BARNSTABLE — Fifteen years ago, when Sandra Bolton and her husband bought their three-bedroom Cape overlooking the serene waters of Shubael Pond, their view was like “heaven on Earth,” she said. Few summer days passed when they didn’t take a dip.
But in recent years they began to notice a guacamole-colored scum marring the previously clear waters. Last summer, just as temperatures were increasing and the pond beckoned, local officials banned swimming there after finding toxic algae blooms that can be harmful if ingested, inhaled, or touched.
Earlier this month, they did the same after dead fish were seen floating on a new layer of slime.
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“It was like paradise, and then it turned into a nightmare,” said Bolton, 79, a retired elementary school teacher who last year resorted to buying a blow-up pool for her grandchildren. “It’s getting worse.”
Shubael Pond is one of 996 small lakes on Cape Cod, freshwater jewels that offer an alternative to the increasingly shark-infested saltwater on the coast. The remnants of melted glaciers from the Ice Age, the mostly shallow kettle ponds are again being transformed by climate change, a blow to those drawn to their secluded beauty.
Scientists have found that the ponds are warming rapidly, sapping their oxygen, making them more turbid, and altering their distinct ecosystems, which include wildlife ranging from microbes to bullfrogs.
The warming temperatures, combined with increased development and more powerful storms that wash fertilizers, wastewater, and other damaging nutrients into the ponds, have created ideal breeding grounds for cyanobacteria, the toxic ingredients of algae blooms that can multiply in dangerous amounts in very short periods.
“The bacteria like warm, calm places, without a lot of water or wind velocity, and these little lakes are their perfect breeding grounds,” said Charles Culbertson, a microbial ecologist with the US Geological Survey’s New England Water Science Center.
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Under the right conditions, cyanobacteria can double their population every 30 minutes, he said.
“There’s no doubt that the intensity and frequency of these blooms is connected to rising temperatures,” Culbertson said. “As temperatures rise, we’re going to see more of this, meaning many of the ponds on the Cape could become unswimmable. You won’t want to be breathing the air around them, either.”
Last year, more than 500 such algae blooms were reported in the nation’s lakes, ponds, and rivers — 18 percent more than the year before and more than seven times the number reported in 2010, according to the Environmental Working Group, an advocacy group in Washington, D.C., that tracks algae blooms.
The dangers aren’t just to people. So far this summer, at least six dogs have died after exposure to toxic algae blooms in the United States, twice as many as during the same period last year, according to the Environmental Working Group. At least 14 dogs died from exposure last summer, the group found.
Not all algae blooms are poisonous, but those exposed to the toxins can experience a range of symptoms, from a sore throat to vomiting. Long-term exposure can lead to cancer and liver failure. Research has also linked cyanobacteria to neurodegenerative diseases such as Alzheimer’s and ALS.
“There are big health concerns,” said Jim Haney, a professor of biology at the University of New Hampshire’s Center for Freshwater Biology, who said toxins can be passed from fish that ingest the bacteria to other species that eat them.
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On Cape Cod, which has an usually high number of kettle ponds because of the way glaciers receded here, local officials have been finding more dangerous algae blooms.
Three years ago, as reports of the algae increased, the Association to Preserve Cape Cod began collecting water samples from dozens of ponds.
Last year, of 35 ponds sampled, the group found high enough levels of cyanobacteria that they issued use restrictions at 11 of them — the highest number since they began monitoring ponds.
So far this year, the association has issued advisories at six ponds, closing four of them. That’s the most blooms they’ve recorded for this time of year, even though volunteers are sampling fewer ponds because of the pandemic.
Late July through mid-October is considered the peak season for the blooms, which thrive on high temperatures, and local officials expect more closures.
“It didn’t used to be like this,” said Andrew Gottlieb, the association’s executive director, who grew up in a home beside Mashpee Wakedy pond and has observed the changes.
It’s far murkier now than when he was a child in the 1960s. It’s not only warmer at the height of summer; it stays warmer into the fall. And in the winters, it doesn’t freeze over as often.
“I put my dock in the water in February this year, because there was no ice,” he said. “Two winters ago, I never took my dock out of the water.”
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Ecologists at the National Park Service have studied ponds throughout the Cape and found they’ve been warming at a concerning rate. A study of 10 ponds from the mid-1990s to 2013 found an average warming of 8 percent.
“It’s reasonable to presume that we remain on the same trend,” said Sophia Fox, an aquatic ecologist with the Cape Cod National Seashore who conducted the study. “The rate at which they were warming was already high.”
While runoff from increased development on the Cape has exacerbated the problem by adding nutrients such as nitrogen and phosphorous into ponds, Fox has found significant algae blooms on parts of the Cape where there hasn’t been any development, such as in ponds at Nickerson State Park in Brewster and Gull Pond in Wellfleet.
“This leads us to think the blooms are more related to the warming temperatures,” she said.
At Santuit Pond in Mashpee, where the scourge of algae has extended past Thanksgiving in recent years, local officials have been using solar-paneled devices to stir up the sediment on the pond’s bottom to boost oxygen and curb the blooms. But without major cuts to global carbon emissions, temperatures are likely to continue rising and creating conditions for more of the bacteria.
For Maura Harway, who has spent summers at a home overlooking Santuit since the 1960s, the changes are painful to witness.
“The pond used to be clear as crystal,” said Harway, 64, who pines for a return to the way it was. “We used to live in our bathing suits.”
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Mats of algae have spread on Santuit Pond nearly every summer for the past five years, making the water unsafe for swimming in late summer.
As the sun set on a recent evening, Harway and her husband walked toward a small beach, where small knots of green algae drifted just below the surface. It wasn’t a good sign.
“Swimming is part of what this place is all about, and I’d like my kids to be able to enjoy it as long as we have,” she said. “It’s just depressing.”
David Abel can be reached at david.abel@globe.com. Follow him on Twitter @davabel.