Eutrophication: Why you should care about pond scum?

Lushani Nanayakkara
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7.32

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When fertilizers from agriculture get into lakes and rivers, big problems can result for aquatic ecosystems, food chains & webs!

Have you ever seen a thick layer of green scum on a lake? Chances are, you were looking at a eutrophic lake.

Eutrophic bodies of water have too many primary producers. Primary producers are organisms that use energy from the sun to create glucose. They later use this glucose for energy. It is a little bit like they are creating their own food. Algae and plants are examples of primary producers.

Primary producers are important to aquatic ecosystems because other organisms depend on them for food. But too many primary producers in a body of water can cause problems.

Did you know?

Eutrophic comes from the Greek word “eutrophos,” which means “well-nourished.”

What do primary producers do in a food web?

A food web is a system of living things that depend on each other for food. Primary producers are central to every food web. They use the Sun’s energy to make sugars through a process called photosynthesis. These sugars are what the primary producers use to live and grow.

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

Animals that eat primary producers are called primary consumers. In lakes and rivers, microscopic animals called zooplankton and small fish are primary consumers. Larger fish that eat primary consumers are called higher-level (secondary and tertiary) consumers.

Graphic of a freshwater food web
Graphic of a freshwater food web (© 2019 Let’s Talk Science).

How does eutrophication happen?

Remember the green layer of scum on the lake? That is a result of a process called eutrophication. Eutrophication can happen when there are too many nutrients in the water. Too many nutrients can make primary producers grow and reproduce much faster than normal.

Sometimes eutrophication happens naturally. But there’s also such a thing as cultural eutrophication. This is eutrophication caused by human activities in a watershed. A watershed is an area of land that drains into a body of water.

Cultural eutrophication happens much faster than natural eutrophication. It also happens on a much larger scale.

Many human activities can cause cultural eutrophication. Agriculture can have some of the biggest impacts. For instance, many farmers put fertilizer on their fields. This fertilizer is an important source of nitrogen and phosphorus. Sometimes, when it rains, there is more water than the soil can absorb. The excess water becomes loaded with nutrients. Then, gravity moves the excess water downward towards bodies of water. This leads to eutrophication.

Beware of cyanobacteria

Have you ever seen (or smelled) a thick layer of scum on the surface of water? This layer of scum is called an algal bloom. Algal blooms can occur in eutrophic bodies of water. When the algae in the algal blooms die, they sink to the bottom of the water. There, microorganisms feed on and break down, or decompose, the dead algae. This leads to more microorganisms that use up more and more oxygen. Eventually the water can become hypoxic or anoxic. Hypoxic zones have little oxygen. Anoxic zones have no oxygen. These areas are sometimes also called “dead zones”. Dead zones are areas where the oxygen is too low for most organisms to survive.

Algal blooms often contain microorganisms called cyanobacteria. Cyanobacteria are primary producers. They can grow extremely fast when there are too many nutrients in the water.

Misconception Alert

Cyanobacteria are often called blue-green algae. But they are not actually algae!

Cyanobacteria from the genus Nostoc
Cyanobacteria from the genus Nostoc (Source: Elif Bayraktar via iStockphoto).

Cyanobacteria have features that other primary producers do not. For example, their cells have vacuoles filled with gas. This allows them to float toward the surface of the water. Up there, cyanobacteria have better access to sunlight. This helps them get the most out of photosynthesis. 

Some species of cyanobacteria also have special cells called akinetes. These cells help them survive harsh environmental conditions. Akinetes have thick cell walls. These cell walls help keep cyanobacteria from drying out and getting too cold.

Some species of cyanobacteria form thick algal mats on the water surface. These mats block out the sunlight for primary producers below the surface. Remember, primary producers depend on sunlight. Without it, they cannot photosynthesize.

Blue green algae bloom on the shore of Lake Erie, 2009
Blue green algae bloom on the shore of Lake Erie, 2009 (Source: National Oceanic and Atmospheric Administration (NOAA) via Wikimedia Commons).

Without sunlight, these other organisms can’t grow—or even survive! This creates a problem for the rest of the food web. The primary consumers won’t be able to find enough primary producers to eat. Eventually, they will start to die off. Soon higher-level consumers won’t have enough food either. Eventually, they will die off, too.

Eutrophication in Lake Erie

In the 1970s, Lake Erie in Southern Ontario was severely polluted. The pollution led to eutrophication. The eutrophication got so bad that newspapers claimed, “Lake Erie is dead”! Governments passed laws to make sure the cities around Lake Erie would dump less pollutants into the lake. This worked for a long time. But there have been times when Lake Erie was threatened again.

Did you know?

Scientists are researching whether climate change can cause more eutrophication. One study suggests it can. That’s because it can change water circulation. This reduces the amount of oxygen that reaches the bottom of lakes.

So, why should you care about eutrophication?

Eutrophication can happen to a water body near you! It can keep you from enjoying your favourite lakes and rivers. It can even affect the types of fresh fish available at supermarkets. Eutrophication is an important water quality issue. It’s an issue for organisms that live in water. It’s also an issue for us humans.
There are ways you can help. If you have a garden at home, don’t use more fertilizer than you need. Check your laundry detergent. It should have as little phosphorus in it as possible. There are also bigger ways you can help. Learn about eutrophication issues in your area. Support groups who are fighting to reduce it! 

Starting Points

Connecting and Relating

  • Have you seen green scum on a pond? What does it make you think about the water when you see pond scum?  
  • Would you be concerned about swimming in a lake or river that had green algae floating on the surface? Why/why not?

