The good, the bad, and the algae: Aquatic plants can be helpful or harmful

Experts explain the different kinds of algae that river users might see on the St. Croix.




5 minute read

Algae on St. Croix River backwater, July 2022. (Photo courtesy David Bowlin)

The St. Croix River is usually pretty clean and clear, perhaps a little orange from the stain of tannins — but sometimes it can get green and goopy. While algae are a natural part of the river ecosystem, and are often present without causing problems, some kinds, and in some conditions, can be a major nuisance.

Last month, St. Croix 360 reported about a bloom of harmful “blue-green algae” observed on the lower St. Croix south of Hudson. It forced the closure of at least two beaches, as this type is known to create toxic chemicals in the water. The bloom has since cleared and the beaches have reopened.

After publishing that report, St. Croix 360 reader David Bowlin reached out about an algae bloom he saw in a backwater on the St. Croix River in Scandia. He said it was nothing like anything he had seen in 50 years experience with that stretch of river. A friend took an aerial photo, and David ultimately paddled over to collect a sample. He brought it to me and I brought it to algae experts at the St. Croix Watershed Research Station in Marine on St. Croix, part of the Science Museum of Minnesota.

Scientists Mark Edlund and Adam Heathcote examined the algae under a microscope, and determined it was not the kind that produces poison. It was actually an interesting and unique species, which can cause problems of its own. Formally called Hydrodictyon reticulatum, the reason for its common name of “water net” is apparent.

Water net creates what looks like a fine mesh of connected strands, and can form big, dense mats like Bowlin saw. It loves the growing conditions in the middle of a hot summer, in the slow waters of a St. Croix River backwater after a long dry spell. The net helps it hold onto its habitat, but it can become a bother if a mat of it floats out of the backwater. Drifting downstream, the grippy mesh can cling to docks, boats, and shorelines.

There are several other kinds of algae loosely related to water net, in the “green algae” family. A related kind, Spirogyra, is also observed on the St. Croix, and has caused similar concerns before. In May 2015, reader Andy Kramer reached out about algae mats he observed near William O’Brien State Park. That one turned out to be Spirogyra, which is the early season cousin to late summer water net.

While water net and Spirogyra can be annoying but not actually harmful, cyanobacteria is another matter. The three common kinds of blue-green algae are nicknamed Annie (Anabaena/Dolichospermum), Fannie (Aphanizomenon), and Mike (Microcystis). It was probably one of those that was seen along the lower St. Croix a few weeks ago, or maybe all of them. In 2013, the three types were found mingling in water samples taken from Black Bass Bar, near the area affected by the recent bloom.

Four types of cyanobacteria, or blue-green algae, in a water sample from Black Bass Bar on the lower St. Croix River in August 2013. (Courtesy Mark Edlund, St. Croix Watershed Research Station)

These plants produce toxic byproducts that can become concentrated in the water. This is particularly dangerous to pet dogs, who might swim in, drink the water, or even lick their wet fur, and several beloved pets have been lost to blue-green algae in Minnesota and Wisconsin in recent years. For people, the chemicals can cause skin irritation as well as liver, kidney, and brain damage.

Edlund says there are a few other types of algae that can cause concerns, too. One involves algae such as Oscillatoria limosa, which grows on the bottom of the river in mats, usually out of sight and unnoticed by people. But when the algae get disturbed or detach the bottom, they can float to the surface, causing an unsightly and smelly mess.

Another are mats of diatoms, which often appear as slimy “gunk,” according to Edlund. It can coat rocks or float in the water, and grow throughout the open-water season.

All the species discussed above are native species that naturally live in the St. Croix and its tributaries. Nonetheless, they can have unnatural impacts due to the influence of humans. Climate change can create better growing conditions, and runoff from farm fields and urban development carries nutrients that feed the algae. 

This year has been a good example of what climate change will probably make more common: A fairly wet spring carried a lot of sediment and nutrients off the landscape and into the river. Then it dried out, the river level dropped, and the water became warm and slow-moving. Algae love it.

Algae problems would also be growing without global warming, as the level of nutrients in the lower St. Croix has nearly tripled since Europeans began settling in the area. Cyanobacteria blooms are becoming more common in many places in the Midwest, and possibly other nuisance algae blooms, too, in big part because of too much to eat.

Of course, algae are not all bad, and are essential to life on Earth, especially in our waterways. There are a handful of types of cyanobacteria that can cause problems, so far sporadically, on the St. Croix. But there are hundreds of species of algae in the river, and they basically form the base of the food chain: photosynthesizing energy, getting eaten by tiny animals that are in turn eaten by larger animals, until the nutrition reaches large species like fish and birds. Globally, algae produce an enormous part of the oxygen we breathe.

Knowing the good and the bad can help you have a better, safer time on the St. Croix. There are several visible species that may be annoying, but aren’t dangerous. Others produce powerful poisons. To avoid exposure to harmful algae, experts’ advice is simple: “when in doubt, stay out.”

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5 responses to “The good, the bad, and the algae: Aquatic plants can be helpful or harmful”

  1. Allison Mcginnis Avatar
    Allison Mcginnis

    Great informative article Greg!

  2. Sarah Lilja Avatar
    Sarah Lilja

    Lots of new information here. Love the algae photos from the SCWRS, too. Thank you!

  3. Troy Avatar

    Algae, cyanobacteria, and diatom buildups were not a problem prior to “free-flow of the river”.

  4. Jay T. Hatch Avatar
    Jay T. Hatch

    The darter in your first diatom photo is a Gilt Darter.

  5. Tim S Crom Avatar
    Tim S Crom

    Great article. Thanks.


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The good, the bad, and the algae: Aquatic plants can be helpful or harmful