Why do we get algae some years and not others?


Why do we get thick swarms of algae in the lake some years and not others? Could it be from someone’s septic system?
—Carol Vineyard, via e-mail

It’s true that malfunctioning or poorly maintained septic systems can increase the amount of phosphorus in a lake and fuel algae growth (other culprits are runoff from lawn fertilizers and putting soap in the lake). But before you start giving your lake neighbours—and their old, metal tank—the stink eye, know that this year’s algae explosion could just as easily be natural.

“The amount of algae seen within a particular lake can vary tremendously from year to year,” says Paul Frost, a professor of aquatic science with Trent University. “It’s a fairly common occurrence even for lakes that are relatively untouched by human activity. The precise explanation would depend on the type of algae growing in the lake and its particular life cycle.”

Along with water chemistry—including the levels of phosphorus and nitrogen—climatic factors such as water temperature, water turbidity, light levels, rainfall, and cloud cover can all affect algae growth, says Jennifer Winter, a research scientist with the Ontario Ministry of the Environment (moe). Since these conditions fluctuate, the algae growth also fluctuates. Don’t panic just because you’ve got algae in your water. Even in large amounts, algae may still be harmless.

However, you probably know that some growths, such as certain dreaded, toxin-releasing blue-green algae (cyanobacteria), can be hazardous to the ecosystem and human health. If your lake has large algal blooms that persist for several days (look for blueish-green water that resembles pea soup, lots of floating particles, or surface films), then—Holy Water Contamination, Batman! Better call the MOE’s Spills Action Centre, at (800) 268-6060, just to be safe.