Would you know blue green algae if you saw it at your local lake?

The municipality wants to help Haligonians identify the potentially harmful bacteria and learn about the risks for their pets and children.

HRM held an information session on the shores of Oat Hill Lake in Dartmouth on Friday. Elizabeth Montgomery, water resources specialist with HRM, gave reporters a primer on blue green algae.

“Cyanobacteria is another name for blue green algae. It’s not actually algae at all. It’s little bacteria, and there’s some of the oldest organisms on Earth. They’re actually the reason we have an oxygen rich atmosphere,” Montgomery said.

“They have always been here, but we’re starting to see them in higher concentrations in our urban lakes and and we just want to give some information about why that is.”

A white woman wearing a white shirt stands in front of a lake on a sunny day.
Elizabeth Montgomery, water resources specialist with HRM, speaks to reporters at Oat Hill Lake in Dartmouth on Friday. Credit: Zane Woodford

Montgomery said blue green algae blooms typically happen in clear, still water that’s exposed to sunlight during the summer months. They like warm water, meaning climate change is likely to lead to more cyanobacteria.

“We often see them more in the morning. And and they can look like green spilled paint, they can look like grass clippings, the water can be like quite green, or it can be … like small little green or blue particulate in the water,” Montgomery said.

“Some of them produce toxins, and some of them don’t. And so I think that’s why we are telling folks if you see something, assume that it’s toxic, but it doesn’t necessarily mean that it is.”

Even experts on blue green algae need to examine it under microscope to tell whether it’s toxic, Montgomery said.

Blue green algae is seen under the surface of water near the shoreline, with a rock poking out and reeds in the foreground.
A blue green algae bloom in Lake Banook in 2018. Credit: HRM

Blooms can also look like pollen or even an oil spill. The difference is that the blue green algae is under water too, whereas pollen and oil sit on the surface.

“They also do better in nutrient rich environments, which is why we’re starting to see these in urban lakes because you have a lot more phosphorus and nitrogen coming through the stormwater system, coming from overland runoff just because it’s a more urbanized area,” Montgomery said.

There could be blue green algae blooms happening in more remote lakes too. They’re noticed in urban areas because there are people around to see them.

The bacteria has killed at least eight dogs in Nova Scotia and New Brunswick in the last 10 years, Montgomery said. That includes one confirmed death this year at Long Lake in Hammonds Plains. In 2021, two dogs died after ingesting blue green algae on Fish Lake, next to Grand Lake.

Benthic mats causing dog deaths

But it’s not the typical blue green algae bloom that’s so harmful to pets; it’s the benthic mats that coat the bottom of lakes and creep up their shores. The mats are something scientists in Nova Scotia have only really been learning about over the last two years.

“They’re a new field of research in Nova Scotia, but they’re also a new field of research worldwide. We just don’t know that much about them,” Montgomery said.

“We’re really fortunate here that we have active research happening in Nova Scotia and New Brunswick, to learn not only about these things in general, but also to learn about them here specifically.”

A blue green algae mat is seen under water, with the sun reflecting off the surface.
Living benthic mat material covering the tributary between Fish Lake and Grand Lake in 2022. Credit: Jamieson Lab/HRM

The benthic mats grow across the bottom of a lake or river. They’re not stringy like algae, and they can bubble.

The blooms, while still potentially harmful, are much less dangerous. They contain liver toxins, not neurotoxins like the mats.

“We haven’t seen instances of serious illness or serious adverse health effects in Nova Scotia associated with the blooms that you think of when you think of blue green algae, like those floating blooms,” Montgomery said.

A person would have to swallow a significant amount of blue green algae in a bloom to experience any effects. Montgomery said blooms are more likely to cause rashes, itchiness, and hives.

Brownish-grey blue green algae material is seen on a shoreline among reeds.
Dead benthic mat material on the shoreline of Grand Lake in 2021. Credit: Jamieson Lab/HRM

But the mats are different.

“We think that they’re sending out a smell that makes dogs interested in them. But also dogs do like to put stuff in their mouth that’s on the shoreline. And so they’re eating mat material, and it’s a pretty potent neurotoxin,” Montgomery said.

“They don’t have to eat that much of the mat material to be exposed to the adverse health effects. And it’s really sad, like it happens really quickly.”

Eating the mat would likely kill a human too, Montgomery said. They’re just less likely to put green sludge in their mouth. But children might, and parents should keep a close eye on them along shorelines.

