On 3 August 2014, residents of Toledo, Ohio, woke to the news that overnight their water supply had become toxic. They were advised not only to avoid drinking the water, but also touching it – no showers, no baths, not even hand-washing.

Boiling the water would only increase its toxicity while drinking it could cause “abnormal liver function, diarrhoea, vomiting, nausea, numbness or dizziness”, read a statement from the City of Toledo, warning residents to “seek medical attention if you feel you have been exposed”.

Toledo sits on the shores of Lake Erie, one of North America’s five great lakes. About half a million residents of the city and surrounding area have relied on Lake Erie for water for hundreds of years.

After the news broke on 3 August bottled water quickly vanished in concentric circles around the city. Eventually, a state of emergency was called and the national guard arrived with drinking water.

Toledo’s water crisis lasted for nearly three days. But the water wasn’t toxic due to an oil spill or high lead levels, as in Flint, Michigan. Toledo’s water was tainted by something altogether different: an algae bloom.



Toledo is not alone. According to scientists, algae blooms are becoming more frequent and more toxic worldwide.



A 14-month long algae bloom in Florida, known as the “red tide”, only ended earlier this year, after killing more than 100 manatees, 127 dolphins and 589 sea turtles. Hundreds of tonnes of dead fish also washed ashore.



In 2018, there were more than 300 reported incidents of toxic or harmful algae blooms around the world. This year about 130 have been listed on an international database, but that number is expected to increase.



Recent reports of a new ‘‘red tide’’ emerging in Florida and more dead wildlife have put the tourist and fishing industries on alert, braced for further devastation.



The causes of the blooms vary, and in some cases are never known, but in many parts of the world they are being increasingly linked to climate change and industrialised agriculture.

What is a harmful algae bloom? Facebook Twitter Pinterest A dead mayfly in a swamp with cyanobacteria on the water surface. Algae blooms may be a danger to humans and animals. Photograph: abadonian/Getty Images/iStockphoto

Algae includes everything from micro-algae, like microscopic diatoms, to very large algae, such as seaweed and kelp. Algae are not officially a taxonomic group of creatures (they don’t fit into general groups like plants, animals or fungi), but the name is generally used to describe marine or freshwater species that depend on photosynthesis.

An algae bloom occurs when a single member of these species – because of certain conditions – suddenly becomes dominant for a time.

Algae are vital to our survival. It’s estimated that at least half of the planet’s oxygen comes from these unsung creatures, who produce it through photosynthesis before releasing it into the water. Algae, like land plants, also sequester carbon dioxide; scientists have explored their potential to draw carbon dioxide out of the atmosphere. They have been used as fertiliser, food sources (such as seaweed), and could be a promising source of biofuel in a more sustainable world.

However, some algae blooms can also be harmful – even lethal.

Harmful algae bloom (HAB), as scientists have come to describe the phenomenon, often manifest by forming a kind of scum over a body of water that can be green, blue, brown or even red. But others are completely invisible. The problem has become increasingly widespread and the impact can be deadly to marine life.

Off the eastern coast of the US, a dinoflagellate – a type of marine plankton named Alexandrium catenella – has the potential to make shellfish lethal. Its appearance routinely shuts down fisheries, crippling local economies. And it’s not just in the US: the same species has shut down mussel farms and recreational collecting of shellfish as far away as New Zealand.

Facebook Twitter Pinterest Signs on the Lake Erie beach warn about the algae in August 2014.

Photograph: Joshua Lott/Reuters

Other blooms wipe out marine life. In 2015, a bloom of various dinoflagellates off the coast of South Africa led to low-oxygen conditions, known as eutrophication, killing 200 tonnes of rock lobster. Freshwater blooms, like those in Lake Erie made up of cyanobacteria or blue-green algae, have not only shut down local water sources but have also been blamed for the death of dogs that had been swimming in them.

It’s difficult to make generalisations about harmful algae blooms since specific species have different causes and impacts. Scientists have identified about 100 toxic bloom species in the oceans. Dozens of potentially harmful species of cyanobacteria are known to affect bodies of fresh water.

During most of the past century, harmful algae blooms were rarely headline news, inspiring little scientific study beyond ecological curiosity. That has changed. Algae blooms are notoriously difficult to predict, but a global monitoring group known as HAEDAT is tracking them across the world as they occur. Harmful algae blooms, such as the one that hit Toledo’s water supply in 2014, are becoming more common and more toxic – and scientists say humans are to blame.

“There’s no question that the HAB problem is a major global issue, and it is growing,” said Donald Anderson, director of the US National Office for Harmful Algal Blooms and a lab director at the Woods Hole Oceanographic Institute. “We also have more toxins, more toxic species, more areas and resources affected, and higher economic losses.”

Hidden cost of Ohio’s corn and soya bean boom Facebook Twitter Pinterest A Nasa image from August 2015 showing the green swirls of algae blooms in Lake St Clair and Lake Erie. Photograph: OLI/MODIS/Nasa

The toxic bloom that took over Lake Erie in 2014 was formed by a cyanobacteria known as Microcystis Aeruginosa, for which farming is at least partly to blame.

