Worcester closes Coes Reservoir after cyanobacteria exceeds recreational threshold

Worcester has closed Coes Reservoir after testing found cyanobacteria densities above the state recreational public-health threshold, according to a city notice issued July 22, 2026.

The closure affects people using the reservoir for water-based recreation. The City of Worcester Department of Sustainability and Resilience instructed residents to stay out of the water, stop boating and fishing, and keep pets away from the reservoir.

The Massachusetts Department of Public Health advised that the lake remain closed until additional testing shows cyanobacteria densities have returned to safe levels. The city said it will continue testing and reopen the reservoir when conditions are determined safe.

What testing found

The city said the testing results it received July 22 exceeded the state threshold used for recreational public-health protection. The notice identified Aphanizomenon and Woronichinia as the dominant cyanobacteria genera found in the testing.

Cyanobacteria are a concern in recreational waters because some can produce toxins that may be harmful to people and animals. The city’s directions apply to swimmers, boaters, anglers and pet owners while the closure is in effect.

However, the city notice did not provide a measured cyanobacteria concentration. It also did not report that toxins had been detected at harmful levels. The reported exceedance concerns cyanobacteria density, not a confirmed finding of harmful toxin contamination.

Closure remains in place

Residents should not treat the July 22 closure as limited to swimming. The city specifically directed the public to avoid swimming, boating and fishing at Coes Reservoir and to prevent pets from entering the water.

Those instructions remain the city’s stated guidance pending further testing. The state health department’s advice ties reopening to testing that establishes safe cyanobacteria densities, and the city has said it will reopen the lake only after conditions are determined safe.

No reopening date was included in the notice. The city also did not give a schedule for additional sampling or identify a date by which the next results are expected. For now, the next known step is continued testing by the city and a reopening announcement only after safe densities are established.

The closure notice does not report illnesses involving people or animals. It is a precautionary restriction based on the testing results and the potential risks associated with some cyanobacteria.