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At 90 feet, Cliff Pond is the deepest pond on Cape Cod. It's located in Nickerson State Park, and is a popular for swimming and boating.
But starting a 1998, toxic algae blooms there became a problem, resulting in the death of at least
two dogs* that had been swimming in the pond.
Closures of the pond to swimming have increased in recent years. Last year, park officials closed the pond for a total of 99 days. This summer, the pond was closed from June 25 to July 9.
The reason was high levels of blue-green algae.
The microscopic plant is present all the time,** but the timing of "blooms," when it multiplies out of control, is unpredictable. Not all blooms result in dangerous toxins. How the weather leads to a toxic bloom isn't well understood. Experts expect more and more blooms as the climate warms.
