---
type: guide
title: "Blue-Green Algae in Minnesota Lakes: How to Spot a Harmful Bloom"
description: "Harmful algal blooms can sicken people and kill dogs. How to recognize blue-green algae in Minnesota lakes and what MPCA and MDH advise."
category: "Swimming & Safety"
updated: 2026-10-01
url: https://www.minnelakes.org/guides/blue-green-algae-minnesota-lakes
---

# Blue-Green Algae in Minnesota Lakes: How to Spot a Harmful Bloom

## Key Takeaways

- Blue-green algae are cyanobacteria. Some blooms produce toxins, and you cannot tell which ones by looking.
- Warning signs: water that looks like pea soup or spilled green paint, surface scum, and a swampy smell.
- Blooms are most likely in warm, calm, sunny weather from midsummer into early fall, especially on shallow, nutrient-rich lakes.
- Dogs are at the greatest risk. Keep them out of scummy water, and rinse them with clean water right away if they get in.
- The agencies' rule is simple: when in doubt, stay out.

Blue-green algae are a real hazard on some Minnesota lakes in late summer, and they are the one water-quality problem that regularly kills dogs. The practical rule from both the Minnesota Pollution Control Agency and the Minnesota Department of Health is short: if the water looks like pea soup or green paint, stay out and keep pets out. Nobody can tell a toxic bloom from a harmless one by looking at it.

## What blue-green algae are

They are not algae. The [MPCA](https://www.pca.state.mn.us/air-water-land-climate/blue-green-algae-and-harmful-algal-blooms) explains that they are bacteria, called cyanobacteria, that live by photosynthesis the way plants do. They occur naturally in Minnesota lakes and are present in small numbers nearly everywhere. Trouble starts when they multiply into a visible bloom.

Some species can produce toxins that affect the liver, nervous system or skin. A bloom that contains those toxins is what agencies call a harmful algal bloom, or HAB. Not every bloom is toxic. Whether one is depends on which species are present, their genetics and where the bloom is in its life cycle. The same lake can have a harmless bloom one week and a toxic one the next, and the only way to know is a laboratory test that takes longer than your beach day.

## How to recognize a bloom

Look for these signs:

- Water that looks like pea soup or spilled green paint. Blooms are usually green or blue-green.
- A scum, film or mat floating on the surface, often pushed into a bay or onto a downwind shore.
- A swampy, unpleasant odor as the cells break down.
- Dried green or bluish crust along the shoreline, which signals a recent bloom.

Things that are commonly mistaken for blue-green algae and are not dangerous:

- **Duckweed**: tiny individual floating leaves you can pick up one at a time.
- **Filamentous green algae**: stringy, hair-like mats that drape over a stick when you lift them.
- **Rooted aquatic plants** growing up from the bottom.
- **Pine pollen** in late spring, a yellow dust on the surface.

If the green is dispersed through the water like paint, or forms a smooth scum, treat it as a harmful bloom.

## When and where blooms happen

According to the MPCA, blooms typically develop in summer and early fall when water temperatures climb above about 75 degrees, and they favor warm, calm, sunny weather. A stretch of hot, still days in August is the classic setup. Cool temperatures, rain and wind tend to break blooms up.

The other ingredient is nutrients, phosphorus above all. That makes location predictable:

- **Most at risk**: shallow, fertile lakes in southern and western Minnesota's farm country, and many Twin Cities lakes fed by urban runoff.
- **Less at risk**: deep, clear, low-nutrient lakes across the Arrowhead, Itasca County and much of the Brainerd Lakes area.
- **Not immune**: bays of large northern lakes. Lake of the Woods has well-documented late-summer blooms, and blooms have been reported on waters people think of as pristine.

Wind matters as much as anything on the day you visit. A bloom spread thinly across a lake can be blown into one bay and concentrated into thick scum within hours, and it can clear out just as fast. A beach that was fine this morning may not be this afternoon.

## Health effects in people

People are exposed by swallowing water, by skin contact, and by breathing in spray or droplets, for example while tubing or water skiing. The [Minnesota Department of Health](https://www.health.state.mn.us/diseases/hab/index.html) lists these common symptoms:

- Rash and blisters
- Eye irritation
- Cough, wheezing, sore throat and congestion
- Earache and headache
- Vomiting and diarrhea

Symptoms generally show up within hours to two days of exposure and typically resolve within a few days in healthy people. Children are more vulnerable than adults: they are smaller, they swallow more water, and they play in the shallow edges where scum collects.

If you become ill after contact with a bloom, call your health care provider and mention the exposure. MDH tracks these cases and takes reports through its foodborne and waterborne illness hotline at 1-877-366-3455.

### What about fish from a bloom lake?

MDH says there have been no confirmed reports of illness from eating fish caught during a harmful bloom, but that it remains uncertain whether fish can accumulate enough toxin to matter. If you keep fish from a lake with an active bloom, the cautious approach is to rinse fillets in clean water and discard the organs. Do not use bloom water to clean fish or wash dishes at camp.

