• Cover Story

Algal Blooms: The Causes, Dangers and Some Solutions

Lisa Ballard and Duane Hovorka, IWLA Agriculture Program Director
Outdoor America 2020 Issue 2
Algal bloom in Lake Erie - credit Ohio Sea Grant & Stone Lab

On August 3, 2014, 400,000 people in Toledo, Ohio had no drinking water. Toxins produced by a massive algal bloom in Lake Erie had contaminated the city’s water supply. Bottled water became instantly more precious than gold. The crisis made national headlines. How could this happen in the United States, where most of us take safe drinking water for granted?

"Algal bloom" is a catch-all name that includes any type of algae that has outgrown its appropriate population size in a body of water.

Toledo is on the southwestern end of Lake Erie. The Great Lakes, particularly Lake Erie, have long been the focus of research on algal blooms, along with the Chesapeake Bay and the Gulf of Mexico. Scientists blame nutrient pollution – excess amounts of nitrogen and phosphorus – that is washed into our waterways.

“All three systems have excess nutrient loads in common,” says Don Savia, professor emeritus of Environment and Sustainability at the University of Michigan and a former aquatic scientist for the National Oceanic and Atmospheric Administration (NOAA). “Phosphorus may be more of an issue in the Great Lakes, but both nitrogen and phosphorus are impacting all three systems from agricultural practices. The difference is that in the Great Lakes and the Chesapeake Bay the problem is local, whereas in the Gulf it comes from as far away as Iowa.”

What Is an “Algal Bloom?”

"Algal bloom" is a catch-all name that includes any type of algae that has outgrown its appropriate population size in a body of water. Like flowers in a garden, they can be as different as roses and daffodils. An algal bloom is just a large accumulation of algae in one spot. Some are harmful to wildlife, domestic animals and people. A harmful algal bloom is commonly referred to as an HAB. Other algal blooms are benign. (More on HABs later in this article).

If you live by water or visit it to swim, fish or paddle, you’ve probably seen an algal bloom. In fresh water, some blooms look like green pillows floating in the water column, near the surface or where it’s relatively shallow. Others form a blue- green scum that looks like half-stirred paint and that clings to the shoreline, buoys, dock pilings and any other stationary objects. These aquatic balloons and layers of muck are not true algae, but a warm-water bacteria called cyanobacteria, which contain chlorophyll and can conduct photosynthesis (turn sunlight into oxygen) like green plants on land.

“Blue-green algae occurs naturally in the Great Lakes and all lakes and ponds,” says Jeff Reutter, retired director of the Ohio Sea Grant College Program and limnologist (freshwater aquatic biologist) at Ohio State University. “Dozens of species of blue-green algae can bloom. Most are native. When a lake is calm, floaters rise to the surface where you see them. If there’s lots of wave action, the bloom mixes into the water column and you can’t see it. You don’t need to see scum to have an algal bloom.”

Reutter points to Sandusky Bay on Lake Erie as an example. Each summer, the bay has an enormous algal bloom, but due to the type of algae, it doesn’t float and there’s no scum on the surface. The water looks clean, but the bloom is there.

Algal blooms are not unique to the United States. It’s a worldwide problem. The solutions are complex, based on how we farm our food and how we deal with climate change, but if we value clean water and our health, we need to prevent these blooms.

“It’s important because algal blooms have a negative impact on the ecosystems we depend on,” says Savia. “They are getting worse, not better.”

What Causes Dead Zones?

Many algal blooms are associated with dead zones in which aquatic life cannot survive. Dead zones have no life in them because there’s no oxygen. They can occur in both freshwater and saltwater ecosystems.

“All [naturally occurring] water has algae in it. It’s the base of the food web. They harvest the sun, make biomass for plankton and produce oxygen through photosynthesis. Everything we appreciate in an ecosystem depends on algae, but it’s a problem when there’s too much,” says Patricia Glibert, professor of Environmental Science at the University of Maryland, a phytoplankton ecologist and one of the nation’s leading experts on algal blooms.

