Many local beaches are shutting down due to bacteria overgrowth. Is climate change to blame?


Fecal bacteria and blue-green algae (cyanobacteria) are common beach menaces. Warmer waters and heavy rain events help them grow.

Shubael Pond in Barnstable was closed due to a dangerous algal bloom in 2020. John Tlumacki/The Boston Globe

Experts are warning that bacteria is a growing threat to New England’s beaches. But while beach bacteria is not a new problem, it has found a new ally in climate change. 

When fecal bacteria and blue-green algae (cyanobacteria) reach a certain level in water bodies, they become dangerous for people, pets, and even fish. As a result, local health boards are forced to close beaches or issue swimming advisories, even in the hottest weather.

Bacteria-caused swimming advisories and beach closures have become so common that both Massachusetts and New Hampshire now have state websites that track them. In July, more than 70 of the Bay State’s beaches were closed due to high fecal bacteria levels.

The warmer waters and heavier rainfall brought by climate change are helping aid bacteria growth, according to experts. If we don’t mitigate these impacts, they warn, New Englanders may see fewer days when it’s safe to swim. 

How beaches reach high bacteria levels

There are two types of bacteria the government and environmental groups test beach waters for — cyanobacteria and fecal coliform bacteria.

Fecal bacteria such as E. coli can cause gastrointestinal issues if ingested, which makes it risky to swim in waters with high levels of fecal bacteria.

Cyanobacteria, on the other hand, produce toxins that can be dangerous to breathe. These toxins can not only cause stomach problems, but also neurological symptoms such as paralysis and death. While there are few accounts of humans being seriously injured by cyanobacteria, every summer, reports circulate of dogs dying from cyanobacteria exposure.

State law requires testing for fecal bacteria so that beaches can be closed if bacteria concentrations reach a dangerous level. While the state doesn’t require testing or beach closure for high levels of cyanobacteria, many municipalities and environmental groups test for it because of the danger it poses to humans and animals. They then post swimming advisories in the hopes that people will choose not to put themselves at risk.

According to UMass Lowell biology professor Sarah Gignoux-Wolfsohn, cyanobacteria naturally exist in water bodies, while fecal bacteria get into water bodies in two ways.

Firstly, she said, it gets picked up by rainwater, which encounters fertilizer, wild animal feces, and pet waste on the ground before reaching a water body. Secondly, when sewage overflows into water bodies, it brings fecal bacteria from human waste along with it.

Rain also helps cyanobacteria grow because the fecal matter in rain runoff and sewage also contains nutrients that help bacteria grow and thrive, Gignoux-Wolfsohn said. Beach closures and swimming advisories are common a few days after heavy rainfall for this reason.

How climate change helps bacteria

There are two ways climate change helps bacteria thrive in water, Gignoux-Wolfsohn said. Firstly, bacteria like warmer waters, and climate change is increasing the average temperature of water bodies.

Secondly, she said, because climate change causes more extreme weather events, when it rains, we get more rain all at once — something New England experienced many times this summer. Heavier rainfall means sewers overflow more often, that the volume of overflow is greater, and that water bodies receive more runoff.

While bacteria blooms are a nuisance for humans who can’t enjoy swimming at their favorite beach, they’re also bad for fish and fishermen. Cyanobacteria can take up too much oxygen in the water, causing fish to suffocate and die, Gignoux-Wolfsohn said. This is commonly known as a “fish kill.”

As of right now, there’s not much we can do to bring down beach bacteria levels once they’ve risen out of control, Gignoux-Wolfsohn said. All we can do is close a beach or post swimming advisories until the bacteria levels return to normal.

It can take a few days to a few weeks for bacteria levels to return to normal, but health officials say two weeks is about average. If heavy rain events cause closures or swimming advisories more than once a summer, this can eat into the time people can use a water body.

What the future looks like

Climate change causes more extreme and polarized weather, which means it might not result in more beach closures every year, Gignoux-Wolfsohn said. For instance, last year, Massachusetts was battling a drought due to less-than-average rainfall over the summer.

Still, we could see more beach closures overall as the effects of climate change take hold, Gignoux-Wolfsohn said.

“These closures are the direct result of climate change, which is caused by humans burning fossil fuels,” she said. “If we want to be able to swim, we need to deal with climate change and reduce our use of fossil fuels.”