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NH Health

Swimmers beware: A record-breaking summer for cyanobacteria

The state has released an updated interactive map showing the latest information about where it’s safe to swim

Both June and July have set new records for the number of cyanobacteria advisories put out by the state’s Department of Environmental Services.Pamela Wright

With hot summer temperatures, it’s tempting to jump into the closest body of water you can find.

But swimmers need to be cautious this summer, according to state officials. So far, both June and July have set new records for the number of cyanobacteria advisories put out by the state’s Department of Environmental Services.

July is also on track to break the month’s previous record of eight advisories, with seven advisories already issued as of Tuesday. The recent heavy rain is only making matters worse: In some places, it’s allowing bacteria to enter the water after sewers and septic systems flood.


To help swimmers keep track of all those advisories, New Hampshire has released an updated interactive map showing the latest information about where it’s safe to swim.

The map includes information about when an advisory was issued, when samples were last collected, the kind of cyanobacteria detected, and pictures of the bloom so people know what to look out for. It’s updated daily, according to the department, and it also includes historical information about advisories that are no longer current.

As of Tuesday, there were four fecal bacteria advisories. Two were issued Tuesday, impacting Corcoran’s Pond Beach in Waterville Valley and Northwood Town Beachin Northwood. The other two were put into place last week for French’s Park Beach in Bradford (on Lake Masseasecum) and for Chase Beach on Post Pond in Lyme.

Ted Diers of the department’s water division said the heavy rains last week and this week have caused flooding that’s introducing bacteria into the water.

“We’re seeing a lot of fecal bacteria in the water that’s been washed in either through sewers or septics that are overflowing,” he said.

“We’re really urging caution for people when they’re out and about recreating and swimming,” said Diers. “I’m very concerned about swimming in areas that are having very high flows from the safety perspective, as well as the contaminants perspective.”


There were four cyanobacteria advisories in effect for Province Lake in Wakefield, Mascoma Lake in Enfield and Lebanon, Keyser Pond in Henniker, and Tucker Pond in Salisbury. Alerts were issued for Silver Lake, Deering Reservoir, and Cobbetts Pond.

An alert is less severe than an advisory, when cyanobacteria levels are approaching the advisory threshold, but haven’t quite reached it yet, according to Kate Hastings. She manages the Cyanobacteria Harmful Algal Bloom Program for New Hampshire Department of Environmental Services.

“It’s been a very busy year for cyanobacteria,” she said, noting the program issued its earliest advisory to date this year in May, which saw a total of six alerts. There were 18 alerts issued in June, the most alerts the program has ever issued in a single month.

Hastings said an alert means the water could be unhealthy: People should avoid swimming or wading in it and keep pets out of the water, too.

Cyanotoxins can cause a host of negative impacts, depending on what kind you’re exposed to and how much exposure occurs, Hastings said. It could range from irritation on the skin, eyes, or nose to gastrointestinal issues, like diarrhea, upset stomach, nausea or vomiting. Some contain neurotoxins that can cause tingling and numbness.

And Hastings said in severe cases it can cause seizures or death. That’s typically seen in pets, like dogs who are more likely to ingest enough of it to cause that reaction. Hastings said there are no confirmed cases in New Hampshire of a dog dying from exposure to cyanobacteria, although there are reports from Vermont of this occurring.


“Two of the major drivers (of cyanobacteria) are climate change, warmer temperatures and changes in precipitation patterns,” Hastings said.

With heavy rains, a lot of nutrients from soil runoff to sewage can enter the water all at once, as opposed to a trickle over time.

“Cyanobacteria takes advantage of that,” said Hastings. “They’re really good at doing what they do.” The extreme rain events and very warm periods this year have favored the bacteria.

One caveat to the record-breaking year: awareness around cyanobacteria is also increasing, which could mean more people are on the lookout and reporting blooms to the state. The state only samples after people report a bacteria sighting online.

There are a few steps people can take to combat cyanobacteria. It all comes down to reducing nutrients in the water. Homeowners can work to reduce runoff through lake friendly landscaping. The nonprofit NH LAKES has resources on how to do this.

Lake associations can create a watershed management plan. They can get help from the department to do so.

Amanda Gokee can be reached at Follow her @amanda_gokee.