Blue-Green Algae

green scum floats on water surface near shore
Surface scum from blue-green algae near the Railroad Swing Bridge above Log House Landing, in August 2012.

Chauncey Anderson

Algal Blooms

Have you ever seen parts of your favorite lake or river turn into pea soup during the summer? Or maybe it looks like a can of green paint spilled? When temperatures climb, conditions are ripe for algae blooms to form on lakes or slow moving portions of rivers.

Why do the blooms form?

Blooms thrive in shallow, warm, non-moving bodies of water. High levels of nutrients like nitrogen and phosphorous, warm water temperatures and high light levels - or a combination of all three factors - may stimulate the rapid reproduction of algae until it dominates the local aquatic ecosystem, forming an algal bloom.

Blooms vary in appearance and can appear as foam, scum, or mats on the surface of the water. These blooms may manifest in a variety of colors, including blue-green, bright green, brown, or red.

While not common on the main channel, algal blooms do occur on the lower St. Croix River and Lake St. Croix, especially from late summer to mid-fall. Typically these algae remain suspended in the water column, but surface scums have been reported from some locations.


How can I help prevent harmful algal blooms?

  • Support efforts by the National Park Service, state and local resource agencies, and others to reduce nutrient inputs to the St. Croix River and its tributaries.
  • Reduce or eliminate the use of fertilizers. If fertilizers have to be used, encourage the use of low- to no-phosphorous products.
  • Check septic systems regularly and make sure the tanks are emptied every two to three years. Improperly functioning septic tanks can contribute to algal blooms by releasing nutrients into the waterway.
  • Eliminate the use of products like soaps and dishwasher detergents that contain phosphates.
  • Wash cars on the lawn so the runoff filters through the soil instead of running straight off of pavement and to the gutters.
  • Encourage the use of rain barrels to reduce runoff.
  • Consider installing pond aeration systems in small ponds and lakes that have had algal blooms in the past.
  • Promote native and natural landscaping around waterways and backyard ponds. Tall grasses help filter some additional nutrient runoff - and they discourage Canada geese from taking up residence. Goose fecal waste contributes to excessive nutrient loading in water.

Tips for staying safe

  • After swimming in a lake, pond, or river with evidence of an algal bloom, always rinse off.
  • Rinse pets that have been swimming.
  • Don't drink water from lakes and rivers, as it may contain algal toxins or pathogens and bacteria. Boiling water will not get rid of the toxins.
  • Make sure pets do not drink from a water source that may have contact with a harmful algal bloom.
  • If anyone becomes ill after swimming, they should seek medical attention immediately. Seek veterinary assistance for pets that appear to be ill.

What are the symptoms of exposure to a harmful algal bloom?

People can experience numbness of lips, tingling in fingers and toes, dizziness, headache, rash or skin irritation, abdominal pain, diarrhea, and vomiting.

Your pet can experience weakness, staggering, convulsions, difficulty in breathing, and vomiting.

water near sandy shore is abnormally yellowish-green in color
Surface scum at Lakeland Beach, near Hudson, WI, in July 2007.

Molly Shodeen

More information on blue-green algae, how to determine if you have blue-green algae, and how to report a possible human or animal illness, is available on the Minnesota Pollution Control Agency Harmful Algal Blooms website.

Last updated: September 7, 2021

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