Every summer, we look forward to certain things: creemees, sunny days at the beach, and cooling off in Lake Champlain. But each summer, cyanobacteria algae blooms make an appearance in Lake Champlain and the surrounding bodies of water.


You’ve probably heard about cyanobacteria algae blooms in the past, most likely right about now. But what are they, and what causes them?

Cyanobacteria, commonly referred to as blue-green algae, are photosynthetic bacteria that occur naturally in waters used for recreation, such as swimming and waterskiing.

Certain environmental conditions, such as elevated levels of nutrients from human activities (e.g., nitrogen and phosphorus), warmer temperatures, still water, and plentiful sunlight can promote the growth of cyanobacteria to higher densities, forming cyanobacterial blooms. When the bloom is formed by a toxin-producing bacteria, it is generally referred to as a harmful algal bloom.

These blooms are considered harmful due to the production of irritants and/or toxins, called cyanotoxins , which can pose health risks to humans and animals.

An Algae Bloom

  • Is like thick pea soup or spilled paint on the water’s surface 

  • May create a thick mat of foam along the shoreline 

  • Is generally green or blue-green in color, but can be brown, purple, red or white
  • Is made of small specks or blobs floating at or below the water surface

An Algae Bloom is NOT:

  • Stringy, bright grass-green, long strands that feel slimy or cottony—this is harmless green algae
  • Mustard yellow in color (this is probably pollen)

Are algae blooms dangerous?

Swimming or wading in water with cyanobacteria may cause minor skin rashes, sore throats, diarrhea, stomach problems, or more serious health problems. Children and pets are at higher risk of exposure because they are more likely to play near the shoreline and drink water while swimming. Pets can also lick and swallow cyanobacteria that may be caught in their fur.


General health effects caused by exposure to cyanobacteria cells (not their toxins) include:

  • Rashes or skin irritation
  • Allergy-like reactions such as a runny nose or a sore throat

Additionally, some cyanobacteria may produce harmful compounds called cyanotoxins. When these toxins are swallowed in large amounts, they can cause the following health effects:

  • Sharp, severe stomach problems like diarrhea and vomiting
  • Liver damage that may take hours or days to show up in people or animals
  • Numb limbs, tingling fingers and toes or dizziness

Pets and other animals

Cyanobacteria toxins can also cause illness and sometimes death in pets and livestock. Possible symptoms animals may show after being exposed to a toxic bloom include:

  • Weakness or staggering
  • Difficulty breathing
  • Convulsions
  • Vomiting or diarrhea

It is not possible to tell if a bloom contains harmful toxins just by looking at it. Only laboratory tests of water samples can confirm whether a bloom is toxic.
Signs that a bloom may be toxic include:

  • Dead fish, ducks, geese or other animals
  • Sudden and unexplained sickness or death of a pet that has been exposed to a bloom
The Health Department partners with other state agencies, environmental organizations, recreational site managers, town health officers, drinking water system operators and hundreds of volunteers to monitor cyanobacteria blooms each summer and test affected waters for cyanotoxins

If you believe that someone has become sick because of exposure to cyanobacteria, get medical attention and call the Health Department at 800-439-8550.

How can you help reduce algae blooms?

Limit your Fertilizer Use

No matter where you live, if you use fertilizer on your lawn, it could be contributing to algae blooms on Lake Champlain. Rain and stormwater will often wash the fertilizer from your lawns into storm drains, leading directly to Lake Champlain. Read about limiting your fertilizer use here.

Redirect your Downspouts

Often times, stormwater runoff picks up fertilizer and other chemicals as it runs to the nearest storm drain. This is especially if your gutters empty out over driveways or impervious surfaces. Redirecting your downspouts to a garden or other plants can help reduce the runoff in your yard and in your neighborhood. Read about this easy fix here.