Needham lifted the public health advisory at Rosemary Lake on Tuesday, Oct. 6, reopening the lake to swimmers and pets after nearly six weeks.

The town announced that water samples tested by the Massachusetts Department of Public Health (DPH) over the previous two weeks showed cyanobacteria levels below state guideline levels. The toxin microcystin was not detected.

The advisory had been in place since Aug. 26, when DPH confirmed a cyanobacteria harmful algal bloom at the lake. Residents were told to avoid contact with the water, keep pets away and refrain from fishing.

The bloom was first spotted by Needham Park and Recreation staff, who reported a bright green film growing on the lake's surface, according to a Needham Board of Health agenda. The Health Department then contacted state officials and the Conservation Commission and posted warning signs before the state confirmed the cyanobacteria.

A Needham Observer report in July estimated the lake draws roughly 300 visitors on a typical summer day and up to 1,000 during heat snaps. The advisory kept those visitors away through the final stretch of warm weather.

Under DPH protocol, two rounds of water sampling must come back below guideline levels before an advisory can be lifted, according to documentation from the Town of Dedham describing the same process used for a separate Charles River advisory. That Charles River advisory, which covered stretches between Needham and Dedham including Millennium Park and Cutler Park Reservation, was issued July 21 and lifted in late August.

The summer of 2026 marked the third consecutive year that the Charles River experienced a cyanobacteria bloom, according to the Charles River Watershed Association. Cyanobacteria, also known as blue-green algae, grow rapidly in water with high phosphorus levels, hotter temperatures and stagnant conditions. Direct contact can cause skin rashes, and inhaling airborne toxins can affect respiratory systems, especially for people with conditions like asthma. Small children and pets are especially vulnerable.

Residents with questions can contact the Needham Public Health Division at 781-455-7940 or [email protected].