I read that bright dock lights can contribute to the growth of blue-green algae. Is that true?—Saskia Hubbard, via email
Yes. Artificial lighting at night can promote algae growth—or any aquatic plant growth. This is because plants don’t know that artificial light isn’t the sun. “It contains photon wavelengths that are required for algae and plants to grow using photosynthesis,” says Andrea Kirkwood, an environmental biology professor at Ontario Tech University in Oshawa, Ont.
But dock lights wouldn’t promote a blue-green algae bloom without the right precursors. “Since the lake’s bloom-forming cyanobacteria start out in the sediments, it’s possible that prolonged exposure to artificial night lighting could supplement their growth but not be the main driver of bloom formation,” says Kirkwood. “There are other factors that contribute to excessive algal growth, such as nutrients and water temperature.”
Cottage Q&A: Why did this slimy algae appear on our lake?
Note that she said “prolonged exposure.” There’s a difference between turning on the dock lights for a couple of hours at night, on a few weekends, compared to leaving them on all night for every single night of the summer.
“Duration of light exposure is directly related to growth rate,” says Kirkwood. “An external lighting scenario supporting algal growth would be multiple spotlights shining down on the water throughout the night.” Plus, “algal growth is light-limited, meaning the lower the intensity and quality of light, the lower the growth rate,” she says. Bottom line? “A select few dock lights used to improve visual safety at the dock would likely have minimal to no effect on algal growth.”
Cottage Q&A: Why do we get algae some years and not others?
We assume that you—and hopefully your lake neighbours—are using all exterior lights this way: for safety when necessary. Because nobody wants the lake lit up like a hospital operating theatre. Well…maybe the algae does.
Got a question for Cottage Q&A? Send it to answers@cottagelife.com.
This article was originally published in the Early Spring 2026 issue of Cottage Life.
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