We have a man-made frog pond at our cottage, which has been home to many frogs since we bought our place in 2007. This year, we arrived in the spring to find the water murky, with green slime floating on top. There are no frogs to be found. We have never done maintenance of any kind from year to year, but I did trim out some cedars that were filling in too close to the pond. What could be the cause of the disappearance, and how do we go about restoring the quality of the water to bring back our frogs?—Sandy Spence, Tobermory, Ont.
You’re right that something’s up with the water. “The green slime is likely algae, which could mean the water quality in the pond has decreased because of excess nutrients,” says Rachelle Fortier with the Toronto Zoo’s Adopt-a-Pond program. Frogs are extremely sensitive to changes in their environment. “They often die or leave the area if conditions are not ideal,” she says.
So what happened? It’s hard to say for sure, but excess nutrients can often come from external sources—fertilizers, for example. We’re not suggesting that you’re using fertilizers. But a neighbour might be. Depending on the size of the pond, even a small amount of contaminant could make a difference in water quality.
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“There are so many factors at play when it comes to eutrophication—excess plant and algal growth in a body of water—including nutrient availability, temperature, light…” says Fortier. “It is a complicated process.”
Don’t feel bad, but, unfortunately, you trimming the cedars may have contributed to the algae problem. “The removal of natural vegetation could have an impact in various ways, for example, increasing direct light or increasing the potential for overland runoff,” says Fortier.
You, can of course, investigate what might have caused the excess nutrients in the pond—check with your neighbours or your lake association. Has anyone else seen changes on their properties? But regardless of what you learn, Fortier suggests adding more native aquatic plants to the pond, for example, floating-leaf species such as water lilies. “They’ll provide shelter and help uptake excess nutrients.” You can’t untrim the cedars, but you can plant more native vegetation around the water’s edge—the more the better. Check with a local native plant nursery for suggestions. Here’s hoping your froggy pals return!
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This article was originally published in the May 2024 issue of Cottage Life.
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