Forty-five years ago, Lake Erie was so polluted that TIME magazine warned it was “in danger of dying by suffocation.” A massive clean-up effort, involving everyone from local businesses to state and national policymakers, brought the lake back from the brink. Recently, algal blooms have once again tainted western Lake Erie with unhealthy, sometimes toxic, green slime. Why is this happening?


For the past forty years, scientists at Heidelberg University’s National Center for Water Quality Research have been measuring pollution in the rivers and streams that feed Lake Erie. Decades of monitoring have led to an inescapable conclusion: phosphorus runoff, primarily from agricultural lands, is feeding explosive cyanobacterial growth in the warm, shallow waters of the western basin.


Understanding the source of the phosphorus pollution that threatens the health of Lake Erie, and of the communities that depend on it, is the first step toward finding solutions.

The History Of Lake Erie’s Troubles

How Do We Know Phosphorus Is The Culprit?


In this video, research scientist Laura Johnson of the National Center for Water Quality Research at Heidelberg University explains how 40 years of data points to dissolved reactive phosphorus as the primary culprit behind Lake Erie’s harmful algal outbreaks.

What is Toxic Algae?

Not all algae are harmful. Algae are natural components of marine and fresh water ecosystems, and form the foundation of most aquatic food chains. The most commonly occurring groups of freshwater algae are diatoms, green algae, and blue-green algae, which are more correctly known as cyanobacteria.   Some cyanobacteria can produce potent toxins known as cyanotoxins.


Outbreaks of cyanobacteria that release toxins are commonly called “toxic algae” blooms. In recent years, toxic algae blooms involving a cyanotoxin called microcystin have led to several temporary “do-not-drink” warnings around the western Lake Erie basin.


A Tale of Two Rivers


The differences in how heavy spring rainfalls affect phosphorus loads in two watersheds – the Maumee and the Cuyahoga – show the different impacts of non-point sources (like the primarily agricultural lands in the Maumee River basin) and point sources (like the urban and industrial lands in the Cuyahoga River basin, which houses nearly 15% of Ohio’s population). Both watersheds occasionally have combined sewer overflows (CSOs), but research suggests these volumes pale in comparison to the river volume during storms.


The charts below illustrate the spring 2015 phosphorus concentrations in the two basins. Note that in the Cuyahoga River basin, phosphorus concentrations (shown as brown dots) remained mostly level despite large spikes in water flow (shown as blue spikes) after heavy rains. In the Maumee River basin, by contrast, heavy rains led to dramatic spikes in phosphorus concentrations.


Chart showing Cuyahoga River Phosphorus concentrations and discharge
Chart showing Maumee River Phosphorus concentrations and discharge

The phosphorus “concentration” is defined as the total amount of phosphorus in a defined volume of water – in this case, milligrams of phosphorus per liter of water. Learn more below about the different kinds of phosphorus, and which are most responsible for Lake Erie’s algae outbreaks.

Point Source vs. Non-Point Source Pollution

There are two categories of pollution that contribute to Lake Erie’s phosphorus loads: pollution from point sources like pipes from factories or water treatment plants, and non-point sources like runoff from fields. In general, point sources are associated with water use for domestic, commercial and industrial purposes, while nonpoint sources are associated with the interaction of land use activities and the hydrological cycle.

Illustration Showing Point Source Pollution

“Point source” pollution is defined by the U.S. Environmental Protection Agency (EPA) as “any single identifiable source of pollution from which pollutants are discharged, such as a pipe, ditch, ship or factory smokestack.” In the case of water pollution, factories and sewage treatment plants are the most common point sources of pollution.

Illustration Showing Non-Point Source Pollution

“Non-point source” pollution is defined as any source of water pollution that does NOT meet the legal definition of a “point source.” Common sources of non-point source pollution include water running off from surface lands like cropland, urban runoff from streets and lawns*, faulty septic systems, and sediment from construction sites.


*NOTE: Some municipalities send some of this surface runoff into combined sewer systems that lead to sewage treatment plants. However, these systems are very expensive and not infallible.

The Role of Tile Drains


A large portion of the western Lake Erie watershed was originally the Great Black Swamp. Thus, farming and settlement was only possible after the widespread construction of drainage ditches and installation of “tile drains” beginning in the early 1900s. Tile drains are simply drain pipes buried beneath fields to help lower the water table and collect surface water after it has filtered through a few feet of soil and then whisk it away from the fields to avoid standing water and surface runoff.

