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What Is Pond Algae and What Causes It?

What is Pond Algae and What Causes It

Algae is a common issue that arises in all ponds from time to time. Its unsightly and often has us frustrated with our ponds.

If you see algae forming in your pond, don't panic! Algae is a normal part of any organic ecosystem. If there is a small amount of algae, it's completely fine; it's when it grows excessively that it may become difficult to manage.

In this article we'll go over what algae is and what causes it. This information will help treat and prevent it from being an issue in the future.

What are Algae?

Algae are a primitive (predominantly aquatic) organisms often found in bodies of water with water quality issues. Their photosynthetic pigments are more varied than plants and their cells have features that are not found in plants and animals. Algae are oxygen producers however they can also remove oxygen from water. They combine carbon dioxide and water to form sugars for energy and growth. When sunlight is not available at night they quickly respire, which uses stored sugars and oxygen to form carbon dioxide, which in turn depletes the oxygen in the pond causing further issues. Algae play an important role on earth as a food source for most aquatic life, they are also used as a source of crude oil and used for some pharmaceutical and industrial applications.

The Two Main Types of Pond Algae

There are 2 main types of algae found in ponds. 1. Filamentous String Algae which forms clusters of hair-like algae that attaches to rocks, plants and other surfaces and 2. Green Water - Suspended Pond Algae which floats in the water causing water to look cloudy and green.

1. Filamentous Algae

Other Common Names: String Algae, Hair Algae, Carpet Algae, Spirogyra, Water Silk

Description: Long, thin, hair-like strands of algae. Together, these strands can form thick mats. They anchor to rocks and other surfaces before floating to the water surface. This algae can grow quickly and impact the appearance and health of a pond. This algae causes no major issues in small quantities. It should be kept under control.

Toxic: No

String Algae - Hair Algae

2. Green Pond Algae

Other Common Names: Pea Soup Algae, Green Water Algae, Suspended Algae, Chlamydomonas, Chlorella, Closterium

Description: One of the most common algae's; these free floating algae are microscopic and give the water a green colour and cloudy appearance. It forms around the bottom, edges and surface of ponds and is visible during blooms.

Toxic: No

*This algae is not what is found in a pond experiencing "New Pond Syndrome" in the first 1.5 months of a ponds life.

Green Pond Algae - Pea Soup Algae

Is Pond Algae Dangerous?

There are types of pond "algae" that are dangerous, the most common in North America is Blue-Green Algae (a.k.a. cyanobacteria). These are actually a type of bacteria found in many lakes, ponds and reservoirs. They resemble pond scum that lies on the surface of the water.

Characteristics: Most commonly a blue-green or greenish-brown colour. It has a musty or grassy smell. Some bloom forming blue-green algae product toxins, when the blooms die and decay, these toxins are released into the water. The toxins can cause a variety of harmful effects and can even kill livestock if severe enough. You can't tell if a bloom is harmful simply by looking at it. If you see such an algae-bloom, its best to take the appropriate precautions as if it were toxic.

Children and animals are most at risk to harmful affects from blue-green algae.

Health Effects of Toxic Blue-Green Algae


Sore throat

Sore, red eyes

Skin irritation or rash

Swollen lips

Hay fever or allergy-like symptoms (stuffy nose)

Drinking the Water:




Liver Damage

Fever (around 101°F)

Nausea and Vomiting

Abdominal Craps

Muscle and Joint Pain

What Causes Pond Algae?

Both String Algae and Green Water Algae are typically caused by water imbalances. Too much direct sunlight and excess nutrients lead to water quality issues and cause this algae to grow. Green Water algae is more likely to form in spring when the weather begins to change.

Factors That Cause Pond Algae

1. Excess nutrients

  • Fish waste (fish poop)

  • Decomposing tissues (dead plant, fish, animal material)

  • Fertilizers and runoff from yards / lawns

  • Overfeeding fish

  • Storm water runoff

  • Having too many fish

  • Lack of pond filtration

2. Direct Sunlight

  • Sunlight

  • Heating a pond over 90°F

Now that we understand what algae is and what causes it, we'll have a better understanding of how to prevent it and how to get rid of it for good!

Got Algae Issues?

If you have algae in your pond, chances are you're looking to get rid of it.

To learn more about preventing algae and getting rid of algae

To Book a Pond Clean Out in the Winnipeg Area


Q: When does algae typically form in your pond? | Comment Below

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