Residents who had hoped to take a dip in Bare Hill Pond over the Labor Day weekend will be disappointed to learn that the swimming ban first imposed Aug. 20 remains in place. According to the advisory issued by the Board of Health, swimming at the town beach and Thurston’s Cove is forbidden, and swimming off private beaches and in any part of the pond is strongly discouraged, as are on-water activities. Visibility in the water remains poor throughout the pond.
A water sample taken Aug. 24 showed the pond is not toxic, but swimming is nevertheless banned. The reason is the high potential for toxicity, based on cyanobacteria level (sometimes loosely referred to as blue-green algae) measured over the past three weeks at the pond. Toxicity is not preordained but is much more likely when cyanobacteria levels are where the pond’s are now, and it can happen very quickly.
According to state and local standards, readings above 70,000 cells per milliliter require local boards of health to issue no-swimming orders and suggest the water be tested for cyanotoxins. Harvard’s readings between Aug. 3 and Aug. 24 ranged from less than 50,000 cells/mL on Aug. 3, to 205,000 two weeks later, to 175,000 on Aug. 24. Weekly testing will be carried out by the Harvard Board of Health and Nashoba Associated Boards of Health until mid-September according to new protocols developed during an algal bloom last August. The pond will be closed to both swimming and all on-water activities if the cyanobacteria reaches a level in excess of 1,000,000 cells/mL.
Signs will be posted at the beach if that happens, according to Board of Health Chair Libby Levison.
Resident links blooms to drawdowns, but that is not established
As the swimming ban continues, Brenda Baer who lives on the pond’s shore claims the yearly drawdowns—meant to reduce weed growth—are contributing to algal blooms and called for a “moratorium” on the drawdowns, encouraging people on the social media site Nextdoor to write to the Conservation Commission to “stop the drawdown” in time for the objections to be heard at a Sept. 2 commission meeting.
But Bruce Leicher, chair of the Bare Hill Pond Watershed Management Committee, warned against conflating the causes of cyanobacteria overgrowth and excess weed growth. “As was pointed out by [committee member] Pablo Carbonell, we effectively had two years without a drawdown, but had algal blooms last August and this August,” he told the Press last week. In 2019, the drawdown happened in the absence of a freeze, so the weeds couldn’t be killed off by the cold. And in 2020 the pump broke in November and wasn’t fixed until spring, effectively eliminating that year’s drawdown, too.
Some common factors affecting both cyanobacteria and weeds include low oxygen in the water, high temperatures, and erratic rainfall, but causal relationships must be sorted carefully, he said.
The main cause of excess weed growth, Leicher told the Press, is phosphorus, which is primarily linked to development around the pond, including lawns and disturbed vegetation. Higher water and air temperatures, drought, and erratic rainfall also contribute to the growth, and likely contribute to the conditions that also promote algae.
The committee is investigating whether the drawdowns will continue to be effective and has hired a wetlands biologist to figure it out. Over the longer term, the committee’s biologist will also study whether any other methods could reduce the rate at which phosphorus is released from mud at the bottom of the pond.
A rain garden planted near the pond absorbs some of the runoff that contributes to an overload of harmful phosphorus. (Courtesy photo)
