Cyanobacteria bloom, Blessing of the Bay, Somerville, 2016. Photo by Wilder Huckleberry.

Cyanobacteria bloom, Blessing of the Bay, Somerville, 2016. Photo by Wilder Huckleberry.

Since 2007, the Cyanobacteria Monitoring Program has monitored cyanobacteria levels throughout the Mystic River Watershed and alerted authorities of the potentially harmful bacteria. Trained MyRWA interns take water samples in recreational areas where there is a higher chance of contact with cyanobacteria.


Cyanobacteria, also referred to as blue-green algae, are photosynthetic bacteria that grow in all types of water and are usually not visible to the naked eye except when they form colonies. Blooms often occur from the late summer to early autumn and can appear overnight. According to the Centers for Disease Control and Prevention, “some [blooms] can look like foam, scum, or mats on the surface of freshwater lakes and ponds.  The blooms can be blue, bright green, brown, or red and may look like paint floating on the water.”  Read more at the EPA website.


Cyanobacteria blooms can starve marine plant and animal life of oxygen, sicken humans and animals, and in some cases can result in damage of the nervous system and liver. To decrease the chance of exposure to harmful blooms, avoid recreational activities such as swimming and boating in areas where the water looks discolored or there is surface scum and, of course, areas where notice of high cyanobacteria levels are posted. If you think you might have come in contact with cyanobacteria, rinse off with fresh water as soon as possible. It is important to note that pets can also be affected by cyanobacteria and the same precautions should be taken.

For more in-depth information about what Cyanobacteria is, how to spot it, and the potential health risks, visit the Centers for Disease Control and Prevention webpage or the Massachusetts Department of Public Health webpage.

Current conditions - Updated 8/15/17

On August 15th, MyRWA staff collected samples at 6 locations along the Mystic River to test for phyocyanin, the photocynthetic compound indicative of cyanobacteria blooms. The results of the tests showed that none of the sites have any indication of a cyanobacteria bloom.

Recent testing by the Massachusetts Department of Public Health (DPH) on July 26th confirmed that there are no longer cyanobacteria cell counts above state thresholds in the Mystic River near Blessing of the Bay in Somerville and in Upper Mystic Lake.  Shannon Beach on Upper Mystic Lake is now open and both of these sites now have had their cyanobacteria advisories rescinded. See the DPH website for more information. 

Track Cyanobacteria!

You can now track cyanobacteria blooms using a smartphone! Check out the BloomWatch App