Lake Erie Harmful Algal Blooms: 2025 Season Review and 2026 Phosphorus Reduction Progress

Lake Erie’s western basin experiences harmful algal blooms (HABs) each summer, driven primarily by phosphorus-laden agricultural runoff entering the lake through the Maumee River and other tributaries draining intensively farmed landscapes in Ohio and southwestern Ontario. These blooms, dominated by the cyanobacterium Microcystis, produce microcystin toxins that threaten drinking water supplies, close beaches, and disrupt the lake’s commercial and recreational fisheries.

The 2025 Bloom Season in Review

NOAA’s Great Lakes Environmental Research Laboratory measured the 2025 Lake Erie bloom at a severity index of 6.5 on a scale of 10, categorizing it as a significant bloom event. The bloom developed in late June, approximately two weeks earlier than the 2010-2020 average onset date, and persisted through mid-October. At its peak extent in late August, satellite imagery from the European Space Agency’s Sentinel-3 satellite showed cyanobacteria covering approximately 2,100 square kilometres of the western basin.

The City of Toledo, Ohio, which draws its drinking water from Lake Erie’s western basin, activated enhanced treatment protocols on 23 occasions during the 2025 bloom season when raw water microcystin concentrations exceeded 1.0 microgram per litre. Toledo’s upgraded water treatment plant, which added granular activated carbon filtration after the 2014 water crisis that left 500,000 residents without safe drinking water for three days, successfully maintained finished water quality within Health Canada and EPA guidelines throughout the season.

On the Canadian side, the Ontario Clean Water Agency reported elevated microcystin levels at the Union Water Supply System intake near Leamington on 14 days during the 2025 bloom season. The utility’s treatment processes maintained finished water quality below Ontario’s drinking water standard of 1.5 micrograms per litre for total microcystins.

Phosphorus Loading: The Root Cause

The scientific consensus, documented in extensive peer-reviewed research and confirmed by the IJC’s Science Advisory Board, identifies excessive phosphorus loading as the primary driver of Lake Erie HABs. The Great Lakes Water Quality Agreement established a target of reducing total phosphorus loading to the western and central basins of Lake Erie to 6,000 metric tonnes per year, a 40 percent reduction from 2008 levels.

Progress toward this target has been disappointing. Heidelberg University’s National Center for Water Quality Research, which operates the most comprehensive tributary monitoring network in the western Lake Erie watershed, reports that dissolved reactive phosphorus loading from the Maumee River has shown no statistically significant downward trend since monitoring intensified in 2009.

Agricultural sources account for an estimated 80 to 90 percent of phosphorus loading to the western basin. The primary pathways include surface runoff from fields where phosphorus fertilizer or manure has been applied, subsurface drainage through tile drain systems that connect directly to tributaries, and erosion of phosphorus-enriched soil particles during intense rainfall events.

Management Approaches

Both the United States and Canada have developed phosphorus reduction strategies for the Lake Erie watershed. Ohio’s H2Ohio initiative, launched in 2019, provides financial incentives for farmers to adopt phosphorus-reducing practices including cover cropping, precision fertilizer application, and constructed wetlands. The program has enrolled over 2.2 million acres across the western Lake Erie watershed.

On the Canadian side, Ontario’s Lake Erie Action Plan commits the province to reducing phosphorus loading from Canadian sources by 40 percent by 2025, consistent with the binational target. The plan emphasizes voluntary adoption of beneficial management practices through the Ontario Soil and Crop Improvement Association’s programs, supplemented by regulatory measures including the province’s Nutrient Management Act.

Despite these programs, progress remains slow. Agricultural economists at Ohio State University have noted that the practices most effective at reducing phosphorus loss, particularly conversion of cropland to permanent vegetation and significant reductions in fertilizer application rates, impose the highest costs on farmers and have the lowest voluntary adoption rates.

Forecast for 2026

NOAA typically releases its Lake Erie HAB seasonal forecast in early July, based on spring phosphorus loading data from Heidelberg University’s monitoring network. However, preliminary indicators suggest that the 2026 bloom could be significant. Precipitation across the western Lake Erie watershed from October 2025 through February 2026 has been above the long-term average, increasing the risk of substantial spring phosphorus runoff when fields thaw and spring rainfall begins.

Long-term projections from the University of Michigan’s Cooperative Institute for Great Lakes Research suggest that climate change will likely increase the frequency and severity of Lake Erie HABs over the coming decades, as warmer water temperatures extend the algal growing season and more intense rainfall events increase phosphorus loading from agricultural land.

Current bloom conditions and forecasts are available from NOAA’s Lake Erie HAB Tracker at coastwatch.glerl.noaa.gov. The Ontario Ministry of the Environment publishes beach water quality advisories at ontario.ca/beachwater.