Warmer weather, intense storms boost dangerous cyanobacteria growth


Keene Sentinel

Published: 08-03-2023 10:20 AM

The state has issued record numbers of warning advisories for high levels of potentially dangerous cyanobacteria in lakes and ponds this year as warm and rainy weather worsened this pollution, according to the N.H. Department of Environmental Services.

These bacteria are marked by surface scum, green streaks or blue-green flecks. It can produce toxins that irritate skin and mucous membranes as well as cause tingling, numbness, nausea, vomiting, seizures and diarrhea. Chronic effects include liver and central nervous system damage. It can also harm or kill pets.

When cyanobacteria are measured at levels exceeding state standards, the DES issues an advisory warning people to avoid swimming or wading in the affected area. It lifts the advisory when water quality improves.

“We’ve broken the record for the number of advisories in May, June and July and we’ve broken the record for the total number of advisories issued in one month, in June, so it has been a heck of a year,” Kate Hastings, a program manager with the department, said Friday.

There were six advisories in May, breaking the old record of five for the month. June saw 18 advisories, doubling that month’s record and setting a single-month record. There have been 10 advisories to date in July, breaking the old record of eight for the full month.

In 2022, the DES issued 46 advisories for the entire summer, which was a record. Hastings said this year’s number will likely surpass that.

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Storm runoff and inefficient or failed septic systems can carry phosphorus into waterways, which is a nutrient for the bacteria and stimulates its growth.

Weather changes associated with climate change exacerbate the problem, she said.

“We’re seeing more frequent, intense rainstorms, which bring in a lot of nutrients all at once instead of the more regular trickle throughout the summer,” Hastings said. “Cyanobacteria are really good at what they do, so they tend to be the ones in the ecosystem that take up those nutrients and grow out of control.”

Also, warmer weather reduces ice coverage in ponds and lakes and makes them susceptible to cyanobacteria earlier in the year, she said.

The DES maintains a “healthy swimming mapper” that lists advisories for cyanobacteria and fecal bacteria, which can also sicken swimmers, causing diarrhea, nausea, vomiting and sometimes fever.

On Friday, the map showed fecal bacteria advisory on Picnic Beach at Greenfield State Park on Otter Lake.

There were no cyanobacteria advisories in the Monadnock region on the map on Friday, but two area water bodies were included in a separate “alert” category for when cyanobacteria were approaching a level exceeding state standards.

An alert can also be called if photos of a cyanobacteria bloom have been provided prior to a state water test.

Alerts were also in place Friday at Pool Pond in Rindge and Silver Lake in Harrisville.

In early June, the DES issued a cyanobacteria advisory for Lake Monomonac in Rindge.

A cyanobacteria advisory is issued lake-wide when cyanobacteria cell concentrations exceed 70,000 per milliliter.