Green Lake, Wis. — For the first time, a lake-wide harmful blue green algae bloom has been confirmed in Green Lake, stretching along the north and south shores and across the lake, including at the surface in the deepest part of the lake at more than 200 feet deep. This marks the first documented offshore harmful algal bloom in Green Lake, extending across the entire surface. Blue-green algae blooms are a warning sign of degraded water quality in Green Lake, and because they can produce toxins harmful to people and pets, the Green Lake Association (GLA) advises the community to avoid boating, swimming, fishing and water sports in areas with visible algae scum, streaks or discolored water.
On Sept. 16, a GLA member reported paint-like streaks and suspicious water conditions along their shoreline. Thanks to GLA’s blue-green algae monitoring program, the team collected samples the same day and confirmed harmful blue-green algae (cyanobacteria) in both the shoreline and the middle of the lake. Using on site microscopes and toxin test strips, GLA can deliver results within hours—rather than waiting weeks for outside lab testing.
Since then, the GLA team has been on the water daily to sample and assess the bloom’s extent. Environmental factors such as wind and wave action can shift the bloom’s exact location and density, but conditions remain widespread. GLA has notified the Green Lake
County Public Health Department and the Wisconsin Department of Natural Resources (WDNR). Once a harmful blue-green algae bloom appears, there’s no way to fix it or make it safe—we can only wait for conditions to change. The only real solution is prevention: reducing the nutrients, like phosphorus, that fuel these blooms in the first place.
“Seeing Green Lake covered by a harmful bloom like this is deeply alarming,” said Stephanie Prellwitz, CEO of the Green Lake Association. “As Wisconsin’s deepest natural inland lake, experiencing something of this scale is unprecedented. The fact that we’re now seeing a lake-wide harmful algal bloom underscores the urgent need for stronger action. Local efforts are not enough—this is a statewide water quality challenge. The State of Wisconsin and the WDNR must increase their commitment to tackling nutrient pollution in Green Lake and other impaired waters if we want our lakes to stay a thriving part of our communities.”
Protecting Wisconsin’s waters cannot fall on local communities alone—it will take all of us, from residents and farmers to shoreline owners and decision-makers, working together to safeguard the lakes that define our state’s identity.
The GLA will continue monitoring the bloom and provide updates as conditions change. For the most current information, follow the Green Lake Association’s Facebook page: https://www.facebook.com/GreenLakeAssociation
ABOUT THE GREEN LAKE ASSOCIATION
The Green Lake Association is a science-driven nonprofit dedicated to protecting Wisconsin’s deepest natural inland lake. Through phosphorus reduction, invasive species prevention and watershed restoration, the GLA is working to improve water quality and build long-term resilience. By engaging volunteers, partnering with local schools, and fostering hands-on lake experiences, we’re strengthening the community’s connection to Green Lake and inspiring generations of stewards.
