The Boston Globe

A clump of the algae was found near a boat dock not far from the Longfellow Bridge.

A sudden bloom of blue-green algae that has the ­potential to release toxins harmful to people and dogs has turned portions of the lower Charles River a scummy greenish color, and it has started several weeks earlier than in previous years.

People and pets should avoid contact with water that looks green and should not ingest it, state public health officials warned. Toxins that can be produced by the algae may cause skin rashes or stomach problems, and can seriously sicken or kill pets.

The bloom, the first in the Charles River since 2010, has been spotted downstream of the MIT boathouse, and signs have been posted near the water to warn boaters, dogwalkers, and others of the potential hazard.

People enjoying the summery weather along the river Friday said they were aware of the warnings and would take precautions if they came in contact with the water, such as washing ­exposed skin, but weren’t worried.

“We’ll be sure to wash our hands when we get home,” said Gian Fabbri, who was fishing on a dock near the ­Esplanade with his two children, A.J. and Alexandra.

‘Obviously we are happy attention is being paid to water quality, and we are glad to have information about it.’

The algae is called aphanizomenon, according to Julie Wood, a senior scientist with the Charles River Watershed Association, and can look like green paint spilled in the river or strands of green silly string.

Suzanne Condon, director of the state Bureau of ­Environmental Health, said that on Monday, levels of the algae, also called cyanobacteria, were well below the threshold that would trigger a public health issue. But on Wednesday a measurement taken near the ­Esplanade showed the population of algae had exploded, ­although no toxins were detected. She said health officials will continue to monitor the algae levels and watch for toxins.

“This doesn’t mean people shouldn’t go out in kayaks and canoes,” Condon said. Instead, people should be mindful of the potential for health risks, avoid contact with water, and keep pets away from the water.

Community Boating executive director Charlie Zechel demonstrated what to do if ­exposed to the water: He stuck his hand in the river, which had a slight greenish tinge, then simply washed it off. He said the ­organization has warned boaters and has suspended children’s activities in which they are sure to get wet, such as windsurfing or kayaking.

“Obviously we are happy ­attention is being paid to water quality, and we are glad to have information about it,” Zechel said.

At one of the docks along the Charles, Nick Derby tossed a ball into the water and watched Maverick, his chocolate Labrador, splash in to retrieve it. ­Derby said he wasn’t worried and doesn’t let Maverick into water that is stagnant or ­appears greenish, which typically happens in the lagoon side.

Wood said the algal blooms are unpredictable and poorly understood. They appear to be fueled by low river levels and warm weather, and the warm and dry spring may have created the right conditions for the bloom to occur in June, rather than later in the summer, as has been typical in years past.

“We really don’t know what causes the cyanobacteria blooms and why exactly we have them some years and not other years,” Wood said, although she added that nutrient runoff is clearly an issue.

Condon said the state has been monitoring algae levels through a grant from the Centers for Disease Control and Prevention and hopes to track algae blooms in various waterways, including the Charles, to learn how to predict when a bloom is about to occur.

The other important question — knowing how long blooms will last — is also far from straightforward. The 2010 bloom lasted nearly two months, Wood said, but the episodes can vary in length. The current advisory will last for at least two weeks because levels of the algae need to decline below a threshold for two weeks before it is deemed safe.

The blue-green algae are ­capable of producing a variety of toxins, but do not always produce them, and the factors that trigger their release are not well understood.

People who do come into contact with the water should wash any exposed body part.

“The river is not closed,” Wood said. “People should pretty much use their own discretion, avoid direct contact as much as possible, obviously avoid ingestion, which is a good idea anyway.”

To help the association monitor the bloom, people who see scummy water are encouraged to take a photo, note the location, and e-mail the image to ­Julie Wood at jwood@crwa.org.