Blue-green algae, or cyanobacteria, are photosynthetic bacteria that live and grow in aquatic environments. One of the common blue-green algae found in the Charles River is Microcystis, which grows naturally in calm, warm, shallow water that is rich in nutrients like nitrogen and phosphorous.
When present in large numbers, these organisms can give the water a green, paint-like appearance (see image above) and can produce toxins which are harmful to humans, dogs, and other mammals in large doses.If you see water that looks bright green or contains bright green strands, avoid contact with the water as much as possible and wash contacted areas after exposure.
A combination of factors, such as excess nutrients like phosphorus and nitrogen, warm temperatures, and sunlight encourage blue-green algae growth. In the Charles, severe algal blooms are usually caused by heavy rainstorms that bring an enormous influx of nutrients to the river, followed by hot weather that leads to warm water temperatures, thus creating perfect conditions for algae growth. Direct discharges of heated water likely contribute to blue-green algae growth as well.
The Massachusetts Department of Public Health defines a blue-green algae bloom as the presence of any of the following:
When algae die and begin to decompose, they secrete a toxin, and accumulations of this toxin can pose a health risk to humans and animals. Common reactions to these toxins include skin rashes, eye irritation, and possible gastrointestinal distress. Additionally, blue-green algae become a serious problem when it grows in abundance, preventing light and oxygen from getting into the water, thus shading and smothering other aquatic organisms.
Unfortunately, there is no direct correlation between visible algae in the water and toxin levels, so it can be difficult to determine when the water is safe.
If you see water that looks bright green or contains bright green strands, avoid contact with the water as much as possible and wash contacted areas after exposure.
While there are currently no recognized methods to eliminate the cyanobacteria from the water, blooms can be prevented by reducing the river’s nutrient levels and water temperature. CRWA has developed several projects to help monitor and address blue-green algae growth in the Charles River:
Water Quality Monitoring - CRWA manages a team of over 80 volunteers, or "citizen scientists, who monitor multiple sites along the Charles River and collect water samples, measure depth and temperature, and record river conditions. This data is used to identify problems in the river and trends in water quality, and to track the progress of efforts to clean up the Charles.
Blue Cities Initiative - CRWA designs and advocates for green infrastructure systems, like rain gardens and permeable pavement, to treat stormwater runoff naturally before it enters the Charles - all while enhancing neighborhoods and connecting existing open spaces.
Environmental Advocacy - Through reforming state water law, policy and practice, CRWA works to protect aquifers and maintain streamflow, and to prevent excessive discharges of hot water into the river.
Every year from July through October, CRWA provides daily water quality alerts of the Charles River lower basin, through our Daily Water Quality Notification Program. Also known as the “Flagging” Program, CRWA and participating boathouses provide daily flag colors based on bacteria levels, blue-green algae counts, and rainfall-based prediction models.Return to top