According to Glibert, when the amount of algae is greater than what other aquatic organisms can eat and that algae dies, they no longer produce oxygen and fall to the bottom of the waterway to decompose. The bacteria that causes decomposition uses oxygen for that process, and if the oxygen is not replaced, the area becomes hypoxic – the water column suffocates from the bottom up, causing a dead zone.

Why Do Nitrogen and Phosphorus Cause Algal Blooms?

Algal blooms need the same nutrients that plants need, particularly nitrogen and phosphorus. Both are naturally occurring elements that are necessary for life on our planet, but there can be too much of a good thing. Currently, there are excessive amounts of nitrogen and phosphorus bleeding into our waterways, which super-fertilize the algae. The result is an increasing number, size and toxicity of algal blooms.

“Nitrogen and phosphorus make a cornfield or a lawn grow faster. The same chemical soup causes algae to grow fast, too,” says Glibert.

While failing septic tanks, overflowing sewers, sewage treatment deficiencies and excess use of lawn fertilizer account for some of this nutrient pollution, most of it – 85 percent or more depending on the location – comes from agricultural runoff. Changes in farming practices, particularly large-scale meat production and the amount and timing of manure spreading, have not only increased farming efficiencies but also the amount of nutrient runoff.

The situation has worsened as our country’s food supply has come to depend on industrial-scale farming operations. These mega-farms concentrate thousands of hogs or chickens together, which in turn produce a tremendous amount of manure.

“Farmers spread manure not just as fertilizer but as a method of waste disposal,” says Reutter, “Some large animal feed operations apply waste at four or more times the concentration needed for crops, and they might broadcast it when the ground is frozen, so a lot of it runs off into streams. This form of phosphorus is 100 percent bio-available to algae…The more phosphorus, the bigger the algal blooms.”

Harmful algal bloom - credit Ohio Sea GrantA biologist with the U.S. Geological Survey discovers a hamrful algal bloom in Harford County, Maryland.

Are All Algal Blooms Toxic?

The short answer is no, but according to Reutter, 35 percent of the lakes in the United States have too much nitrogen in them, also as a result of agricultural runoff. That’s why HABs have become more pervasive.

“We’ve learned that phosphorus levels determine how big a bloom gets, whereas nitrogen levels determine how toxic it gets,” says Reutter.

There are many types of algae, and they produce numerous types of toxins. In the case of Lake Erie, the most common blue-green algae is a cyanobacteria called microcystis, which produces a toxin called microcystin. The Environmental Protection Agency (EPA) requires the level of microcystin in drinking water to be less than .3 parts per billion for children, an extremely small amount.

“Sometimes a news report will say an algal bloom wasn’t as toxic as expected, but it’s still toxic,” warns Reutter. “When the news says the bloom is not as toxic because the algae is producing 3,000 parts per billion instead of 10,000 parts per billion, it’s still very toxic. At that level, if you fall in, you can be in real trouble. If your dog jumps in, swims around and drinks the water, it might die. A dog’s liver can’t process the toxin.”

Microcystin is just one of several toxins that blue-green algae can produce, all of which are more toxic than cyanide. Given the fact that the liver is the human body’s defense against microcystin poisoning, it comes as no surprise that researchers have seen a strong correlation between HABs and an increased occurrence of liver cancer.

“We’re also seeing links to Parkinson’s and Alzheimer’s,” says Reutter. “Some of the toxins in algal blooms are neurotoxins. Others seem to cause rashes and respiratory problems. We don’t have enough data yet, but it’s highly likely that these toxins play a role.”

The challenge is reducing nitrogen and phosphorus. Glibert believes efforts to reduce phosphorus use in agriculture may decrease the size of the algal blooms, but the algae that’s still in the water then becomes more toxic. In other words, if the same amount of nitrogen is available to fewer organisms, they get a bigger dose of it and thus produce a greater concentration of toxins.

“In fresh water, the argument has been to control phosphorus to control the growth of cyanobacteria,” she says. “That works, but the problem is, the cyanobacteria that remains will be more toxic. The challenge is how to control both when we use nitrogen to grow food.”

Why Does Stormy Weather Make It Worse?

