Press Release: 10/29/2025
DPH Lifts Cyanobacteria Advisory in Charles River
FOR IMMEDIATE RELEASE: October 28, 2025
Contact: Stefan Geller, Charles River Watershed Association
Phone: 781-572-9341
Email: sgeller@crwa.org
BOSTON, MA – The Massachusetts Department of Public Health (DPH) last week announced its recommendation to lift the public health advisory for the cyanobacteria bloom in the Charles River, 11 weeks after first recommending it.
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.
On October 21, DPH collected follow-up samples at three locations in the Charles River. Laboratory analysis showed cyanobacteria concentrations below DPH’s guideline value of 70,000 cells/mL at all three locations:
MIT Richard J. Resch Boathouse – 2,200 cells/ml
Broad Canal – 2,600 cells/ml
Community Boating – 3,200 cells/ml
The toxin microcystin was not detected in any sample.
DPH requires two rounds of sampling, one week apart, with cell concentration and cyanotoxin values below their guideline levels to recommend rescinding an advisory. As this was the second consecutive result with all locations below DPH guidelines, the advisory was lifted for the entire Charles River Lower Basin.
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.
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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.