By Tiffany Williams –
WORCESTER, Mass. — The City of Worcester has closed Coes Reservoir to recreational use after testing found cyanobacteria levels exceeding the state’s recreational public health threshold, prompting officials to warn that the water may contain toxins harmful to humans and pets.
The Worcester Department of Sustainability and Resilience announced that all persons should stay out of the water until further notice.
Officials are also advising the public to discontinue boat use and fishing and to keep pets out of the water while the advisory remains in effect.
According to the city, cyanobacteria are naturally occurring but can become harmful when present in high densities.
On July 22, 2026, the municipality received cyanobacteria density testing results from Coes Reservoir that exceeded the Massachusetts recreational public health threshold.
The dominant cyanobacteria present were identified as the genus Aphanizomenon and the genus Woronichinia.
The city said both genera have the capacity to produce toxins.
Based on those testing results, the Massachusetts Department of Public Health is advising that Coes Reservoir remain closed until additional testing shows cyanobacteria densities have returned to safe levels.
City officials said the municipality will continue testing the reservoir for cyanobacteria and will reopen the lake when it is determined safe to do so.
The closure affects all recreational activity on the reservoir.
Residents are being instructed to stay out of the water, discontinue boating and fishing, and prevent pets from entering the reservoir until the advisory is lifted.
The city said it maintains an active cyanobacteria monitoring and management program intended to keep Worcester’s lakes safe for recreation and wildlife.
Officials are continuing to monitor conditions at Coes Reservoir as additional testing is conducted.
The advisory will remain in place until testing confirms cyanobacteria densities no longer exceed the state’s recreational public health threshold.
The city is encouraging residents seeking additional information about cyanobacteria and their toxins to view its educational video series, “The Blue Space Minute.”
Residents with questions may contact the Worcester Department of Sustainability and Resilience at 508-799-8324 or visit the city’s Blue Space webpage.