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City treats Indian Lake for dense native vegetation; herbicide use followed by temporary fish losses, staff to adjust public messaging
Summary
DSR described a fast-acting herbicide treatment for dense elodea growth at Indian Lake, a subsequent drop in dissolved oxygen and a small fish kill, and plans for an 18-inch winter drawdown and clearer public communications.
WORCESTER, Mass. (July 28, 2025) — City environmental staff described a sequence of lake management steps at Indian Lake this summer: alum dosing to reduce phosphorus, rapid growth of the native plant elodea, a low-dose herbicide treatment to remove dense mats, an associated dip in dissolved oxygen that caused a limited fish kill, and follow-up mechanical and aeration efforts that staff said have restored lake conditions.
What staff said: John (DSR staff) explained the sequence and the trade-offs. He said the alum dosing station improved water clarity by binding phosphorus and reducing material that can feed cyanobacteria, which then allowed more light into the water and contributed to an elodea surge. "What alum does is it binds to the phosphorus and other material and pulls it down to the bottom of the lake," John said. When elodea became excessive and impeded boat use, the department applied a low-dose herbicide and used a lake mower and an aerator to remove vegetation and restore oxygen levels.
Consequences and adjustments: John told the committee the treatment worked but "the water oxygen, dissolved oxygen, rather, in the lake, went down, which, caused a number of fish to die, but small number fortunately, relatively speaking." Staff said oxygen levels have recovered and the lake appears clear and usable at present. The department will perform an initial 18-inch winter drawdown to expose shoreline vegetation and reduce nuisance growth, and will adjust further management as needed.
Public communications and internal review: Committee member Ted Connor raised concerns about a city statement that said, "The application of herbicides is a common practice to reduce weed density and poses no risk to humans and animals after application is complete." Connor said that phrasing was misleading in light of the fish loss and asked staff to refine outgoing messaging. John agreed to clarify the distinction between direct toxicity and indirect ecosystem effects that can follow vegetation die-off.
Why this matters: Indian Lake is a recreational resource for boaters, anglers and swimmers. Managers said balancing nutrient controls (alum) with vegetation management is complex and can produce unintended short-term impacts even when long-term water-quality benefits are the goal.
Ending: Staff said conditions have improved and that the department will continue monitoring water quality, oxygen levels, and vegetation, and will pursue the planned drawdown this winter as one management tool.
