We got this weird green stuff in our lake this year. What is it?—Name Withheld, via email
Without analyzing a sample under a microscope, it’s impossible to know for sure, but the experts we asked all believe that it’s likely filamentous green algae.
Don’t panic: filamentous green algae—composed of hair-like strands that accumulate in floating “mats”—isn’t necessarily dangerous.
“It’s part of the ecosystem,” says Paul Frost, a professor in the biology department at Trent University, in Peterborough, Ont., who studies freshwater ecology. “There is an ecological function to it. But it’s probably not something that you want floating around your dock.”
Neither does the wildlife living in your lake. Never mind the ick factor, “there are issues associated with such a large mass of algae when it dies off and oxygen is consumed by decomposers,” says Andrea Kirkwood, an associate professor in the biology department at Ontario Tech University. “This creates inhospitable conditions for invertebrates and fish,” she says. “And it could smell pretty bad.”
6 ways to protect your lake’s water quality
So, where did this mass of slimy hair come from? “Excess nutrients are usually implicated as a main driving factor,” says Kirkwood. “It’s difficult to speculate, but it is safe to say that algae need a lot of food to get to that level of growth, and fertilizers or sewage are like candy to filamentous green algae.”
No one is suggesting you’ve been feeding the algae candy, er, sewage. The mats could have floated up from somewhere else. Maybe nobody on the lake did anything at all—a winter of heavy precipitation could lead to more runoff during the spring, which increases the level of nutrients in the lake, which eventually causes the amount of algae to grow to an extreme level. Then, months later, there are huge blooms in a lake. “And it’s like, ‘What happened?’ ” says Frost. “But unravelling that is not easy. It’s so circumstance-based. It doesn’t keep me up at night…but sometimes you want to throw up your hands when you don’t have easy or obvious answers.”
One thing you should know: it is possible for a mat of filamentous green algae to contain some cyanobacteria in the mix. If that were the case, “the material would have to be tested, since a variety of cyanobacteria—but not all—can produce toxins,” says Kirkwood.
If you’re ever concerned that your lake has toxic blue-green algae, or is contaminated in any way, report the sighting. Most provinces have hotlines.
Got a question for Cottage Q&A? Send it to answers@cottagelife.com.
This article was originally published in the Sept./Oct. issue 2024 issue of Cottage Life.
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