“You have people that still to this day will only use bottled water,” says Dr Timothy Davis, an expert in plankton ecology at Bowling Green University, five years after the water crisis and even after Toledo spent $132 million (£101 million) on improving its water treatment plant to handle the blue-green algae.

Lake Erie, the shallowest of North America’s Great Lakes, has seen such events in the past. During the 1950s and 60s algae blooms were common, most likely, say researchers, due to poor domestic and industrial wastewater treatment.

“At one point, Lake Erie was considered a dead lake,” Davis says. But by the early 1970s, the “dead lake” was resurrected, due to new regulations from the Clean Water Act and the Great Lakes Water Quality Agreement that capped phosphorus loads into the lake at 11,000 tonnes. Phosphorus provides nutrients to plants and is commonly found in manure and produced for fertiliser.

Then in the late-1990s, blooms began to reappear. A cyanobacteria bloom requires two things: nutrients and heat. In the case of Lake Erie, nearby farms have become increasingly reliant on large inputs of synthetic fertiliser.

“We went from agriculture that was small farms [and a] variety of crops to larger commercial farms that were harvested for essentially two row crops, corn and soya beans,” says Davis. Today, corn and soya beans are Ohio’s top crops.

Facebook Twitter Pinterest Water collected from Lake Erie in August 2014. Photograph: Aaron P Bernstein/Getty Images

Employing more fertiliser to feed a global market, the farms’ excess phosphorus and nitrogen, another plant nutrient, washed out during storms and into the river and streams that feed Lake Erie. About 80% of the nutrients running into Lake Erie are from sources around the Maumee River, which in this case means agricultural runoff from the surrounding farmland.

“If you have an agricultural system where the farmer can only survive by polluting Lake Erie, then there’s something fundamentally wrong with that system,” says Dr Thomas Bridgeman, director of the Lake Erie Center.

Since the 1990s, Lake Erie has seen a bloom every year – and they appear to be lasting longer and getting larger. This year’s bloom in Lake Erie was the fifth largest since 2002 – when monitoring began in earnest. It was 620 square miles at its largest after growing throughout August, before dissipating in September.

Meanwhile, climate change has heated up our planet substantially. Nearby Lake Superior, the most northerly of the Great Lakes and the world’s largest, has had its first documented cyanobacteria blooms over the past decade. Before climate change, the lake simply would have been too cold for a long-lasting bloom.

Facebook Twitter Pinterest The Lake Erie algae bloom in August 2019.

Photograph: Zachary Haslick/Aerial Associates Photography/NOAA

It’s now almost a certainty that blooms will continue to appear every summer, say researchers, unless Ohio changes its agricultural practices and the global community finally tackles the climate crisis.

“We have to look around and say, ‘Look, what do we grow here?’” says Bridgeman. “We grow corn and soya beans. Where does the corn go? It goes into our gas tanks. Where do the soya beans go? They go to China, they go to hogs. Is that really what we want to be doing with our watershed?”

Dead manatees and sick sea lions Facebook Twitter Pinterest A sick manatee is treated at Lowry Park Zoo Manatee Hospital in Tampa, Florida.

Photograph: Skip O’Rourke/Tampa Bay Times/ZUMA/Alamy

Algal blooms are also becoming more common and severe in many parts of our oceans, harming wildlife and posing potentially dangerous health impacts for local communities.

Scientists say the “red tide” that stuck around the Florida coast from 2017 through to this year may now be a semi-normal part of the ecosystem.

These blooms are pumping poison into the air, known as brevetoxin, which may be harmful to humans if inhaled. Anyone breathing it in can suffer from uncontrollable coughing and a sore throat. “It doesn’t make for a pleasant day at the beach,” says Malcolm McFarland, a researcher into algae blooms with the Harbor Branch Oceanographic Institute in Fort Pierce, Florida.

It may have long-term health implications as well – one study found that brevetoxin attacked the DNA of lungs in rats, but further research is needed to understand the impact on human health.

Scientists are less certain about the causes of these red tide marine blooms, but both nutrient runoff and climate change may play a role. “The red tide seems to initiate and peak in the rainy season when runoff from the land is highest, and nutrient inputs to freshwater and coastal water bodies spike,” says McFarland.

Facebook Twitter Pinterest A pregnant California sea lion poisoned by domoic acid found in blooming algae in Santa Monica in 2011.

Photograph: Jonathan Alcorn/Zuma/Alamy

Meanwhile, on the other side of the north American continent, a different red tide is attacking a different species: California is seeing more sick sea lions taken in by rescue centres; pups and adults are dying.

Scientists believe they are suffering from eating fish tainted by Pseudo-nitzschia australis algae. The highly toxic algae are fatal at high doses, both to sea lions and humans.

Unlike Florida’s red tides, those in California appear to be a very recent arrival. Until the turn of the millennium, large-scale toxic blooms were rare off the coast of California. Then something changed.

“From [2000] forward, we had a very significant bloom every single year with ecosystem impacts in California, and that has never stopped. Not only that, it seemed as though things were getting more and more toxic,” says Clarissa Anderson, the executive director at Southern California Coastal Ocean Observing System, nothing that in less than 20 years of research, scientists have seen toxin numbers multiply by 200 – from 500 to 100,000 nanograms per litre of sea water.



