As temperatures rise, toxic bacteria blooms in Mass. ponds prompt 'stay away' notices

Warm weather and sunlight are attracting more than just summer vacationers to ponds in Massachusetts. These conditions are also contributing to "blooms" of cyanobacteria that public health officials warn can be harmful to humans and pets.

This week, harmful blooms were detected in nine Massachusetts ponds, prompting the state to issue an updated advisory. On July 26, the advisory was updated again to 11 ponds and one lake.

Experts including from the EPA and the Centers for Disease Control say climate change increases the occurrence of these harmful blooms in the U.S. Studies point to bodies of water used for recreation in the Northeast as among the areas at risk of these blooms.

“Warmer water coupled with the sunlight in the summer is just a perfect breeding ground for them,” said Lucia Ross, chief marketing officer at BlueGreen Water Technologies, a company that works to reduce cyanobacteria.

While algal blooms are not always toxic, experts warn that cyanobacteria blooms in particular are a cause for concern. They can release “cyanotoxins” that — when touched, ingested or inhaled — can cause stomach aches and other illnesses.

Cyanobacteria are found naturally in all types of water, but blooms signal an unbalanced growth that usually happens when they feed off nutrients from fertilizer and human and animal waste runoff.

Ross adds that cyanobacteria thrive in still water and, when exposed to sun, can double in size every five hours. “So that biomass will keep getting larger and larger,” she said.

According to the CDC, climate change can “make blooms occur more often and be more severe.” Hot weather driven mainly by climate change has caused record-high temperatures across the globe. Last month was the hottest June ever recorded on Earth, according to the National Oceanic and Atmospheric Administration.

In Massachusetts, the number of cyanobacteria blooms typically peaks between August and October, according to the state Department of Public Health. But state data shows the blooms appear earlier in the year: Between Jan. 1 and July 21, the department was aware of 16 harmful cyanobacteria advisories issued in 2023. In 2022, 23 advisories were issued over the same time period.

Blooms often appear as extra cloudy water or greenish foam or scum floating on the surface. The CDC says it’s not possible to tell if a bloom is toxic just by looking at it. But the the Massachusetts Department of Public Health says it is best practice to assume all cyanobacteria blooms pose a health risk, so people should stay away if they see one, whether or not an advisory has been issued.