Nutrients cause blooms, less known about mats

Nova Scotia wasn’t officially aware of the blue green algae mats until recently. But Montgomery said when HRM met with members of the Oathill Lake Conservation Society following a recent discovery in the lake, they said they’d been seeing them for years.

“When those folks were out with the researchers, and they were showing what the mats looked like, they were like, ‘Oh, that’s been here forever,'” Montgomery said.

“We’re not sure how much of these mat material are influenced by human activities that are sending nutrients into the water, whether they’re affected by climate change, whether they’re affected by rainfall, what time of year they come. Those are big questions, right?”

Added nutrients in a lake are known to cause blue green algae blooms. Less is known about the benthic mats.

“The science is so new, we are not even 100% sure that they’re caused by the same thing that’s causing the bloom,” Montgomery said.

Either way, making sure nothing harmful is entering the lake is worthwhile. That could be action from residents, or government intervention like climate action, land-use zoning, and reducing run-off from roads.

“If the nutrients aren’t getting in the lake, there’s a lot less for these algae to eat and so they’re less likely to bloom in concentrations where you’re seeing toxin,” Montgomery said.

HRM hopes to learn more about lakes generally through its LakeWatchers program, started in 2022. It’s still looking for volunteers with a goal of testing more than 70 lakes twice a year.

How to avoid blue green algae

At HRM’s supervised beaches, lifeguards are monitoring for blue green algae daily.

“You may think well, they’re kids, they’re not experts, but they’re there every day and the best way to really notice when there’s something different is just having eyes on it all the time,” Montgomery said.

Any time a lifeguard spots what could be blue green algae, staff examine the photos to see whether they can rule it out. If they can’t, the municipality closes the beach pending testing for toxins. It’s a conservative approach, Montgomery said.

Earlier this summer, for example, HRM closed Springfield Beach in Lower Sackville.

“And once we sent it to our man with a microscope, there was nothing there that could be producing toxins,” Montgomery said.

HRM reopened that beach in two days. Cunard Lake Beach on Williams Lake has been closed for a week due to blue green algae.

“We don’t know that there were toxins but there are species there that could produce toxin and we are just being super cautious at this time,” Montgomery said.

People who want to be sure they’re not going to come into contact with blue green algae can stick to those supervised beaches.

At other lakes and rivers, Montgomery said it’s just about looking before you go in for a dip.

Provincial list doesn’t mean closure

The provincial government has a list of lakes with blue green algae blooms on its website. Montgomery said that list is based on reports from residents.

“I’m not sure that it’s been super clear to folks, just based on some of the questions that we’re getting through 311, that it’s not telling you not to swim,” Montgomery said.

“It’s telling you that you should be more mindful or careful before you swim like someone has reported that they might have seen blue green algae in the lake.”

The province doesn’t remove lakes from the list, either. For instance, Oat Hill Lake is on the list, but there was no blue green algae present on Friday.

Montgomery stressed that there are greater risks than blue green algae at lakes, drowning topping the list.

For more information and photos, check the municipality’s blue green algae website.

Zane Woodford is the Halifax Examiner’s municipal reporter. He covers Halifax City Hall and contributes to our ongoing PRICED OUT housing series. Twitter @zwoodford

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  1. This is a very helpful explanation and it’s nice to know that I can narrow my paranoia about my dog ingesting blue green algae a little bit!

  2. I think what needs to be emphasized is that these nutrient inputs are NOT a default that comes along with development.

    We can have sustainable development along waterways that have appropriate vegetative buffers and runoff management that absorb nutrients etc. It just takes the will of the province and the municipalities to 1) create good regulations around vegetation management above the high water mark 2) apply them and 3) enforce the rules when violations occur. It’s also important to look throughout the watershed that feeds the lake throughout it’s watershed and ensure these nutrient inputs aren’t coming from far away and causing problems.

    If people want good water quality on their lake they should be working hard to get policies in place that eliminate as much hardscaping, lawns, rock walls and storm runoff etc. around their lake and in all waterways that feed into it.

    We know from studying natural low nutrient lakes in NS that these algal and cyanobacteria blooms are relatively rare without a huge influx of nutrients or very specific conditions of sustained high temperatures with little mixing of the water column.
    Some lakes in NS have naturally high levels of nutrients but a very large proportion of lakes in NS are naturally low nutrient lakes. Experts have looked back at sediment cores and seen that these lakes have been low nutrient environments for hundreds to thousands of years.