It’s a simple fact that the wetter the weather, the more nutrients wash into a waterway.

“In Lake Erie, we predict the severity of a bloom based on the amount of phosphorus in the Maumee River from March 1 to July 31 each year,” explains Reutter. “We also know that 80 to 90 percent of it comes during the 10 largest storm events of the year from agricultural runoff. If it’s not raining or only gently raining, there’s no runoff, but when there’s a hard rain, the drainage tiles may clear water quickly off the fields, but it dumps into waterways which eventually drain into Lake Erie.”

What’s more, large storm events cause lots of wave action. As those waves crash into the shoreline, the mist and spray thrusts HABs into the air, which can travel up to four miles inland. If a person breathes them in, it can cause respiratory problems.

Savia, who has modeled algal blooms for many years to help forecast their severity, also looks closely at the weather in the spring as a key element in those predictions.

“The size of the blooms in a particular year is greatly influenced by weather,” he says. “If it’s a wet spring, it’s going to be a bad summer for algal blooms. If there’s a drought, the blooms will be reduced.”

What Is the Impact of Climate Change on Algal Blooms?

A warming climate doesn’t cause algal blooms, but it certainly exacerbates the problem. Blue-green algae is thriving because of warmer water temperatures.

“For blue-green algae to bloom, the water temperature must be above 60 degrees [Fahrenheit],” explains Reutter. “If you give it nutrients in warm water, you will have a bloom. That’s why these blooms always occur during the summer.”

Moreover, warmer air can hold more water, producing more rain that washes more nutrient pollution into waterways.

“Farming practices have changed from the family farm to the large industrialization of agriculture. This started in the 1980s, so that’s not new,” says Savia. “What’s really changed is the climate. Spring precipitation has increased, which makes the flushing of agricultural lands a bigger problem.”

“Last summer was the wettest on record for the Chesapeake Bay,” adds Glibert, “Species that grow in warm water, like algae, grow well under these conditions.”

Fish kill - credit Adrian JonesA toxic algal bloom in the Chesapeake Bay during the summer of 2003 was responsible for a large-scale fish kill.

Are There Any Other Ways HABs Can Affect Us?

As mentioned earlier, there’s growing scientific evidence linking HABs to liver cancer, respiratory problems and other serious diseases like Alzheimer’s and Parkinson’s. But the most urgent issue is the impact of HABs on drinking water. When large cities, like Toledo, Ohio, have a compromised water supply, thousands of people are affected for days. Boiling water doesn’t get rid of the toxin, which causes diarrhea and vomiting. The crisis raised awareness of the issue, which can happen to any water supply that relies on above-ground sources like a lake, reservoir or river.

Algal blooms impact us in other important ways, too. The dead zones they cause kill fish, limiting where we can cast a line. For those who like to eat oysters, crabs and other seafood, algal blooms take these favorite seafoods off the menu. They also make swimming and boating potentially harmful to your health. Bottom line: these blooms are definitely not beautiful.


In freshwater, bivalves – clams and mussels – are an important part of a waterway’s natural filtration system. They suck in algae as they feed, helping to keep it under control. Unfortunately, non-native zebra and quagga mussels, which crowd out native bivalves, stop filtering when they suck in blue-green algae and spit it out. What’s more, they remove other beneficial elements from the water, like microscopic bugs that eat algae, and they excrete soluble phosphorus. They don’t cause algal blooms, but they contribute to the problem.

Oysters are the equivalent bottom filters in salt water. They used to be plentiful in the lower Chesapeake Bay, but now their numbers have declined for various reasons, including HABs. When HABs are present, oysters don’t spawn or grow.

“It’s a vicious cycle,” says Glibert. “The presence of an algal bloom hinders the recovery of the oysters, yet oysters are the natural filter that helps keep algae in check.”


