Heat Wave Spurs Harmful Algal Blooms in Some Minnesota Lakes

Two weeks ago, Woodbury resident Anna Barker noticed something unusual while canoeing on Carver Lake. Floating amidst the normal mix of pondweed and coontail were clumps of brown gunk that looked an awful lot like poop. Around the same time, in Forest Lake, the Comfort Lake Forest Lake Watershed District received a number of calls from concerned residents asking about algae growing in the lake. Could it be harmful to people and pets?

Blobs of blue-green algae in Carver Lake, Woodbury – July 2021. Photo by Anna Barker.

During the hottest months of summer, many Minnesota lakes transition from blue to green as duckweed, lilies, algae and other aquatic plants proliferate. The change is most pronounced in shallow, nutrient-rich lakes where phosphorus and nitrogen act as fertilizer in the water. Usually, the algae and aquatic plants are merely a nuisance, but sometimes a form of cyanobacteria known as blue-green algae can bloom and release toxins that make humans and animals sick.

After collecting and sending samples to a lab for testing, watershed managers came back with good news for Forest Lake and bad news for Woodbury. The water samples from Forest Lake did not contain cyanobacteria; in the case of Carver Lake, however, the poop-like clumps were found to contain concentrated bundles of blue-green algae. As a result, the City of Woodbury has now closed the swimming beach at Carver Lake Park and is advising people to avoid swimming, wading, or letting their dogs drink from the lake.

For the past five years, a team of researchers from University of Minnesota St. Anthony Falls Laboratory (SAFL), University of Minnesota Duluth’s Natural Resources Research Institute, Minnesota Extension, Minnesota Sea Grant, Minnesota Pollution Control Agency, and the Science Museum have been working to learn more about how and why harmful algal blooms form. We know that runoff pollution and excess heat can be triggers, but experts aren’t entirely sure why some blue-green algae produce toxins while others remain harmless.

Last summer, a team of researchers from UMD’s Natural Resources Research Institute collected and analyzed samples from six sentinel lakes, stretching from southwestern Minnesota, to the Twin Cities, to the far northeastern corner of the Arrowhead. Led by Dr. Christopher Filstrup, the team is using DNA and RNA sequencing and toxin screening approaches to examine environmental conditions that may cause blue-green algae to produce toxins. The six lakes studied represent a range of latitudes, climates, and land uses representative of lakes in Minnesota. The study is still in progress.

Dr. Christopher Filstrup is an applied limnologist studying harmful algal blooms. Photo from University of MN – Duluth.

To avoid harmful algal blooms, stay out of lakes, rivers, ponds and bays that are green and smell “swampy.” Blue-green algae blooms often look like pea soup or spilled green paint, but are sometimes restricted to portions of a water body or appear as clumps in the water (as is the case with Carver Lake). Blue-green algae thrives in water that is warmer than 75°F.   

Algae in Little Rock Lake, MN, which is part of the Mississippi River-Sartell Watershed. Photo from Minnesota Pollution Control Agency.

You can become sick if you swallow, have skin contact with, or breathe airborne water droplets while swimming, boating, waterskiing, or tubing in water that has cyanobacteria. Symptoms generally begin with a few hours to two days after exposure and may include vomiting, diarrhea, rash, eye irritation, cough, sore throat, and headache.

Animals can experience symptoms such as vomiting, diarrhea, weakness, difficulty breathing, and seizures within minutes of exposure to the toxins. If you suspect a blue-green algae bloom, keep your dog out of the water and do not let them drink.

Dogs can get sick or even die after swimming or drinking from water with blue-green algae blooms. When it doubt, keep people and pets out.

In the long-term, our best defense against harmful algal blooms is to minimize runoff pollution to our lakes and rivers. You can help by using less fertilizer on your lawn, maintaining buffers of native plants along lakes and streams, inspecting and maintaining septic systems, and adopting a storm drain near you (www.Adopt-a-Drain.org).

The Minnesota Pollution Control Agency tracks reports of potential harmful algae blooms. Email pictures of suspected blooms to algae.mpca@state.mn.us

In addition, report human health effects to the Minnesota Department of Health (MDH) Foodborne and Waterborne Illness Hotline at 877-366-3455. For health questions, visit www.health.state.mn.us/diseases/hab/index.html.