Above: Image © ErikaMitchell, iStockphoto.com

Have you ever looked at a lake and wondered why there was a thick layer of green scum on the water? Chances are, you were looking at a eutrophic lake.

Eutrophic bodies of water have too many primary producers. Those are organisms that create their own food, like algae and plants. Primary producers are important because other organisms depend on them for food. However, when a body of water has too many primary producers, it can cause problems ranging from dead fish to powerful toxins.

Understanding primary producers

A food web is a system of living creatures that depend on each other for food. Primary producers are central to every food web. They are photosynthetic. This means they use the Sun’s energy to make sugars. These sugars are what the primary producers use to live and grow.

Primary producers also need nutrients to survive. Two important nutrients are phosphorus and nitrogen. Phosphorus helps make photosynthesis possible. It also helps the primary producers’ cells grow and reproduce. Nitrogen helps make proteins and produces new plant tissue.

Animals that eat primary producers are called primary consumers. In a lake or river, two examples are the microscopic animals called zooplankton and certain types of fish. Then, higher-level consumers, such as larger fish, would feed on the primary consumers.

Patomac River
Eutrophication in the Patomac River near Washington, D.C. (Alexandr Trubetskoy, Wikimedia Commons).

How eutrophication happens

So how does a body of water like a lake end up with too many nutrients? It can happen naturally, often over a long period of time. Sometimes it even takes thousands or tens of thousands of years.

However, there’s also such a thing as cultural eutrophication. That’s when eutrophication is caused by human activities in a watershed. A watershed is an area of land that drains into a body of water. Cultural eutrophication happens at a much faster rate and on a much larger scale than natural eutrophication.

Many human activities can cause cultural eutrophication. However, agriculture—especially intensive or industrial agriculture—can have some of the biggest impacts. For instance, the fertilizer that crop farmers put on their fields is an important source of nitrogen and phosphorus. When farmers use too much fertilizer, the extra amounts dissolve in rainwater. Heavy rain causes excess water to run along the surface of the watershed. As a result, huge amounts of nutrients can get picked up and dumped into nearby bodies of water.

Remember, lakes and rivers are located below other parts of a watershed. So gravity moves the nutrient-loaded water downward, toward these bodies of water. The increased amount of nutrients can cause primary producers to grow and reproduce at a much faster rate than normal.

Did you know? Some species of cyanobacteria have akinete cells that help them survive harsh environmental conditions. Akinetes are filled with food reserves and thick cell walls that keep them from drying out.

Beware of cyanobacteria

Have you ever seen (or smelled) a thick layer of scum on the surface of a pond or lake? This layer of scum is called an algal bloom. The bloom occurs when extra nutrients enter the lake and cause the algae to reproduce and grow much faster than normal. These blooms often contain cyanobacteria, which is also called blue-green algae (even though they are not algae). They’re an example of a primary producer that can grow extremely fast when there are too many nutrients in the water.

Cyanobacteria have several features that set them apart from other primary producers. For example, they have gas vacuoles that allow them to float toward the surface. This gives them better access to light, which helps them get the most out of photosynthesis.

Some species of cyanobacteria form thick algal mats on the water surface. These mats can block sunlight from reaching other primary producers that live below the surface of the water. Without sunlight, these other organisms can’t grow—or even survive! This also creates a problem for the rest of the food web. The primary consumers won’t be able to find enough primary producers to eat. The primary producers will start to die off. Soon higher-level consumers won’t have enough food either and they will die off.

When the algae in the algal blooms die, they sink to the bottom of the water. There, they will start to decompose (rot) and this causes the water to smell bad. As the algae rot, this uses up dissolved oxygen from the water. This can cause hypoxic (low oxygen) or anoxic (no oxygen) waters. Many aquatic organisms can’t survive unless there’s enough oxygen in the water. When water has little or no oxygen dissolved in it, this can kill large numbers of fish.

Not only do cyanobacteria smell bad, they’re also dangerous. They can produce powerful toxins (poisons). These toxins can prevent people and their pets from using natural bodies of water for recreation.

Did you know? As water temperatures increase, the concentration of dissolved oxygen in the water goes down.

Should you care about eutrophication?

Yes, you should. Eutrophication can keep you from enjoying your favourite lakes and rivers. By killing off fish, it can even affect what types of fresh fish is available at local supermarkets. It’s an important water quality issue that you should keep in mind. Can you find any ways that you can help reduce eutrophication? Are there any programs or regulations in your area to deal with the issue?

This article was updated by Let's Talk Science staff on 2017-01-25 to improve readability by reducing the reading grade level.

Learn More!

About eutrophication:

Eutrophication: Causes, Consequences, and Controls in Aquatic Ecosystems (2013)
M. F. Chislock, E. Doster, R. A. Zitomer & A. E. Wilson, Nature Education Knowledge 4

Water Quality: the Impact of Eutrophication (2001)
United Nations Environment Program

About Eutrophication
World Resources Institute

Interactive Map of Eutrophication and Hypoxia
World Resource Institute.

About aquatic organisms:

A biological synopsis of walleye (Sander vitreus) (2009)
G. F Hartman, Department of Fisheries and Oceans

Cyanobacteria (2009)
W. F. Vincent, Laval University, Quebec City, Canada.

About agriculture and eutrophication:

Lushani Nanayakkara

Lushnai Nanayakkaea

Originally from Colombo, Sri Lanka I did an undergraduate in Zoology at Ohio Wesleyan University in Delaware, Ohio. After that I lived in Florida for a short-time working with Asian elephants. Then I moved to Maryland where I completed my MSc in Environmental Sciences and Policy at Johns Hopkins University. Wanting to experience living yet another country I moved to Canada to pursue my PhD at the University of Regina in Saskatchewan. I am currently finishing my PhD while living in Toronto. During my free time I enjoy traveling, trying new types of cuisine and attending concerts/plays and interesting lectures.

Starting Points

Connecting to Content on CurioCity

Connecting to Careers on CurioCity

To see the complete Starting Points and free educator resources for this content, please log in or register.

b i u quote

Save Comment