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Vermont DEC: Many once‑clear lakes showing rising phosphorus; monitoring and alum treatments highlighted
Summary
Vermont Department of Environmental Conservation officials told a committee that long‑term monitoring shows rising phosphorus in many oligotrophic lakes, and the agency described monitoring programs and targeted alum treatments used to reduce in‑lake phosphorus.
Jenny Austin, the new lakes and ponds program manager at the Vermont Department of Environmental Conservation, told a state committee that long‑term monitoring shows phosphorus concentrations are increasing in many of the state’s otherwise clear, cold lakes.
The trend matters because increasing phosphorus raises the risk of algal blooms and long‑term declines in water clarity, recreation and certain cold‑water fisheries. “Lakes and ponds are a focal point of the watershed system,” Austin said, adding that the agency monitors trophic state, shoreland condition and invasive species to gauge lake health.
Austin described the agency’s monitoring framework, including the Vermont Lay Monitoring Program, which trains volunteers to sample lake water for clarity, phosphorus and chlorophyll a. She said the state has “over 800 lakes and ponds,” of which 445 are larger than 10 acres and 220 are larger than 20 acres. The DEC uses volunteer and professional monitoring to establish baselines, track long‑term nutrient trends and assess compliance with Vermont water quality standards.
DEC staff emphasized that lakes are “a sum of their parts,” meaning upstream watershed conditions and shoreline disturbance strongly influence lake phosphorus. Austin said some oligotrophic (low‑nutrient) lakes have shown steady increases in phosphorus over about 40 years, while a small number of eutrophic (high‑nutrient) lakes have stable or declining phosphorus trends. She noted that causes vary by watershed and can include historical land use, shoreline disturbance and agricultural runoff.
The agency described how too much phosphorus can cause harmful cyanobacterial blooms and low dissolved oxygen that stress fish. For lakes where most phosphorus originates from internal loading (legacy phosphorus in sediments), the DEC said it has had selective success with aluminum sulfate (alum) treatments, which bind phosphorus in sediments and can improve water clarity for years or decades. Austin cited Lake Moray, where post‑treatment monitoring showed reduced phosphorus, and Tickleminton Pond as another successful example; DEC staff said a treatment is planned for Lake Carbide.
Austin cautioned that alum treatments are effective only after watershed phosphorus sources are controlled; otherwise the benefit is temporary. She urged more targeted monitoring in underserved watersheds to better attribute sources and inform management.
The presentation included discussion of the Vermont Lakes Scorecard, trophic classification (oligotrophic, mesotrophic, eutrophic) and the role of long‑running volunteer data in assessing trends. Committee members asked for more detail on source attribution and on the agency’s planned follow‑up; DEC staff said they would provide additional monitoring breakdowns and continue refining management strategies.
DEC officials closed the segment by noting ongoing work to protect shorelines, reduce watershed phosphorus inputs and tailor restoration tools to each lake’s conditions.
