Avoid contact with the Charles River for a little bit, and don’t let your pets near the water, either.

The Massachusetts Department of Public Health has confirmed the presence of a large bloom of cyanobacteria in the lower basin of the Charles River.

Commonly referred to as blue-green algae, cyanobacteria is common in rivers, lakes and ponds during the dry season and can release toxins that can cause skin rashes and respiratory issues, which are especially pronounced in children and pets.

“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 swallow water in the area, to keep their pets away, and to rinse off after having contact with the water,” an advisory from the Charles River Watershed Association reads.

“DPH staff identified the bloom downstream of Massachusetts Avenue (a.k.a. the Harvard Bridge) based on visual assessments of the river conducted throughout the week, which was corroborated by sampling and microscope analysis conducted by the Charles River Watershed Association,” the CRWA notice continues.  “CRWA performs biweekly cyanobacterial cell counts at two representative locations in the lower basin and alerts DPH when cell counts approach regulatory thresholds.”

This is the second cyanobacteria bloom found in the Charles in less than a year, with one discovered downstream of the Longfellow Bridge last September, which at that time was the river’s first major bloom in four years.

Cyanobacteria flourish in dry, hot conditions when water goes stagnant, phosphorus levels in the water rise and there isn’t enough rain to wash it away and replenish the fresh water faster than the blooms grow.

“In the Charles River, cyanobacterial blooms are a symptom of excess stormwater pollution and degraded ecology,” the CRWA notice concludes. “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.”

You can check the water conditions at CRWA’s flagging webpage.