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8/24/23 Cyanobacteria Update

On 8/6/23, a purplish bloom was observed on the water surface of Onota Lake near the Thomas Island shoreline. Since then, these blooms have only been seen in parts of the lake’s north basin (north of the sandbar), specifically the area near the southwestern and western shorelines of Thomas Island and extending somewhat further out from there.

Samples were collected, and this material was determined to be Planktothrix, a type of cyanobacterium that can potentially (but not always) release harmful toxins, resulting in what is commonly called a ‘Harmful Algal Bloom’, or ‘HAB’. For more information on HABs, see: NWQP Research on Harmful Algal Blooms (HABs) | U.S. Geological Survey (
The cyanobacteria patches have appeared and disappeared since first reported on 8/6, generally becoming smaller and less ‘concentrated’ with each reappearance. Most recently, a few tiny patches were observed near Thomas Island’s Shore Drive. A sample from these accumulations was collected and has been sent to a laboratory for analysis of cyanotoxins. This will tell us whether or not the Planktothrix cyanobacteria are producing toxins.

The Pittsfield Department of Health has posted warning against water contact for people and pets in the affected area (dogs are especially vulnerable to ingesting the toxins when they drink the water and lick their fur after swimming), and the Thomas Island Association has alerted area residents via email. Organizers of activities at Burbank Park have been informed to keep water contact activities (including kayaking) to the south side of the sandbar. Residents and other lake stakeholders will be informed of the lab test results as soon as they are available, and additional lake recreational use safety guidelines may be warranted. Until such time, all current guidance is in effect.

Anyone observing this phenomenon is urged to contact city open space manager Jim McGrath by email ( or phone (413) 499-9344.