Cyanobacteria Bloom in Charles River Reduced—Head of the Charles Course Not Under Advisory (Press Release)
FOR IMMEDIATE RELEASE: October 16, 2025
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.
(Courtesy – MA Department of Public Health)
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.
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.
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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.