by Valerie Hurley ·
Friday, October 18, 2024
The algae bloom that began in late August is officially over, according to the results of the Board of Health’s water samples collected Sept. 26 and Oct. 1. Swimming is once again safe in Bare Hill Pond for the hardy souls who enjoy autumnal swims.
According to state guidelines, cyanobacteria can become toxic when it registers at 70,000 cells per milliliter (c/mL) in a body of water. At its highest, cyanobacteria in the pond was measured at 360,000 c/mL. Visibility, which must be at 48 inches or more to be considered safe for swimming, was 30 inches at its lowest. Recent tests show levels of cyanobacteria at about 32,000 c/mL, while visibility has improved to 54 inches. Two tests in a row must show acceptable visibility and cyanobacteria levels for the Board of Health to lift the swimming advisory, which it did Oct. 14.
A member of the Nashoba Associated Boards of Health takes the water samples and measures for visibility. Water was taken from the town beach for algae testing; visibility was measured by lowering a weighted disk from the crew dock and measuring the depth at which it was no longer visible.
This year’s algae bloom, which follows two others in 2020 and 2021, has some residents wondering whether the pond drawdowns that have taken place nearly every year since 2002 can still influence outcomes for algae and weed overgrowth.
Deep drawdowns are still effective against both weed and algae overgrowth, said Bruce Leicher, chair of the Bare Hill Pond Watershed Management Committee. “We now have clear evidence of what happens when we do not do drawdowns,” he said, pointing to the 2024 report on the state of Bare Hill Pond by environmental consulting firm Aquatic Restoration. The report shows that in the absence of 6½-foot drawdowns for two successive years (due to equipment failure in 2022 and 2023), fanwort increased 33% from 2023 and 86% from 2022. Milfoil increased by 35% from 2023. The invasive plants simply “repopulated the drawdown zone,” Leicher added.
But are drawdowns also effective in avoiding algae blooms? The consultant’s report concluded they play a role in delaying or avoiding blooms, mainly by influencing the phosphorus available in the water to feed the cyanobacteria.
Leicher pointed to the late start—the end of August—of this year’s bloom compared to the earlier onsets in other lakes statewide. He said an unintended deeper drawdown in 2022–23 postponed the 2024 bloom by keeping phosphorus lower than it had been when blooms occurred. The deeper drawdown happened when the electrical system failed to stop the pump on time, lowering the water by 7 feet, 1 inch instead of the intended 6½ feet. And he noted, as did the report, that blooms were avoided altogether “every year we did a deep, pumped drawdown.”
Predicting algae blooms isn’t something the report tries to do, but it tracks the conditions that can lead to them, including water temperature and levels of phosphorus and dissolved oxygen at certain depths. These conditions can determine the likelihood of “phosphorus loading” from the pond’s sediment. It is this loading–triggered by just the right combination of low oxygen, high temperature, and organic matter such as weeds–that can tip the algae into a bloom.
The dreaded loading appears to be what happened this year. Phosphorus readings, except the one in deep water on Aug. 6, were encouragingly “low,” according to the report. But it warned: “The pond continues to have oxygen conditions that support sediment phosphorus release,” referring to anoxic water in the pond. In fact, water with no dissolved oxygen “was present in water as shallow as seven feet in July, the shallowest depth ever recorded.” Phosphorus loading, the report concluded, was likely given the increasingly “warmer, low oxygenated, nutrient-rich waters” of the pond.
As reported previously, with the pumping system fully repaired, the drawdown began two weeks ago with the goal of taking the water down by 6½ feet. Although the permit for the drawdown calls for it to end Nov. 30, Leicher said that for the first time, if 6½ feet is not achieved by then, the committee will ask the Conservation Commission for permission to keep pumping until it’s achieved–or until the first frost. Doing so would improve the chances of killing the weeds.
With the clearing of the harmful algae, pond committee member Joe Petirossi was able to reschedule the diver-assisted suction harvesting of weeds that had been planned for late August to this week.
Homeowners along the pond who want to hire DASH services for their shorefronts can find guidance by going to the Bare Hill Pond Watershed Management Committee’s webpage, www.harvard-ma.gov/bare
hill-pond-watershed-management. Click on “Diver Assisted Suctioning Harvesting.” Getting a permit can take four to six months.