide: 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.

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 Gulf supplies 72 percent of U.S. harvested shrimp, 66 percent of harvested oysters, and 16 percent of commercial fish. Consequently, if the hypoxic zone [dead zone] continues or worsens, fishermen and coastal state economies will be greatly impacted."

The impact goes beyond the fishing industry, however. Recreational fishing, which is a significant driver of the tourism industry, also suffers from a loss of revenues. And algal blooms can have a severe impact on human health: Not only can humans become seriously ill from eating oysters and other shellfish contaminated with red tide toxin, the dinoflagellate that causes red tides can cause eye, skin and respiratory irritation (coughing, sneezing, tearing, and itching) to swimmers, boaters and residents of coastal areas.

How to Control It

Steps have already been taken to control the spread of eutrophic water: Low-phosphate detergents are rapidly becoming the norm, and their use stops the flow of phosphate nutrients into streams and lakes.

Increasing the size and diversity of wetlands, estuaries and riverside natural areas helps to manage the runoff of nutrient-rich water into streams and oceans. And better sewage treatment facilities and septic tank regulations greatly reduce nutrient flows, resulting in fewer algal blooms.

There is a real concern. The increasing demands on farmland productivity will continue to result in increased use of phosphate- and nitrogen-rich fertilizers, which fuel the growth of eutrophic dead zones. Until this problem is addressed, these dead zones can be expected to multiply and grow.