The Massachusetts Department of Public Health has issued a cyanobacteria (blue-green algae) advisory for the lower Charles River Basin from the BU Bridge to the Museum of Science. (Read our algae explainer here.)

Blue-green algae produces a toxin that is dangerous to ingest, and can lead to stomach cramps and nausea, skin rashes, hay fever-like symptoms or, in extreme cases, death. It often appears as a green scum on the water's surface, but can also look like green paint, bright green strands or pea soup.

People should avoid contact with the water in this area — including swimming, boating and fishing, according to Marc Nascarella, DPH's chief toxicologist.

"When in doubt, stay out," says Nascarella. "Contact with harmful algae blooms can cause mild gastrointestinal upset, it can irritate the skin, it's certainly unpleasant. In the worst case scenario it can cause serious complications, like complications to the liver, kidney, and in some rare events it has caused death."

People who do touch the water should rinse off as soon as possible.

The blue-green algae is especially toxic to pets, says Nascarella, partly because they lick it off their fur. So people should not allow their dogs to swim in the water or drink it.

DPH sampled the water Wednesday to test the concentration of the algae, and will take follow-up samples on July 31. DPH requires at least two samples, one week apart, to rescind the advisory.