The first harmful cyanobacteria blooms of the year were found in a pair of ponds this week. 

A bloom near the bottom of middle Chilmark Pond was found on Tuesday, prompting the Chilmark board of health to issue a warning for people and pets to stay out of the water there. 

Mink Meadows in Tisbury also has a bloom, according to experts.

The two blooms were detected earlier than normal in the year, said Emily Reddington, the executive director of the Great Pond Foundation. But the bloom in middle Chilmark Pond is what’s known as a “benthic” bloom, which is found in green globs along the bottom of the pond rather than the usual bright green algal blooms in the water column. 

Researchers only learned about the benthic blooms last year after finding one in Tisbury Great Pond. Because testers are only now on the lookout for these blooms, it is hard to tell if it is unusual for them to be present this time of year.

The Chilmark bloom was isolated to middle Chilmark Pond. Upper Chilmark Pond and lower Chilmark Pond look healthy, Ms. Reddington said.

The first bloom found on the Vineyard in 2023 was in Tisbury Great Pond on July 19.

Cyanobacteria is a group of photosynthetic organisms in ponds, oceans and estuaries that, when in high concentrations, can produce toxins harmful to humans, dogs and other animals. 

It was once thought that brackish ponds were inhospitable to these types of blooms, and Vineyard scientists are now working to better understand them as they frequently pop up in the coastal ponds across the Island.