Blue-Green Algae Public Notice Lifted - Muldrew Lake

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The Simcoe Muskoka District Health Unit (SMDHU) would like to inform residents and visitors to the Town of Gravenhurst that additional laboratory analysis confirms the blue-green algae bloom impacting the northwest side of Peninsula Road on Muldrew Lake in the Town of Gravenhurst has dissipated. Please be advised that the public health notice regarding precautions for blue green algae issued on November 10, 2022 by the SMDHU for Muldrew Lakes has been lifted.

This public notice advised residents to take precautions and avoid drinking the water from private water supplies that are impacted or draw from the area of the lake where blue-green algae has been identified.

Recent sampling conducted by the Ministry of the Environment, Conservation and Parks, no longer identify the presence of a blue-green algae bloom. Generally, cooler weather, rainfall, and reduced sunshine will lead to the breakup of an algal bloom. Blue-green algae blooms naturally occur in freshwater lakes, bays and inlets, usually during warmer weather. They thrive where water is shallow, nutrient-rich, slow moving and warm, but may also be present below the surface. The best method for managing algal blooms is prevention. Decreasing the amount of nutrients, such as phosphates and nitrogen, entering the water source can effectively reduce the occurrence and severity of algal blooms. This means using phosphate-free detergents, not using fertilizers on lakefront properties, taking steps to impede agricultural run-off, and ensuring that septic systems do not leak into the water source.

For more information, please call the SMDHU Health Connection Monday to Friday 8:30 a.m. to 4:30 p.m. at 1-877-721-7520. Links to general information about blue-green algae are available on the health unit website at:

If you suspect a blue-green algae bloom, please assume toxins may be present, avoid using the water and call the Ministry of the Environment Pollution Hotline at 1-866-663-8477.

Charles Gardner, MD, CCFP, MHSc, FRCPC, Medical Officer of Health