Connecting and Relating

  • Have you seen green scum on a pond? What does it make you think about the water when you see pond scum?  
  • Would you be concerned about swimming in a lake or river that had green algae floating on the surface? Why/why not?

Relating Science and Technology to Society and the Environment

  • Should governments limit the amount of fertilizers that farmers can use? Why/why not?
  • The scale of human population growth suggests that large-scale agriculture will continue to be important. How can we balance the need to use fertilizers for farming with the potentially negative impacts of eutrophication? Explain.
  • What is a Nutrient Management Plan? Why is it a good practice for farmers to have a sound nutrient management plan in place? (Note: this question may require some research.)

Relating Science and Technology to Society and the Environment

  • Should governments limit the amount of fertilizers that farmers can use? Why/why not?
  • The scale of human population growth suggests that large-scale agriculture will continue to be important. How can we balance the need to use fertilizers for farming with the potentially negative impacts of eutrophication? Explain.
  • What is a Nutrient Management Plan? Why is it a good practice for farmers to have a sound nutrient management plan in place? (Note: this question may require some research.)

Exploring Concepts

  • How can eutrophication affect a food web?
  • What factors lead to pond scum growth in aquatic ecosystems?
  • Explain how natural eutrophication differs from cultural eutrophication.
  • What makes cyanobacteria so good at forming algal blooms?
  • Why do fertilizers increase plant growth?

Exploring Concepts

  • How can eutrophication affect a food web?
  • What factors lead to pond scum growth in aquatic ecosystems?
  • Explain how natural eutrophication differs from cultural eutrophication.
  • What makes cyanobacteria so good at forming algal blooms?
  • Why do fertilizers increase plant growth?

Nature of Science/Nature of Technology

  • Should manufacturers of farm fertilizers be held accountable for large-scale eutrophication when excess nitrogen ends up in local watersheds? Why/why not?
  • What new technologies or techniques are farmers using to minimize the amount of fertilizer that goes into local watersheds?

Nature of Science/Nature of Technology

  • Should manufacturers of farm fertilizers be held accountable for large-scale eutrophication when excess nitrogen ends up in local watersheds? Why/why not?
  • What new technologies or techniques are farmers using to minimize the amount of fertilizer that goes into local watersheds?

Media Literacy

  • In the movie Erin Brockovich (2000), Julia Roberts portrays a legal assistant who spearheads a fight against intentional pollution of groundwater by a large corporation. Why would a person or organization intentionally pollute a watershed? How can such actions affect human populations?
  • The article says that some newspapers in the 1970s claimed, “Lake Erie is dead!” Is this headline scientifically accurate? How important is it for media headlines to be completely scientifically accurate?

Media Literacy

  • In the movie Erin Brockovich (2000), Julia Roberts portrays a legal assistant who spearheads a fight against intentional pollution of groundwater by a large corporation. Why would a person or organization intentionally pollute a watershed? How can such actions affect human populations?
  • The article says that some newspapers in the 1970s claimed, “Lake Erie is dead!” Is this headline scientifically accurate? How important is it for media headlines to be completely scientifically accurate?

Teaching Suggestions

  • This article can be used in Biology and Earth & the Environment to support teaching and learning related to human impacts on freshwater and aquatic ecosystems, nitrogen and phosphorus cycles, food chains and food webs. Concepts introduced include primary producers, higher-level consumers, nutrients, fertilizer, natural and cultural eutrophication, watershed, agriculture, blue-green algae, vacuoles, akinetes, cell walls, algal mats, decompose, anoxic and hypoxic. 
  • Before reading the article and viewing the embedded video, students could conduct a Vocabulary Preview learning strategy to introduce key terminology. Ready-to-use Vocabulary Preview reproducibles can be downloaded in [Google doc] or [PDF] formats.
  • As further consolidation, teachers could have students complete a Concept Definition Web for the term “cultural eutrophication.” Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats. 

Teaching Suggestions

  • This article can be used in Biology and Earth & the Environment to support teaching and learning related to human impacts on freshwater and aquatic ecosystems, nitrogen and phosphorus cycles, food chains and food webs. Concepts introduced include primary producers, higher-level consumers, nutrients, fertilizer, natural and cultural eutrophication, watershed, agriculture, blue-green algae, vacuoles, akinetes, cell walls, algal mats, decompose, anoxic and hypoxic. 
  • Before reading the article and viewing the embedded video, students could conduct a Vocabulary Preview learning strategy to introduce key terminology. Ready-to-use Vocabulary Preview reproducibles can be downloaded in [Google doc] or [PDF] formats.
  • As further consolidation, teachers could have students complete a Concept Definition Web for the term “cultural eutrophication.” Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats. 

Learn more

Why is eutrophication such a serious pollution problem?

A more detailed description from the United Nations Environment Program on the consequences of eutrophication. 

Interactive Map of Eutrophication and Hypoxia

An interactive map from the World Resource Institute of coastal areas that have or are experiencing eutrophication or hypoxia. Requires Flash Player.

Nutrients and Water Quality

Government of Canada provides links to a variety of reports both technical and aimed at the public about eutrophication in various regions across Canada.

Water Pollution from Agriculture: a Global Review

A 35 page PDF summary from the United Nations' Food and Agriculture Organization, which includes a short section on excess nutrients.

What is eutrophication? (2018)

Brief video (1:03 min.) from the US National Ocean Service which explains eutrophication

References

National Geographic. (2019). Dead zone.

Vincent, W. (2010). Cyanobacteria. In Likens, G. Plankton of Inland Waters. (pp. 226-232). Academic Press.