Press Release: 10/16/2025

Cyanobacteria Bloom in Charles River Reduced–Head of the Charles Course Not Under Advisory

FOR IMMEDIATE RELEASE: October 16, 2025

Contact: Stefan Geller, Charles River Watershed Association

Phone: 781-572-9341

Email: sgeller@crwa.org

 

(Courtesy – MA Department of Public Health)

BOSTON, MA – The Massachusetts Department of Public Health (DPH) on Thursday announced

its recommendation to rescind the cyanobacteria bloom advisory in the Charles River upstream of

the MIT boathouse, no longer affecting the Head of the Charles Regatta course.

DPH collected follow-up samples at four locations in the Charles River on Tuesday, all of which

laboratory analysis showed cyanobacteria concentration below its advisory guideline value of

70,000 cells/mL, and levels of the toxins microcystin and anatoxin-a below DPH’s toxin guideline

levels.

Location

Cyanobacteria

Concentration

Anatoxi

n-a

Microcyst

in

1. Magazine Beach 3,200 cells/ml 0.19 µg/L ND (<1

µg/L)

2. BU DeWolfe Boathouse

2,800 cells/ml 0.25 µg/L

ND (<1

µg/L)

3. MIT Richard J. Resch Boathouse

5,400 cells/ml 0.26 µg/L

ND (<1

µg/L)

4. Community Boating

34,000 cells/ml 0.37 µg/L

ND (<1

µg/L)

DPH requires two rounds of sampling, one week apart, with cell concentration and cyanotoxin

values below guideline levels to recommend rescinding an advisory.

● For Magazine Beach and the BU DeWolfe Boathouse, this is the second round of low

results, therefore, the advisory is lifted for these areas.

● For the MIT Boathouse and Community Boating, this is the first round of low results,

therefore the advisory will remain in place for areas downstream of these locations. DPH

will sample these locations again next week to see if the whole advisory may be lifted.

DPH initially recommended a public health advisory for a significant cyanobacteria bloom

downstream of Massachusetts Avenue (a.k.a. the Harvard Bridge) in late July. DPH staff identified

the bloom based on visual assessments of the river, which was corroborated by sampling and

microscope analysis conducted by the Charles River Watershed Association (CRWA). CRWA

performs weekly cyanobacterial cell counts at two representative locations in the Lower Basin

and alerts DPH when cell counts approach regulatory thresholds.

Cyanobacteria grow rapidly when bodies of water experience high phosphorus levels, hotter

temperatures and stagnant conditions. In the Charles River, cyanobacterial blooms are a symptom

of excess stormwater pollution and degraded ecology. Current stormwater regulations mandate

an over 50% reduction in nutrient loading from watershed communities by the year 2038. While

progress has been made, this bloom highlights the work to be done and the importance of

addressing municipal stormwater runoff.

For ongoing water quality updates, please visit CRWA’s flagging webpage.

###

Charles River Watershed Association’s mission is to protect, restore and enhance the Charles River and

its watershed through science, advocacy, and the law. CRWA develops science-based strategies to

increase resilience, protect public health, and promote environmental equity as we confront a changing

climate.