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Heat wave leads to early reports of blue-green algae on some Minnesota lakes

The August-like weather could mean a green summer on local lakes.
Algae in Prairie Lake in Sherburne County, which killed a family dog after he fetched a tennis ball from the lake, the MPCA says.

Algae in Prairie Lake in Sherburne County, which killed a family dog after he fetched a tennis ball from the lake, the MPCA says.

Conditions are ripe for algal blooms on Minnesota lakes thanks to the recent record-breaking heat wave, and that could mean we're in for a green summer. 

Hot, calm conditions with little rain create the perfect environment for algae growth in lakes, ponds and slow-flowing streams, the Minnesota Pollution Control Agency (MPCA) says

Algae blooms can occur at any time during the year. However, Minnesota's lakes usually don't see major harmful algae growth until later in the season, when the weather is dry and hot for days on end and water temperatures are higher than 75 degrees.

But the recent stretch of 90-degree days along with little to no rain (most of the state is abnormally dry or experiencing a moderate drought) has led to a few reports of harmful algal blooms on Minnesota lakes, the MPCA told Bring Me The News on Monday.

"This is an early start to our algal bloom season. Usually, this is something that occurs a little closer to the Fourth of July or even really — the weather we've been having feels a lot like August — so this is something where we have a number of calls in the past couple of weeks, so it is picking up," Pam Anderson, who manages MPCA's surface water monitoring, told Bring Me The News. "We're seeing calls as far north as Brainerd and Grand Rapids areas and then certainly across central and southern Minnesota as well."  

This has the MPCA reminding people: "If in doubt, stay out."

That's because while some algae blooms are harmless, others can be deadly to pets and can sicken humans — and there is no way to visually tell which particular bloom contains harmful toxins (see photos of algae blooms here). The MPCA does not routinely monitor for algal toxins due to the fact that Minnesota has more than 11,000 lakes.

The most well-known is blue-green algae (it's not even algae — it's microscopic organisms called cyanobacteria). And certain types of blue-green algae can contain toxins that are deadly to dogs within hours and can make humans sickm, which symptoms including vomiting, diarrhea, rash, eye irritation, cough, sore throat, and headache.

This type of algae can have a "thick, cloudy appearance" that can look like green paint, pea soup or floating mats of scum, the MPCA says.

State health officials and the MPCA say people should avoid or minimize spending time in water that appears to have a blue-green algae bloom, but if you do go in the water, wash off quickly and don't drink it. 

People should keep their pets out too — if they do get in the water, you should hose them off right away before they can lick themselves clean. (They get sick by ingesting the toxins in the algae.)

If you or your pet get sick from a possible harmful algae bloom, you can report it to the Minnesota Department of Health by calling 1-877-366-3455.

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A green summer

Algae are a natural part of a lake's ecosystem and can come and go quickly, so it's best to stay away from the water until it rains, the wind shifts or temperatures cool down, which all disrupt algae growth, the MPCA says.

Algal blooms can develop in all Minnesota lakes but are most often found in lakes that have excessive levels of nutrients, like phosphorus, and in areas of a lake that are downwind or in a secluded bay or shoreline, the MPCA says.

"What algae like are hot, dry, calm conditions. They're an organism that likes hanging out on the surface of water and have perfect conditions for growth," Anderson said. "So most lakes have available food in them — phosphorus, the nutrient that's hanging out in our lakes — but when we get a lot of sun and a lot of hot temperatures, that's when these harmful algal blooms really thrive."

And continued hot weather could mean a green summer on Minnesota's lakes if it doesn't cool down. 

"The fact that we warmed our lakes up early certainly gives the blue-green algae an advantage over other algae that would be found in the water. So, if we continue with the hot, dry weather — we don't cool off or have long periods of rain — we're likely going to see this persist across the summer," Anderson said.

"And that means the opportunity to run into green water is going to be higher than it would be on a normal year," Anderson added.

What lakes get algal blooms?

There are no real statistics on how many Minnesota lakes have harmful algal blooms each year but the MPCA does measure phosphorus levels, which is a good indicator on whether a lake could develop an algal bloom. 

"In Minnesota, we have a lot of water. We struggle to be able to monitor all of it," Anderson said. The MPCA and its partners sample lakes for phosphorus and then measure the algae that's on a subset of lakes — a few hundred — each year.

The MPCA has a good idea of what's out there across the state but there is no easy way to measure harmful algae blooms because you can't tell by looking at them, Anderson said. Plus, the MPCA notes conditions change quickly, which can lead to the development of algal blooms or their disappearance. 

"So it's really a message of awareness and making sure that people watch out for water that looks green, gray, blue — kind of that scummy color — and to make sure that you avoid that and keep your kids and pets and livestock out," Anderson said. 

At least 150 Minnesota lakes have tested positive for harmful toxins (microcystin and anatoxin-a) that can occur in blue-green algae, the MPCA's website shows. Here's a map of the lakes that were tested as part of the National Lakes Assessment survey.

And EWG has an interactive map that shows results of state tests for algae bloom toxins in Minnesota, Iowa and Wisconsin, as well as news stories of where algae blooms were reported between 2010-2020, with EWG noting just because an algae bloom is marked on the map doesn't mean it was toxic because testing is limited in the three states. 

"What we're able to track is how many phone calls we get of reports of blooms," Anderson said. "We know we have impaired waters across the state, and we've already identified those as having too much phosphorus, so we would expect those lakes to be green more of the summer than we would prefer to have.

"But there are other lakes that are in really good shape where we get these high temperatures — those algae really can start taking off even if there's not a lot of phosphorus on the water," Anderson added.

You can report algae blooms to the MPCA's lake monitoring staff via email at or by calling 651-757-2822. If a lake doesn't meet water quality standards (if it's on the impaired waters list), the MPCA "will make note of the bloom." If the lake isn't considered impaired, the MPCA uses bloom reports to "help prioritize lakes for water quality monitoring when the watershed is next visited."

Preventing algal blooms

The best way to prevent algal blooms in the long term is to reduce the amount of nutrient runoff into lakes from fertilizers and organic materials, like leaves and yard waste. 

Anderson said the recent heat wave is going to cause stress to our lakes, noting fish will be stressed because the water is warming and can't hold as much oxygen, and lakes will be producing more algae, which becomes an issue for people recreating on lakes, as well as for pets and livestock. 

But there are things people can do to help prevent this. 

"The weather plays a driving role and this year we're going to have algae blooms, but it's also important to remind people that it does matter if you sweep up your grass clippings, clean up after your pets, if you can put in a rain garden — all those little pieces help," Anderson said. "There are things people can do to prevent nutrients from getting into our lakes. Once they're there and you get a nice warm stretch like this, we're going to have algae blooms."  

The MPCA's website explains this further: 

"We can't eliminate blue-green algae from a lake — they are an inherent part of the overall algal community. What we really want to do is control their overall intensity and the frequency of the blooms. Since we can't control the water temperature, the best thing we can do is to reduce the amount of nutrients getting into the lake. This can best be accomplished by reducing the amount of phosphorus and nitrogen from man-made sources such as lawn fertilizer, and runoff from cities, cultivated fields, feedlots, and a myriad of other sources.

"Though a reduction of nuisance algal blooms will not be immediate, it is the best long-term solution to minimizing the frequency and intensity of algal blooms."

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