Past blue-green algae blooms in Halifax have not been dangerous to humans, research finds

Blue-green algae can be a real buzzkill for those looking to cool off with a dip at one of Halifax's 20 supervised beaches, but new data from researchers at Dalhousie University suggests the risk to humans who go swimming when the algae is present has been limited over the past few years. 

The team monitored eight recreational beaches over three years and found that though the genes that produce blue-green algae were present at each site, the toxins detected were well below Health Canada's acceptable levels for toxins in drinking water, let alone recreation.

"We don't know that that's the conditions all the time, but through our research, it's looking really promising that even where we're seeing these blooms, we're not seeing toxins that would be considered a risk to people who are swimming," says Elizabeth Montgomery, a water resource specialist for the municipality's environment and climate change team.

The sites monitored include Lake Banook, Kearney Lake, Penhorn Lake, Chocolate Lake, Cunard Pond and several spots in Shubie Park. Cunard Pond, in particular, has seen many beach closures due to blue-green algae blooms in recent years.

Blue-green algae occurs in all parts of Nova Scotia and once it appears in a body of water, it will bloom again when the conditions are right.

The algae produces toxins that can cause illness in humans and be fatal to pets.

All Halifax Regional Municipality supervised beaches are monitored for blue-green algae from July 1 to Aug. 31.

As per the HRM's current policy, if blue-green algae is spotted at one of the city's supervised lakes, the beach is shut down until further testing can be completed. 

On Tuesday, Montgomery told CBC News the research gives the municipality a better picture of what's happening in city lakes and may help them make faster decisions around reopening beaches.

"We're looking at being more reactive, using the information we're getting from this research, and testing more quickly, and depending on what those test results say, opening the beach," she said. 

Overall, Montgomery sees the results as encouraging. 

"It's a relief because we advise people not to swim at our beaches when they're closed but we know that they do anyway," she said. 

She said people still shouldn't let their guard down while swimming, making sure to avoid drinking the water and to dry off vigorously and shower when they're done. 

But dogs face more of a risk, she said, though not typically from accidentally drinking lake water when blue algae is present. 

They can be harmed by accidentally eating benthic blue-green algae mats — slimy sheets that can wash up on shore and are often fatal when ingested, as was likely the case when two dogs died at Shubenacadie-Grand Lake in 2021.

The only way to make sure you're swimming in water that has been tested, Montgomery said, is to visit one of HRM's supervised beaches.

"If you're looking for that extra level of precaution, I do recommend coming … they're really nice," she said. 

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