Last month hundreds of swimmers took to the Charles River in triumph, celebrating the water’s remarkable recovery from infamous pollution. A time when cooling off in the Charles might become a regular summer tradition didn’t seem far away.
But on Thursday, in a reversal that underscored the river’s delicate balance, officials announced that an algae bloom had taken hold in the Lower Charles River Basin, stretching from the Weld Boathouse in Cambridge to the Charles River Dam. Tests indicated that bacteria concentrations were roughly twice the recommended limit, prompting an advisory from health officials.
Known as cyanobacteria, the blue-green outbreak turns water murky and gives off an unpleasant smell. The blooms are relatively common in the heat of the summer, and an outbreak recently caused Jamaica Pond in Boston to be closed.
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But as environmental advocates work to improve the river’s image, the bloom served as a reminder that progress can be halting.
“With the Charles, it can change in a day,” said Julie Woods, project director for the Charles River Watershed Association, an advocacy group. “Let’s clean it up, so it is made to swim all the time.”
Marc Nascarella, who directs the Environmental Toxicology Program at the state public health department, said that the bloom does not appear to be toxic, and that symptoms of exposure would generally be “mild and reversible.”
“You’d have to consume an appreciable amount of that water to get ill,” Nascarella said.
Still, officials posted warning signs Thursday along the river in Boston and Cambridge, urging people to use caution, and noting that drinking the water could cause gastrointestinal distress for humans and and their pets.
“Do not swallow the water. Rinse off after contact,” the advisory stated.
Health officials have issued 150 algae advisories across the state since 2009, 11 of them involving the Charles River.
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Algae blooms have become an increasing problem across the country. Last summer an outbreak disrupted the water supply in Toledo, Ohio, a crisis that left hundreds of thousands without safe tap water for three days.
Nascarella said the outbreak on the Charles, the second since the beginning of June, is more an indication of climate change than the condition of the river.
“I don’t think it necessarily speaks to anything about the health of the Charles,” he said.
Last Friday, the watershed association responded to reports of “gross, greenish” water at the Weld Boathouse near Harvard Square, and testing confirmed elevated bacteria levels. Warm weather, combined with heavy rain that carries polluted runoff into the river, creates the perfect environment for the bacteria to grow, Woods said.
“It’s in the water pretty much all the time at low levels,” she said. “Even when we have the swim events, we have to take the precautions to make sure it’s safe.”
A bloom in early June caused the Charles River Swimming Club to postpone a race, and another last month prompted restrictions at the Brookline Reservoir.
Sam Lipson, director of environmental health for the public health department in Cambridge, spent part of the afternoon posting warning signs near boat and yacht houses along the river.
The signs caution against swallowing the water and encourage people who have come into contact with it to rinse off. But Lipson said the advisory would be temporary, and downplayed the risks.
“Algae blooms are not uncommon and not highly hazardous if people take the right precautions,” he said. “It won’t be like this the entire summer. You can still enjoy the river.”
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Nascarella said most advisories last less than two weeks.
Swimming is generally discouraged in the Charles River, but special events have been held when the water meets clean standards. Last month, Matthew Beaton, the state’s secretary of energy and environmental affairs, joined more than 200 swimmers, the third year the community event was held.
Last year, the Charles River met bacterial water quality standards for boating 91 percent of the time and for swimming 65 percent of the time, according to the US Environmental Protection Agency.
That was a dramatic turnaround from 1995, when the river met swimming standards just 19 percent of the time.
But the perception of the Charles as polluted remains strong.
“I’ve always heard it’s not safe to swim,” said Maria Gomez, 23, who was walking on Magazine Beach in Cambridge. She was surprised people would even canoe.
At Charles River Canoe & Kayak, employees were fitting people with life jackets and helping them into canoes as a yellow flag hung, indicating moderate risk. Manager Sean Turcotte said he tells customers to avoid contact with the water.
Yasmeen Abu Nada, 20, was eager to try kayaking with a group of friends but was worried about falling in.
“You see the decomposed trash, the algae ... it doesn’t look healthy.”
Abu Naba, a student at the Berklee College of Music, described the water as “murky.”
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“I know it’s a lot cleaner than it used to be, but it could be better,” she said.
Environmental groups say high levels of phosphorus from storm water is to blame for the algae blooms.
“Let’s restore the Charles so we can have a reliable, healthy, safe river all the time,” Woods said.
Peter Schworm can be reached at schworm@globe.com. Sara DiNatale can be reached at sara.dinatale@globe.com.