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Toxic Blue-Green Algae Blooms Spread in Cape Ponds

Gerald Beetham; Association to Preserve Cape Cod
Cyanobacteria bloom in Walkers Pond, Brewster, shot on June 25, 2020.

In five ponds across the Cape, high concentrations of blue-green algae blooms are raising concerns about dangerous levels of toxins they produce.

Blue-green algae blooms, also known as cyanobacteria scums, can make the water of a pond look like pea soup, though usually the growths are natural and harmless parts of ecosystems. 


That is, until they dominate and release dangerous toxins, as they have at times this summer in Walkers Pond and Cliff Pond in Brewster; Mares Pond and Deep Pond in Falmouth; and Long Pond in Marstons Mills, according to the Association to Preserve Cape Cod (APCC). 


“The concern is it’s early,” said Andrew Gottlieb, executive director of APCC. “Ponds have not really warmed up that much and yet we’re seeing toxic blooms in a number of different ponds and that gives concerns about …  what we might see as the summer deepens and the warmth continues.” 


Exposure to these toxins can pose health risks that range from skin irritation, to fevers, to major organ damage. Town boards of health are responsible for restricting access to these ponds and have already done so to keep people and their pets safe. 


At Long Pond, in particular, Gottlieb said he was disturbed by the level of toxicity. 


“We found that the bloom was made up almost exclusively of a type of cyanobacteria that produces the highest toxicity,” he said. “And it’s among the highest concentrations we’ve seen since we started monitoring five years ago.” 


This time last year, Gottlieb said, toxic blooms were found in far fewer ponds. The question now is whether the early blooms are an anomaly, or an indication that blooms are becoming toxic earlier in the summer. 


What’s relatively clear is the role that climate change plays. 

“The two things that drive these blooms are nutrients and warming,” Gottlieb said. “So as the temperature regionally and globally warms we’re generally creating conditions that are conducive to cyanobacteria blooms.” 


But because these toxic blooms are also driven by nutrient loading, there are solutions: people can use less lawn fertilizer, and upgrade to nitrogen-reducing septic systems.


In the meantime, people, especially homeowners around the ponds, Gottlieb noted, should check in with their local board of health before diving in. 


Eve Zuckoff covers the environment and human impacts of climate change for CAI.