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Early Toxic Algae Blooms Prompt Warnings For N.H. Lake Beaches

A lake with a bright green slick across it, demonstrating the presence of cyanobacteria in the water.
Wikimedia Commons

New Hampshire's lakes are seeing more toxic algae blooms than normal this early in the season, according to state officials. 

Over a dozen cyanobacteria blooms were reported in the week after Memorial Day, the N.H. Department of Environmental Services says. There are normally just a few reports during this time of year.

These blue-green algae blooms look like scum on the water surface and can produce neurotoxins. People and their pets and livestock should avoid contact: No swimming, fishing, wading or drinking affected water. 

Three of those advisories are still in place as of Tuesday: for Strafford Town Beach on Bow Lake, Elm Brook in Hopkinton and Swains Lake in Barrington.

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The state says the rain on Memorial Day weekend, followed by hot, calm weather, likely helped cause these growths. Beach advisory coordinator Amanda McQuaid said in an email that this was a "perfect recipe for cyanobacteria blooms." 

She said a short winter ice period and mild spring combined with near-drought conditions across the state, causing slow currents and stagnant water, could also be to blame. 

Each of those weather patterns are expected to increase with climate change. Warmer waters also tend to lead to harmful algae blooms. 

A state report on lake health released last year said New Hampshire's annual number of beach advisories for cyanobacteria and E.coli rose overall between 2003 and 2018. It also found that average spring thaws on New Hampshire lakes are now about two weeks earlier than in the 1930s, when reliable record-keeping began.

The state says in general, people are also becoming more aware of cyanobacteria and reporting it more often, leading to an uptick in numbers.

If you think you've spotted a cyanobacteria bloom, you can report it to the state at (603) 848-8094 or For more information, click here.

Annie has covered the environment, energy, climate change and the Seacoast region for NHPR since 2017. She leads the newsroom's climate reporting project, By Degrees.
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