Illustration Showing Tile Drains

Unfortunately, in the western Lake Erie region, heavy clay soils often develop cracks and pores that act like the drain in your bathtub: surface water quickly drains through the soil into the tile drain, carrying high concentrations of phosphorus with it. Although surface runoff can lead directly into ditches during large storm events carrying even higher concentrations of phosphorus, a majority of field runoff exits through tile drains. Both surface runoff and tile drains empty into ditches that lead to streams, which lead into rivers, which ultimately empty into Lake Erie, delivering damaging loads of phosphorus.

Why is dissolved phosphorus such a problem for Lake Erie?


There are two types of phosphorus that enter Lake Erie from the surrounding watersheds. Dissolved phosphorus is the phosphorus that remains in water after that water has been filtered to remove sediment and other particulate matter. Phosphorus that is attached to sediment or particulate matter is called particulate phosphorus. Together these two forms of phosphorus make up the total phosphorus concentration in a water sample.


Dissolved phosphorus is a special problem in Lake Erie because it is much more “bioavailable” to algae than particulate phosphorus. In other words, algae can easily consume dissolved phosphorus, leading to rapid algal growth. This type of phosphorus is sometimes called “dissolved reactive phosphorus.” Dissolved phosphorus also tends to remain in the water, while particulate phosphorus settles to the lake bottom where it may no longer be available to algae. The amounts or loads of dissolved phosphorus entering Lake Erie have more than doubled since the mid-1990s.


P forms

Total Phosphorus includes all forms of phosphorus found in the water sample. Particulate Phosphorus will generally settle out of the water column and not be bioavailable to cyanobacteria and other organisms.

Dissolved Reactive Phosphorus is bioavailable to organisms and feeds harmful algal blooms.  Dissolved reactive phosphorus comes from agricultural fertilizer and nutrient polluted runoff.

The Way Forward

Dissolved phosphorus from agricultural runoff is the primary driver of Lake Erie’s harmful algae outbreaks, and a recent report from the USDA-NRCS shows that 84% of phosphorus applied to agricultural land in the Lake Erie Basin is from commercial fertilizers, and 16% is from manure. While we can be quite certain about the sources of the majority of the phosphorus pollution, the solutions are harder to pinpoint. In part this is because agriculture is exporting a small amount of the phosphorus that is applied and many farms are at recommended soil test phosphorus levels. Also, agricultural lands in the Lake Erie watershed vary in the size of the operation, composition of their soil, even the amount of rainfall they receive. What works for one farm or livestock operation might not work for another, and changing the way farmers do business could have large economic costs. Making the wrong kinds of changes might not make things better for Lake Erie – it could even make things worse.


Here are some of the many opportunities for reducing phosphorus pollution while preserving farmland productivity.

Focus on Adaptive Management


Just as there’s no “one size fits all” solution, the options for reducing nutrient pollution can change over time. Active monitoring and interpretation of the current conditions on farmlands is crucial to managing nutrient applications and keeping nutrient pollution out of waterways.

Follow the “4Rs” for nutrient stewardship


The 4Rs promote best management practices designed to ensure that the Right fertilizers and manures are applied at the Right rate, at the Right time, and in the Right place. These practices can include avoiding fertilizer and manure application to frozen fields; injecting fertilizer beneath the surface of the soil; and testing soils to know how much fertilizer will be needed for healthy crops. Learn More

Slow down the water

Reducing the rapid loss of water and concurrently stripping excess phosphorus out of water before it reaches waterways can help reduce pollution levels. Opportunities may include controlled drainage structures with bioreactors, blind inlets instead of tile risers, and saturated buffer strips. Cover crops and gypsum application, which would improve water infiltration and soil water storage may also help reduce the rapid delivery of water to drainage ditches and ultimately Lake Erie.


Opportunities may include restoring natural wetlands, installing artificial wetlands, or filtering water that exits tile drains. Cover crops and buffer strips may also reduce runoff from surface soils.

This is a fixable problem. If people work together to reduce phosphorus pollution, Lake Erie can and will recover.

Dr. Jeffrey Reutter

Former Director of and Special Advisor, Ohio Sea Grant Program and F.T. Stone Laboratory

Forty years of water monitoring data has made a huge difference to our understanding of what’s happening in Lake Erie, and can help us make smart decisions about the steps we need to take.

Senator Randy Gardner

Co-chair of the Lake Erie Caucus, Ohio Senate, District 2

Farmers are ready and willing to do the right thing and USDA is working hard to provide incentives focused on dissolved phosphorus reductions, such as holding back runoff water and better fertilizer management. 

Steve Davis


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