A 'major' bloom of harmful cyanobacteria discovered in the Charles River

People and pets should avoid touching the Charles River water in the area of the Longfellow Bridge due to a major bloom of harmful cyanobacteria.

The Massachusetts Department of Public Health posted a public health advisory for the river between the bridge and the Cambridge Dam. The Charles River Watershed Association said this is the first "major bloom" of cyanobacteria in the river in four years.

Cyanobacteria is a blue-green algae that naturally occurs in fresh water. It can release cyanotoxins, which can be fatal to pets and young children exposed to contamination.

Direct contact with water contaminated with cyanobacteria can cause skin rash and airborne toxins can have harmful affects on respiratory systems. Swimming or swallowing contaminated water can also cause nausea, cramping or liver failure. The public department advises people to rinse off if they come into contact with the water. (Read more about it from WBUR correspondent Barbara Moran.)

The public health department initially identified the bloom through a visual assessment of the Charles and confirmed the growth Thursday afternoon based on water quality data collected by a U.S. Environmental Protection Agency sensor and watershed association testing.

Boston experienced a significant stretch of dry weather from the end of August through mid-September, which likely likely contributed to the bloom, the water association said.

"Cyanobacteria is known as very mobile and so when it rains, it has the ability to kind of break up the cyanobacteria," said Marielena Lima the association's river science program manager. "Think of it as like a lazy river versus one with really choppy waves."

The lack of rainfall created ideal conditions for the growth of the algae when paired with warm weather and phosphorus from stormwater runoff, the association said.

The watershed association and the Conservation Law Foundation sued the EPA in 2022, alleging the agency failed to issue permits aimed at reducing stormwater pollution into the Charles, Mystic and Neponset rivers. The groups claim that the lack of permitting has contributed to the growth harmful bacterial blooms in those rivers. A judge stayed the case until November.

For those who are often near the water, the association has an email alert system that send out notifications when there are water quality issues in the Charles.