County Of Sherburne issued the following announcement.
With heavy spring rain and recent warm, sunny weather, the conditions are optimal in some area lakes to produce blue-green algae. As the name suggests, blue-green algae has a bright, blueish-green color and may resemble an oil slick or spilled paint as it sits on the water’s surface.
Blue-green algae can sometimes (but not always) produce toxins that are harmful to humans and pets. Thus, the Minnesota Pollution Control Agency (MPCA) advises that if blue-green algae is suspected, people and animals should be kept out of the water for several days to weeks until the algae has broken down.
Original source can be found here