Swimmers are being advised to stay away from a popular Victoria-area lake after the discovery of toxic bacteria suspected in the deaths of several dogs last year on Vancouver Island.

Beaver Lake tested positive earlier this month for a type of blue-green algae that produces harmful cyanotoxins, prompting the Capital Regional District to warn visitors against swimming or allowing their animals to splash in the water.

Toxic chemicals produced by blue-green algae — also known as cyanobacteria — have been found in hundreds of lakes across Canada in recent years, particularly in the prairies.

The increasing prevalence of these single-celled organisms is a bit of a mystery, according to biologist Charles Trick at Western University in Ontario.

"We've been seeing it a lot more over the last decade," Trick said.

"We kind of thought we'd solved the problem in the 1960s, but somehow we're getting more nutrients [in the water] and maybe warmer lakes."

blue-green algae

A bloom of blue-green algae as shown by researchers near Edmonton. (University of Alberta)

Cyanobacteria thrive in phosphorus-rich water

Last year, blue-green algae were suspected in the deaths of at least four dogs in the Cowichan area. The animals all became sick after swimming in Quamichan Lake.

Swimmers were also warned to avoid Beaver Lake and nearby Elk Lake last fall because of blue-green algae.

Cyanobacteria lives in phosphorus-rich waters, and the mineral can leach into lakes from agricultural runoff, fertilizers or sewage.

Growing populations near lakes, combined with the human love of a pretty view, could be one explanation for the growing presence of blue-green algae in Canadian lakes.

"Usually, we've had forests and soils to kind of scavenge or bind [nutrients] on the way, but we like looking at lakes, so we take out many of the plants," Trick said.

"All that brush underneath in the lower part of your canopy ... they're really the workhorses of stopping nitrogen and phosphorus from getting into the lake."

Searching for a solution

Staff at the Capital Regional District are working on a plan for dealing with the cyanobacteria in Beaver Lake, but they're not expected to report back until the fall.

One option could be something known as high-efficiency oxygenation, a process that feeds dissolved oxygen into the water.

That option could work well in a relatively healthy body of water like Beaver Lake, Trick said. Theoretically, the oxygen would cause the phosphorus to sink into the sediment at the bottom of the water, instead of being available to cyanobacteria at the surface.

A task force charged with reducing blue-green algae levels at Quamichan Lake are testing out a different solution, adding barley straw to some of the streams that feed into the water.

"It does help the lake a little bit because it makes an anti-cyanobacterial compound that seeps into the lake," Trick said.

"That kind of works but only for a while."

Conditions in more damaged lakes, like Quamichan, make them poor candidates for the oxygenation method, according to Trick.