Algae & Toxic Algae

Algae are small plant-like organisms that can be found in nearly every body of water including lakes. There can be hundreds of algae species in a single lake. Most algae are microscopic but some large macroalgae are often confused with plants. All algae have pigments that allow them to create energy from sunlight through photosynthesis.

Benefits of Algae

Algae are incredibly important to the health of lakes. They serve as the base of the food web providing energy directly or indirectly to nearly all aquatic life. They feed tiny animals knows as zooplankton that, in turn, are eaten by aquatic insects and fish.  Algae also replenish the lake and atmosphere with oxygen through photosynthesis.

Problems with Algae - Algae Blooms

Most algae are harmless and aren’t even noticed by lake users. But sometimes algae can grow rapidly or “bloom.” A bloom refers to prolific growth of one or two algae species in the lake. Algae blooms can appear differently.

Blooms of free-floating algae, called phytoplankton, may cause the water color to turn different colors such as green or golden brown. With dense blooms, you may observe floating clumps of blue or green algae or surface scums that appears as paint spilled on the lake surface.

Other types of algae, called filamentous algae, grow stringy green filaments that can coat the lake bottom, rocks, structures and even aquatic plants. When dense, it can appear as stringy balls or mats in the water which may float to the surface and appear bubbly or turn brown.

Potentially toxic algae blooms

While algae blooms may look unsightly, most do not present a health risk. However, a few species of blue-green algae or cyanobacteria cause harmful algae blooms. They can produce toxins that are dangerous to animals and pets. Refer to our toxic algae webpage to learn how to respond to these potentially harmful blooms.

We have created a “Visual Guide to Algae & Other Scums” to help you learn about the different types of blooms including potentially toxic blooms.

View our "Visual Guide to Algae and Other Scums"


Causes of Algae Blooms

Most lakes will naturally have an occasional algae bloom. While temperature and sunlight play a role in blooms occurring, often times excessive nutrients - particularly phosphorus - is the primary factor leading to blooms. Frequent algae blooms are a symptom of excessive phosphorus pollution and a sign of declining water quality.

Managing Algae Blooms

If your lake is experiencing an algae bloom there is not much that can be done besides to wait for the bloom to dissipate. Algaecides require extensive permitting and a licensed herbicide applicator. Furthermore, they are not recommended as the cells release toxins as they die back. Decaying algae also release nutrients that trigger a new algae bloom.

If you have excessive growth of filamentous algae on your lake shoreline, you may be able to use a weed rake to remove. Leave rooted aquatic plants undisturbed as they take up nutrients preventing more algae growth and may require permits to be removed. Once removed, the algae should be taken away from the lake to decompose so nutrients don't wash back in causing more algae. When it dries, it makes great garden compost.

Preventing Algae Blooms

The best strategy for managing algae blooms is to prevent them by reducing phosphorus pollution to the lake. Phosphorus pollution mainly comes from homes and yards that are near the lake and is washed into the lake when it rains. Common sources are lawn fertilizers, pet and animal waste, septic systems and any bare soils or dirt which are rich in phosphorus. 

Snohomish County has a program called LakeWise which helps residents take simple, voluntary actions on their properties to reduce phosphorus pollution. Even small reductions in phosphorus can make a big difference for your lake. Learn more or sign up for a FREE educational LakeWise visit at

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