This article is cross-posted from Field Notes, the St. Croix Watershed Research Station’s new blog and a St. Croix 360 partner.

Andy Kramer of Marine on St. Croix was boating on the river near William O’Brien State Park recently when he saw something he thought was suspicious: mats of floating algae. It seemed early in the year to be seeing such a sight. Knowing that algae is often caused by excessive nutrients in runoff, he was worried about what it meant for the St. Croix.

Andy sent a photo and his question over, and then went back out a few days later to get a sample of the bright-green stuff, which he dropped off at the station. Scientist Mark Edlund looked at it under the microscope and determined it was almost entirely the genus Spirogyra, and a “handsome” example at that.

While globs of algae might be alarming, Edlund says it is common during the spring along the river. It is very slimy, which makes it hard for other critters to eat it, and the thick mucus-like layers trap air bubbles it produces during photosynthesis, causing the algae to float up to the surface, especially on nice sunny days.

“It forms large mats of unbranched filaments in nearshore and quiet backwaters that will persist until the water warms up a bit, at which time it will have sex and produce specialized cells called ‘zygospores’ that will lie dormant on the bottom until later in the fall or next spring,” Edlund says.

Unlike “cyanobacteria,” or blue-green algae, Spirogyra does not produce toxins. Blooms of that harmful algae are often associated with excess nutrients, and usually occur in warmer weather. The St. Croix River occasionally experiences such blooms, which is one reason the lower river is listed by the Environmental Protection Agency as impaired.

River users are some of the best people for detecting algae blooms and helping increase our understanding of the river’s ecology. If you see algae, please collect a sample and bring it to the station. Our scientists will be glad to put it under a microscope and share what they find.

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