Many regions of the U.S. are confronting problems with algal blooms, and the League is promoting solutions tailored to reflect the regional causes. Where the blooms result from excess nutrient runoff from farmland, there are tested solutions to reduce that polluted runoff. They include planting grass buffer strips between farm fields and streams, building healthier soils by planting winter cover crops, eliminating plowing and tilling, using more diverse crop rotations, and reducing chemical fertilizer and pesticide use. Where livestock manure is part of the problem, better management of manure from feedlots and factory farms and rotational grazing on grasslands will reduce polluted runoff.

Although these conservation practices have proven effective, most are in use by only a minority of farmers. The League is advocating for federal and state programs that educate farmers and help them adopt better conservation systems on their farms and ranches. We are also educating policymakers about the benefits of farm conservation systems.

In the Chesapeake Bay estimates are that 42 percent of the nitrogen, 55 percent of the phosphorus and 60 percent of the sediment entering the Bay comes from agricultural sources. In Congress, the League advocates for Farm Bill conservation programs that now invest more than $200 million per year in the Chesapeake Bay states of New York, Pennsylvania, Maryland, Delaware, Virginia and West Virginia to help farmers reduce polluted runoff into waterways. According to the U.S. Department of Agriculture, those conservation programs provide $6 billion nationwide to help farmers improve their conservation systems.

The League has been a leader in a coalition of groups working together to build a network of volunteers who monitor streams and gather data that is used by government agencies and others to understand the pollution problems in the watershed. The League recently added testing for nitrates and phosphorus to our Save Our Streams protocol to provide additional data.

In the Great Lakes region, farmers have been reducing their use of phosphorus fertilizer, but Lake Erie has seen an increase in phosphorus load because of the huge increase in the amount of livestock manure spread on farm fields in the region. A 2019 report by the Great Lakes Water Quality Board highlighted big differences in how the eight states surrounding the Great Lakes and the Canadian province of Ontario regulate livestock operations.

The Board recommended that the state agencies involved use Ontario’s system as a model of stronger standards. It also suggested that states adopt common minimum standards that include no spreading of manure on snow or frozen ground or during a heavy rain, no aerial spraying of manure, limits on the amount of manure spread that reflect the ability of crops to use the nutrients, and requiring permits for mid-sized animal confinement facilities, not just the largest ones. The report gives the League a recipe for promoting common-sense modernization of outdated livestock management laws at the state level.

In the Upper Mississippi River region, the League is educating policymakers about the benefits of soil health in reducing polluted runoff and advocating for state funds to help local conservation districts educate farmers about practices that will reduce polluted runoff. In 2019, the League expanded our ability to recruit and train volunteer stream monitors in Iowa and surrounding states, and we are finding innovative ways to use volunteer data to make our case for changes in state policy.

When the nutrient pollution is not reduced at the source, the costs of treating the problem downstream can be significant. A 2018 report by researchers at Iowa State University found that Iowa communities had already invested at least $1.8 million to remove nitrates from drinking water, and Des Moines Water Works is expected to spend $15 million more in the next few years to combat increasing nitrate pollution in source water. Those costs are passed on to water utility customers and costs are on the way up because nitrate levels in the state’s surface and groundwater continue to rise.

Algal blooms create problems from local lakes and ponds to our nation’s great waters, but there are solutions. The League is working to engage volunteers, educate policymakers and the public, and put in place common-sense solutions that will restore the health of our nation’s waters.


Water quality monitoring with Save Our Streams (SOS) chemical parameters is an easy way to detect potential sources of algal blooms! SOS monitoring includes tests for nitrate, phosphate and temperature, all of which can facilitate harmful algae growth or worsen a preexisting issue. Keep a close eye on your streams and rivers. Should you notice an increase in nitrate and phosphate levels, followed by algae growth on your local lakes and ponds, it is clear that your watershed (or area of land that drains to the body of water) may have excess nutrients, typically from agriculture, suburban lawns, golf courses or sewage treatment outflow. The solution to this problem lies in changing that land use, improving soil health, and increasing buffers around streams and rivers.

Learn more about how to get involved in Save Our Streams monitoring


Writer/photographer Lisa Ballard is an Ike from Red Lodge, Montana. www.LisaBallardOutdoors.com

Duane Hovorka is IWLA's agriculture program director located in Gaithersburg, Maryland.