The 2025 western Lake Erie cyanobacterial bloom had a severity index (SI) of 2.4, which is considered a mild bloom, and less intense than 2024 (SI of 4.2). The SI captures the amount of biomass over the peak 30-days of the bloom, calculated using satellite imagery (determined from three 10-day composites with the maximum amount of biomass).
Bloom severity fell at the lower end of the seasonal forecast issued on June 26, which predicted a SI between 2 and 4. The Microcystis bloom started in early July in far western Maumee Bay. Unlike recent years (2020–2024) when the bloom was well developed in July, the 2025 bloom slowly intensified and was not fully established until early August. The bloom reached peak biomass and area in late August before declining rapidly in early September. This is earlier than in recent years, when the bloom did not decline until the end of September. A weak bloom did persist into October before ending in late October following seasonally cooler temperatures and mixing from stronger winds.
At its peak, the bloom covered 441 square miles. Through the season, the bloom stayed closer to the U.S. coast, primarily from Stony Point, Michigan, to Port Clinton, Ohio. The location, biomass, and spatial extent of the bloom are impacted by wind direction and speed.
The seasonal forecast uses an ensemble of different models, which consider phosphorus loading into the lake during the spring and early summer. NOAA scientists will examine the differences between observed and predicted bloom severity in comparison with forecasts for previous years to evaluate the models.
Since 2009, NCCOS has used satellite data from sensors across three different satellite missions to monitor the severity and impacts of the annually occurring bloom. Following the 2024 bloom season, NCCOS reprocessed the full time series with updated satellite calibrations, improved algorithms and additional data, generating a more complete, consistent and continuous data set for HAB monitoring in the lake.
For the full final bulletin and seasonal assessment, click here.
For more information on the Lake Erie HAB forecast, click here.
This work is authorized by the Harmful Algal Bloom and Hypoxia Research and Control Act (HABHRCA) 33 U.S.C. §§ 4001 et seq.
