What Is Eutrophication? - Definition, Causes & Effects

Lesson Transcript
Instructor: Elizabeth Friedl

Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

Eutrophication occurs when an aquatic system has too many nutrients in it, which can be just as harmful as having too few. This lesson delves into the causes, effects, and prevention of eutrophication. Updated: 08/27/2021


All living things need specific nutrients to survive. Usually, nature does a pretty good job of providing just the right amount of nutrients, because too many or too few can cause problems. This is especially true in aquatic ecosystems because they are so dynamic. When too few nutrients are present, the water is oligotrophic. It makes sense that when there is not enough nutrition available for the variety of organisms living in an aquatic environment, serious problems will arise.

However, problems can also arise when the aquatic system has an overabundance of nutrients. When this happens we get eutrophication. A eutrophic stream, river or lake occurs when too many nutrients, like nitrogen and phosphorous, are present, usually as a result of runoff from the surrounding land. Algae, plankton and other microorganisms love these types of nutrients, and when they are plentiful, these aquatic organisms can take over. When a lake, river or other aquatic system becomes eutrophic, it can have serious negative effects on other organisms like fish, birds and even people. But first, let's look at what causes eutrophication.

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  • 0:01 Definition
  • 1:18 Causes
  • 2:10 Effects
  • 3:57 Prevention
  • 4:15 Lesson Summary
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Eutrophication is most often the result of human activity. Farms, golf courses, lawns and other fields tend to be heavily fertilized by people. These fertilizers are the perfect type of nutrients to feed hungry algae and plankton, and when it rains, these fertilizers run off into lakes, streams, rivers and oceans. Concentrated animal feeding operations (CAFOs) are also a major source of polluting nutrients.

Eutrophication can also come from natural events. If a stream, river or lake floods, it may wash away any excess nutrients off the land and into the water. However, eutrophication is less likely to occur in areas that are not surrounded by fertilized lands.


Eutrophication can have serious, long-term effects. The most notable effect of eutrophication is algal blooms. When a bloom occurs, the stream, river, lake or ocean becomes covered with algae, which is usually bright green. In addition to looking pretty ugly, it also blocks light from reaching the water. This prevents the aquatic plants from photosynthesizing, a process which provides oxygen in the water to animals that need it, like fish and crabs.

Algae growth from eutrophication

If an algal bloom is so bad that it causes wide-spread death in the water, the organisms that die will all sink to the bottom and start to decompose. The microbes that break down these dead organisms use oxygen to do their work. So, in addition to the lack of oxygen from photosynthesis, there is also now a lack of oxygen from the decomposition of dead organisms.

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