Anderson says the current best working theory is that increasing carbon sequestration by the oceans – due to the huge increase in greenhouse gas emissions since the industrial revolution – is behind the sudden regularity of these deadly blooms and an increase in their toxicity. She says the study of these events and their toxins is so new that there may be incidents of illness from eating affected fish or shellfish that are misdiagnosed because these poisons are not on the radar of many health organisations.

The Baltic’s dead zones Facebook Twitter Pinterest Blue-green algal blooms in the Baltic Sea in 2018. Photograph: ESA/Finnish Environment Institute

Europe has had its own experience of deadly algae blooms that now threaten the future of its fisheries. Last year, the Baltic Sea experienced a bloom so large it could have encompassed Manhattan, and it closed beaches from Finland to Poland.



Finland has been systematically sampling its area of the Baltic since 1979, giving us a clearer idea of the spread and growth of the problem, and what’s to blame. In that time, blooms have become larger and longer-lasting, creating dead zones and depleting Baltic fisheries.

Like the example in Lake Erie, the Baltic bloom is caused by an influx of nutrients from agriculture and warming waters.



Scientists are regularly tracking nutrient loads from Finland’s rivers into the sea. Data from 2014 in the HELCOM Pollution Load Compilation database, the best currently available, found that more than three-quarters of the nutrient load coming into the Archipelago Sea is from agriculture. The number is surprisingly similar to the proportion coming from industrialised agriculture in Ohio.

The Baltic is a brackish water body, thus supporting blooms typical of both fresh and salt water. But, as in Lake Erie, of real concern are cyanobacteria: several species have been known to produce blooms here.

Below the sea’s surface there has been a decline in the more nutritious phytoplankton – and food for fish – and an increase in the potentially toxic species in the more southerly parts of the sea since the early 1980s.



Milder winters and increased rainfall pushing more nutrients into the sea, along with higher surface water temperatures – all due to the climate crisis – are also exacerbating factors, say researchers. Blooms usually begin in July and disappear by August or September. But last year a species particularly resistant to coldremained until November.



“The ice was blue-green because of the cyanobacteria under it,” says Sirpa Lehtinen, an expert on plankton for the Marine Research Centre with the Finnish Environmental Institute, who adds that scientists are still trying to work out what this all means for the marine ecosystem, and whether fisheries in the Baltic are in serious long-term peril.

Fixing an ecosystem out of balance Facebook Twitter Pinterest Scientists conducting research into Lake Erie’s harmful algal bloom. Photograph: David J Ruck/Great Lakes Outreach Media/NOAA

So how can we solve a problem like algae? The answer, says Davis, will be part-regional, part-global solutions. For Lake Erie, it will require agricultural changes – including regulations to reduce the nutrient load – and tackling the climate crisis. But solutions elsewhere may be different, for example, blooms in developing countries might require better wastewater treatment.

The 2014 water crisis in Ohio forced the issue politically which hasn’t happened in many other places. Governor Mike DeWine recently announced an initiative called H2Ohio, which is expected to include hundreds of millions of dollars for Lake Erie and other Ohio water bodies over the next 10 years. However, scientists say this is not enough.

“It’s going to take a lot more money and a lot more political will than what’s happening right now,” says Bridgeman.

At the Ohio Department of Agriculture,director Dorothy Pelanda said the department was primarily looking at voluntary programmes based on marketing and education for potential solutions such as cover crops and smarter use of fertiliser. In 2014, Ohio passed new regulations on fertiliser use for farms near the lake: such as not spreading before a storm or on frozen fields.

“We know from science that there is not one solution to every farm … It’s about education, it’s about being sensitive to what works, what doesn’t work,” she said. She’s hoping to provide increased access for farmers to use high-tech, but often expensive, equipment that can give them a better idea of what parts of their land may need fertiliser and how much.

Pelanda said she’s also seen interest in diversifying crops beyond corn and soya beans, to grapes, chestnuts and maple sugar. Asked if voluntary programmes will go far enough, Pelanda says: “That’s our challenge. We have to get beyond. We’re doing these things … but we’re not doing enough of these things. We need to really increase the voluntary adoption of these practices.”

Others are more sceptical of voluntary approaches. “We have a long history in this country of a farmer does what he wants on his land. You can choose to take advantage of a programme or something, but you can also choose not to,” says Bridgeman, who believes local and federal governments can no longer afford to ignore the climate emergency.



“We need to do something about climate change and we’re either going to be paying for it by reducing greenhouse gases or we’re going to be paying for it by additional treatment of water,” says Bridgeman, adding that most of the blooms around the world have a human element to them.

One thing is certain. Algae blooms aren’t going away but are yet another sign – like ocean acidification, vanishing Arctic sea ice, and mass extinction of the Anthropocene – “of an ecosystem that is out of balance,” says McFarland.

• This article was amended on 8 January 2020 to remove a reference to “toxic cryptophytes”. Cryptophytes are not toxic. Also, the text was changed to clarify that baltic bloom in Lake Erie is caused by a range of nutrients, not just nitrogen from farming.







