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Blue-green algae blooms present at three Minneapolis lakes

Friday, May 22, 2020

June 2, 2020 Update: The Minneapolis Park and Recreation Board (MPRB) has documented improved conditions at the three lakes impacted by blue-green algae blooms. See below for the latest information from MPRB about the status of these harmful algage blooms.

On May 22, the Minneapolis Park and Recreation Board (MPRB) issued an alert for a "harmful algae bloom" on three Minneapolis lakes: Nokomis, Cedar, and Isles. Recent visitors to the lakes may have noticed the discolored material floating in the water. 

The issue is caused by a "cyanobacteria" bloom —more commonly known as a blue-green algae bloom, though the color can vary. While blue-green algae blooms are more common later in the season, MPRB laboratory analysis has confirmed that this particular species thrives in cool conditions and began proliferating under the ice.

The blooms in these lakes are attributed to the existing presence of algae that is taking advantage of this season's unique combination of early ice-out, a cooler-than-average spring, and the existing levels of high nutrients within the lake. For example, it appears that Cedar Lake experiences its highest annual phosphorus concentrations between April and May of each year.  This combination of factors provided optimal growing conditions early in the season when other plants are not competing for the same limited resources. 

MCWD will continue to evaluate the situation as it evolves in close coordination MPRB, which is actively monitoring and publishing information to keep park users informed and safe.

Below is more detailed information from MPRB about these blooms, their underlying causes, and the associated safety considerations. 


Update from MPRB 

Posted on June 2, 2020

Conditions have improved since May 22, when the Minneapolis Park and Recreation Board (MPRB) confirmed blue-green algae blooms at Cedar Lake, Lake Nokomis and Lake of the Isles.

Signs that these algae blooms have subsided include:  

  • Improved water clarity, based on measurements at Cedar Lake and Lake of the Isles last week.
  • At all three lakes, MPRB water quality staff have observed that the brown color indicative of these algae blooms has faded. 

Based on those indicators, in the coming days MPRB will be removing the blue-green algae warning signs at the lakes. 

Testing water samples is a third way to confirm the clearing of a harmful algae bloom. MPRB's plan to report definitive test results for all three lakes later this week is now delayed for as much as two weeks. 

In the meantime, we recommend people follow the guidelines below from the Minnesota Department of Health (MDH) to make their own decision about recreating in these lakes. Anyone uncomfortable with that option should stay out of the water.  

MPRB: Blue-green algae blooms present at three Minneapolis lakes

Since May 15, when the Minneapolis Park and Recreation Board (MPRB) reported on a likely harmful algae bloom (HAB) at Cedar Lakeblue-green algae has been confirmed at Lake Nokomis and Lake of the Isles, as well as Cedar Lake. MPRB is advising that people and pets stay out of the water when these blooms are visible. 

Contrary to its name, blue-green algae can turn lake water into a murky brown color, as it has on these three lakes. Blue-green algae is actually a bacteria that is that is normally present in many lakes throughout the state and country. When conditions are right, this naturally forming bacteria can grow quickly, forming blooms throughout all or portions of a lake. 

Blue green algae can produce cyanotoxins, which can make humans and animals sick, particularly if ingested. According to the Minnesota Department of Health (MDH), symptoms of illness from blue green algae can include vomiting, diarrhea, rash, eye irritation, cough, sore throat, and headache. Symptoms generally begin several hours up to 2 days after exposure. 

The MPRB is posting warning signs on the shorelines of all three lakes and at connecting channels.  

About Harmful Algae Blooms (HABs) 

There is no way to tell if an algae bloom is toxic just by looking at it. Therefore, the MDH advises the following: 

  • Don’t swim if you can’t see your feet in knee-high water.   
  • If you’ve recently been in contact with an algae bloom, make sure to wash off with fresh water. 
  • When in doubt, stay out!  
  • Don’t let dogs drink or swim in the water. *
  • For more information on minimizing your risk of exposure to blue-green algae and harmful algal blooms from swimming, boating, or consuming fish, visit the MDH website:

* Note: Dogs are not allowed in Minneapolis water bodies, per MPRB ordinances.

Why is this happening? 

Algae are a natural part of lake ecology, and as weather and lake conditions change, different types of algae can grow. The species of algae currently blooming grows better in cooler conditions. 

Plankton samples MPRB collected last winter show that the algae blooms at these three lakes started under the ice. In a normal year, ice would melt off slowly and the algae would die off before it got a foothold in the lake. This spring, a rapid ice-out and cooler than average temperatures led to conditions that allowed the algae to persist. These blooms will diminish or subside as air and water temperatures rise. 

The MPRB continues to monitor the algae blooms in Cedar Lake, Lake of the Isles and Lake Nokomis, and the water quality at all other lakes in the Minneapolis park system. 

Updates will be shared as new information becomes available, and if algae blooms at any lake in the park system are deemed potentially harmful.  

More information 

MDH: Preventing Harmful Algal Bloom-Related Illness

MDH Fact Sheets