## Protecting your dog

This is the part to take most seriously. The MPCA states that some blooms contain toxins that can kill dogs, livestock and other animals within hours, and that several dog deaths in Minnesota have been attributed to toxic algae.

Dogs are at higher risk than people for simple reasons:

- They are smaller, so a smaller dose matters.
- They wade into scum that a person would avoid.
- They drink the water while swimming.
- They lick their coats clean afterward, swallowing whatever is on their fur.

What to do:

1. **Look before you let the dog out of the car.** Check the shoreline where the dog will actually enter, not the view from the parking lot.
2. **Keep the dog leashed near suspect water.** A retriever will be in the lake before you finish evaluating it.
3. **Bring clean drinking water** so a thirsty dog is not tempted.
4. **If the dog gets into a bloom, rinse it immediately** with fresh, clean water and do not let it lick its fur.
5. **Watch for symptoms**: the MPCA lists vomiting, diarrhea, weakness, difficulty breathing and seizures. If you see any of them, go to a veterinarian right away and say the dog was in a possible algal bloom. Minutes count.

Hunters should note that blooms can persist into early fall, which overlaps with early waterfowl seasons on exactly the shallow, fertile lakes most prone to them.

## If you find a bloom

- Stay out, and keep children and pets out. Choose another beach or another lake. The [swimming directory](https://www.minnelakes.org/swimming) and [clearest lakes list](https://www.minnelakes.org/clearest-lakes) can help you find an alternative nearby.
- Do not boat, ski or tube through thick scum. Spray is an exposure route.
- If you touched the water, wash with soap and clean water.
- Never drink untreated lake water. Boiling does not make bloom water safe, and backcountry filters are not designed to remove these toxins.
- Report it through the bloomWatch app and tell the beach operator or lake association.

There is no statewide program that tests every lake or posts every bloom, so a missing warning sign means nothing. Your own eyes are the monitoring system.

## Why blooms happen, and why there are more of them

Blooms are fed by phosphorus and other nutrients reaching lakes from farm fields, lawns, streets and failing septic systems, and they are favored by warm water. The MPCA has repeatedly tied bloom risk to wet springs that flush nutrients into lakes followed by hot summers. Minnesota's lakes are warming, which lengthens the season in which blooms can form. Keeping fertilizer, leaves and grass clippings out of the street and maintaining shoreline vegetation are the main things property owners can do.

## Bottom line

- Pea-soup green, paint-like streaks or surface scum mean stay out. You cannot judge toxicity by eye, so do not try.
- Dogs die from this. Check the water first, bring clean water, and rinse the dog immediately after any contact.
- Blooms move with the wind and peak in hot, calm weather from late July through September. A different beach or a deeper, clearer lake is usually an easy fix.

## Frequently Asked Questions

### What does blue-green algae look like in a lake?

The Minnesota Pollution Control Agency describes it as looking like pea soup or spilled green paint. It may form a scum or mat on the surface and can give off a swampy odor as it breaks down. Blooms are not always large or thick, and they can pile up along one shoreline while the rest of the lake looks normal.

### Can you swim in a lake with blue-green algae?

You should not swim, wade or let children play where a bloom is visible. There is no way to tell by eye whether a bloom is producing toxins, so MPCA and MDH advise staying out of any water that looks discolored or scummy. Clear water elsewhere on the same lake is a judgment call, and wind can move a bloom quickly.

### How quickly can blue-green algae hurt a dog?

Very quickly. The MPCA says the toxins in some blooms can be deadly to dogs and other animals within hours of contact, and several dog deaths in Minnesota have been attributed to toxic algae. If your dog has been in a bloom and shows vomiting, weakness, difficulty breathing or seizures, get to a veterinarian immediately.

### What are the symptoms of blue-green algae exposure in people?

The Minnesota Department of Health lists rash, blisters, eye irritation, cough, sore throat, congestion, earache, headache, vomiting and diarrhea. Symptoms generally appear within hours to two days of exposure and usually pass in a few days in healthy people. Call your health care provider if you become ill after contact with a bloom.

### When do algae blooms happen in Minnesota?

Mostly in summer and early fall. The MPCA says blooms typically develop when water temperatures are above about 75 degrees, in warm, calm, sunny stretches. They can occur at other times of year if conditions line up, but late July through September is the usual window.

### Who do I report a harmful algal bloom to in Minnesota?

Report a suspected bloom through the bloomWatch app, which the MPCA points people to, and tell the beach manager or lake association. If a person becomes ill, the Minnesota Department of Health takes reports through its waterborne illness hotline at 1-877-366-3455.

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*Source: [MinneLakes.org](https://www.minnelakes.org/guides/blue-green-algae-minnesota-lakes)*