There are over 30 harmful algae blooms in Massachusetts lakes and ponds right now: See the list

Health officials in Massachusetts are spreading the word about harmful blooms of cyanobacteria, also known as blue-green algae, in lakes and ponds around the state.

In Plymouth, the public health director told WBZ-TV that there are six ponds with cyanobacteria blooms that can make people and dogs "very sick." Signs are posted on the beaches to tell people not to swim at the water, and to keep their dogs away from shore.

In Brookline on Tuesday, the town warned that a harmful bloom has been confirmed at the Brookline Reservoir, which is a popular walking area. Fishing is banned at the reservoir and no pets are allowed until further notice, the town said. Anyone who makes contact with the algae should rinse off immediately.

Part of the Charles River near the Harvard Bridge also has an algae bloom. In 2024, the area experienced its first major bloom in four years.

What is an algae bloom?

An algae bloom happens when cyanobacteria multiplies quickly in a highly concentrated area. It can cause the water to become pea soup-colored and sometimes smell bad. Warm weather, sunlight and excess nutrients in the water from runoff can make it grow faster.

The bloom can release toxins that may cause skin irritation or gastrointestinal problems if ingested. Inhaling the water spray may also result in asthma-like symptoms, the state says.

Algae bloom advisories in Massachusetts

Here is the list of harmful cyanobacterial blooms in Massachusetts as of Aug. 6, according to a state dashboard. 

Lake Holland, Belchertown
Chestnut Hill Reservoir, Boston
Charles River--Downstream of Massachusetts Ave (the Harvard Bridge), Boston/Cambridge
Quaboag Pond, Brookfield/East Brookfield
Brookline Reservoir, Brookline
Freeman Lake, Chelmsford
Heart Pond, Chelmsford
Rubber Thread Pond, Easthampton
Seymour Pond, Harwich/Brewster
West Reservoir, Harwich
Schoolhouse Pond, Hyannis
Long Pond, Marstons Mills
Santuit Pond, Mashpee
Clarks Cove, Nantucket
Capaum Pond, Nantucket
Hummock Pond, Nantucket
Lake Cochituate-South Pond, Natick
Crystal Lake, Newton
Carbuncle Pond, Oxford
Clear Pond, Plymouth
Big Sandy Pond, Plymouth
Billington Sea, Plymouth
Great Herring Pond, Plymouth
Little Herring Pond, Plymouth
Russell Mill Pond, Plymouth
Bass Pond, Springfield
Lake Quannapowitt, Wakefield
Hardy Pond, Waltham
Glen Charlie Pond, Wareham
Pequot Pond/Hampton Ponds, Westfield
Blackstone Canal, Uxbridge