Cyanobacteria Bloom In Charles River Expands (Press Release)
FOR IMMEDIATE RELEASE: October 7, 2025
BOSTON, MA – The Massachusetts Department of Public Health (DPH) on Monday announced that the cyanobacteria bloom in the Charles River — now in its 10th week — has expanded to all areas downstream of the Cambridge Street/River Street bridge.
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.
Days after confirming the bloom was still active, DPH on Thursday collected samples at three locations upstream of the Harvard Bridge to determine if the cyanobacteria bloom had expanded beyond the advisory area, and a fourth sample was also collected at Community Boating. Laboratory analysis showed cyanobacteria levels were above the 70,000 cells/ml threshold at all four locations:
Magazine Beach – 82,000 cells/ml
BU DeWolfe Boathouse – 120,000 cells/ml
MIT Richard J. Resch Boathouse – 250,000 cells/ml
Community Boating – 140,000 cells/ml
Based on the test results, DPH has expanded the public health advisory at the Charles River to include all areas downstream of the Cambridge Street/River Street bridge.
Cyanobacteria, or blue-green algae, are naturally occurring microorganisms in rivers, lakes and ponds, and during periods of rapid growth can release harmful toxins. Direct contact can cause skin rashes, and inhaling airborne toxins can affect respiratory systems, especially for those with pre-existing conditions like asthma. Small children and pets are especially susceptible to the effects of cyanotoxins.
Due to these severe health risks, DPH has deemed the area unsafe for people and pets and has advised the public not to swim or ingest water in the area, to keep their pets away, and to rinse off after having contact with the water.
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.