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Ask Lyn: Bright Green Algae

Tuesday, April 27, 2021

TWF Posted by: TWF

Dear Lyn,

It is only April and there is already a bunch of bright green algae in the channel and along the shoreline of my favorite lake. Is it dangerous? Was it caused by lawn fertilizer?

Signed, Algae is Gross

Algae in backyard
Algae in backyard

Dear Algae is Gross,

I agree; the water condition in these photographs does look pretty yucky. In this case, the bright green algae were likely caused by a combination of low water levels, unseasonably warm spring temperatures, and nutrients. Although lawn fertilizer can undoubtedly be a source, most runoff from the watershed contains phosphorus – the primary driver of algae growth in our local lakes.

The good news is that this very bright green early spring algae is not harmful to people or pets and usually dissipates quickly. If you’re interested in learning how you can do a better job of identifying potentially harmful algae blooms, check out this helpful guide. Although harmful algal blooms usually have an unpleasant odor, they can appear in many different colors and forms.

It’s actually quite difficult to be certain about the type of algae without the benefit of a microscope and training. Lucky for our community, our partner – the Lilly Center for Lakes & Streams can provide algae identification. Please contact their lab for testing information.

Everyone, keep sending us your questions and photos about what you are seeing outdoors. We depend on your eyes and ears to continuously monitor our water quality. As always – if you have a question, Ask Lyn at