Tracking phosphorus in Western Lake Erie: Why harmful algal blooms keep returning

Subhasis Giri<girisubh@msu.edu>, Michigan State University, Department of Biosystems and Agricultural Engineering - May 18, 2026

How phosphorus movement from fields to water fuels repeated algal outbreaks.

Shoreline view of green algal blooms floating along the edge of a calm lake or reservoir. Dense green algae collect near grasses and rocks along the shore under a blue sky with scattered clouds. — Harmful algal blooms in a lake due to excessive nutrient loading. Photo by iStock.

Harmful algal blooms (HABs) have become a recurring concern in Western Lake Erie, affecting water quality, recreation and drinking water supplies. These blooms can produce toxins that pose risks to people and wildlife. In 2014, a severe bloom left nearly 500,000 residents in the Toledo, Ohio area without safe tap water for several days. So why do these blooms keep happening?

The role of phosphorus

Excess phosphorus in the water is the primary driver of HABs. Rainfall and snowmelt wash phosphorus off farm fields and urban areas into rivers like the River Raisin and Maumee River, which carry it into Lake Erie.

Alt text: Map of western Lake Erie showing the Maumee River and River Raisin watersheds, major tributaries, and USGS monitoring stations. The Maumee watershed is highlighted in light blue, the River Raisin watershed in pink, and Lake Erie and Lake St. Clair are labeled on the satellite basemap. — Figure 1. Satellite image of a harmful algal bloom in Western Lake Erie, Sept. 3, 2011. Large blooms can be seen from space, showing how widespread the issue can become. Photo by Subhasis Giri, 2026.

Phosphorus is transported from the land to nearby streams and rivers primarily in two forms:

Particulate phosphorus: where phosphorus is attached to soil particles and moves through soil erosion
Dissolved phosphorus: where phosphorus is soluble in water and moves through surface runoff and tile drainage

Total phosphorus represents the sum of both particulate and dissolved forms. Out of these, dissolved phosphorus is the key driver of HABs, as it is readily available for uptake by plants and algae, promoting rapid growth.

Efforts over the past few decades have reduced overall (total) phosphorus in some areas. For example, the River Raisin has seen declines in total phosphorus partly due to better wastewater treatment and reduced erosion. But there’s a catch: Dissolved phosphorus has increased by nearly 40% in recent decades.

Bar chart titled “Average Seasonal and Annual Loads (Dissolved Phosphorus (tons))” comparing average dissolved phosphorus loads for 1981–1999 and 2000–2018 across winter, spring, summer, fall, and annual averages. Loads increased in winter, spring, summer, and annually during 2000–2018, with the largest annual increase from about 22 to 34 tons per year, while fall loads decreased slightly. — Figure 2. Dissolved phosphorus trends in the River Raisin. Photo by Subhasis Giri, 2026.

Even as total phosphorus declines, the form that drives blooms is rising, helping explain why HABs persist. Several factors are driving this increase, including more intense rainfall that increases runoff, tile drainage systems that quickly move water (and nutrients) off fields, and legacy phosphorus stored in soils over time. These conditions speed the movement of dissolved phosphorus from land to water. In Western Lake Erie’s shallow, nutrient-rich waters, even small increases can trigger significant algal growth.

We’ve made progress, but the challenge is evolving. Understanding the rise of dissolved phosphorus is key to developing solutions that protect Lake Erie moving forward.

Learn more

This article highlights just part of the story. To learn more about the total phosphorus loading trends from the River Raisin and phosphorus loading trends from the Maumee River (the largest tributary to Western Lake Erie), please refer to the full Michigan State University Extension bulletin E3544: Harmful Algal Blooms in Western Lake Erie.

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