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What is a Blue-green Harmful Algal Bloom?

Blue-green algae HABs in freshwater lakes and ponds, occur when specific types of photosynthetic bacteria (cyanobacteria aka blue-green algae) form blooms (visible, dense build-ups). Under certain conditions, these may produce harmful toxins.

Not all blooms produce toxins

It is not easy to tell if a bloom will produce toxins harmful to human health or animals. Large populations of blue-green algae may be required to produce toxins at a high enough level to affect those using the water for drinking or recreational use. Laboratory analysis of a water sample is needed to confirm the presence of toxins.

Environmental conditions that may trigger a bloom

Blooms may be triggered by a combination of water and environmental conditions, including:

A blue-green algae bloom may have the appearance of green and white spilled paint.
Blue-green algae may have the appearance
of green and white spilled paint.
  • excess nutrients (phosphorus and nitrogen)
  • sunlight
  • low-water or low-flow conditions
  • calm water
  • warmer temperatures
  • upwellings

Are some lakes more likely to have a blue-green algae HAB?

Some waterbodies appear to be more likely than others to have frequent blooms. Studies conducted in New York State and elsewhere continue to look at the specific triggers for each lake exhibiting blue-green HABs. Find out what waterbodies have a blue-green algae bloom notice.

How long does a bloom last?

Blue-green algae HABs may be short-lived, appearing and disappearing in hours, or long-lived, persisting for several weeks, depending on the weather and the characteristics of the lake.

What time of year do blue-green algae HABs occur?

Blue-green harmful algal blooms are most likely to occur between July and October, but have been reported in NYS as early as March and as late as November.

How are lakes affected by blue-green algae?

Blue-green algae blooms can alter lake ecosystems by:

Thick mats of blue-green algae on the lake surface block light needed for the growth of beneficial algae. Photo: Ohio Environmental Protection Agency.
Thick mats of blue-green algae on the lake surface
block light needed for the growth of beneficial algae.
Photo: Ohio Environmental Protection Agency.
  • reducing oxygen levels
  • preventing the growth of beneficial algae
  • producing toxins that are directly harmful to fish and other organisms

Reduce oxygen levels

As the bloom dies and decomposes, the amount of oxygen in the water decreases and can threaten fish and other aquatic life.

Prevent beneficial algal growth

The large thick bloom mats can block sunlight needed for other beneficial algae to grow and reduce the quality and amount of food resources available to the food web.

Toxic to fish and other organisms

Blue-green algae cells may produce toxins, however the toxins are not always produced and what controls toxin production is poorly understood.

The toxins are released when the cells break open; this can happen naturally during die off, when algaecides are applied, or in the stomach when ingested by people or animals. The toxins may be harmful to fish and other organisms. These nerve, liver and skin toxins can also sicken people, pets or livestock if consumed or ingested in large quantities, and can cause rashes or other allergic reactions.