Blue-Green Algae Is Filling Rivers With Toxic Sludge

Harmful algal blooms are taking over as the world warms and grows richer in carbon dioxide—and there’s no easy fix.

Standing on the marina, Rob Skelly peers into the darkness of the river where bright speckles of algae drift in the water. A neon green invader. “It’s starting to build,” he says. “Tomorrow, you’ll find that there’s clumps like that all over the river—and then the day after that there’ll be more and more.”

Until this summer, Skelly had never seen algae wax and wane like this in the River Bann, a major waterway in Northern Ireland. The owner of the Cranagh Activity Centre set up his thriving water sports business 27 years ago, and it has been in this location since 2015. The algae has killed it. Following news reports of toxin-producing blue-green algae in lakes and rivers around Northern Ireland, people began canceling their bookings for water-skiing lessons and similar activities in droves.

Skelly doesn’t blame them. “How can I put customers into that?” he says, looking at the mottled water below us. The season ruined, Skelly has decided to close his business for good. “You know, it’s heartbreaking.”

Blue-green algae is coming to a river or lake near you, almost without doubt. The scourge of toxic blooms is becoming increasingly problematic worldwide, in part due to the climate crisis. Despite the name, blue-green algae aren’t actually algae, but a group of photosynthesizing bacteria called cyanobacteria. Under the right conditions, they can suddenly proliferate across huge expanses of water, leaving characteristic grayish-blue marks at the edges of lakes or rivers. Often, a highly unpleasant, rich, drain-like smell pervades in affected areas.

These microbes sometimes produce huge quantities of toxins—cyanotoxins—which can cause diarrhea, vomiting, breathing difficulties, and occasionally even death in humans. Outbreaks have been associated with pet and livestock fatalities. People in the United States living near lakes where cyanobacteria regularly bloom have a higher risk of liver cancer, and some research suggests that cyanotoxins might even cause motor neurone disease, though further investigations are warranted to prove that particular connection. Research suggests that cyanotoxins can likely be aerosolized and breathed in when water is kicked up during recreational activities or fishing.

Genetic analysis hints that cyanobacteria have been around for roughly 3 billion years. While the dumping of sewage and nutrient runoff from farms have long been known to swell the bacteria’s ranks in bodies of water, the cyanobacteria seem to be really flourishing now as global temperatures and atmospheric levels of CO2 rise.

“That, I think, is the really compelling evidence for the link to climate change—we’re seeing these increases in places where there hasn’t been a really substantial increase in urbanization or fertilizer application,” says Hans Paerl at the University of North Carolina-Chapel Hill’s Institute of Marine Sciences. “It is a global problem.”

Cyanobacteria are proving to be a menace practically everywhere—from Florida to Africa and China, to name a few examples. In China’s Lake Taihu, the blooms are so bad that authorities have battled for years to physically remove the sludge with special machines that chew it up using hundreds of tiny teeth.

This year in Northern Ireland, some of the most serious blooms have occurred in Lough Neagh, the largest body of fresh water by surface area in the UK and Ireland. Some locals have described algal blooms on the lough as the worst they have seen in their lifetimes, and there have been reports of multiple dog deaths possibly caused by cyanotoxins. From Lough Neagh, water flows into the River Bann and heads north toward the town of Coleraine, where Rob Skelly’s water sports business was located until recently. Finally, the Bann enters the sea on the north coast of Northern Ireland. Warnings about blue-green algae were put up on beaches there earlier this summer.

WIRED showed Paerl pictures of a blueish residue above the waterline at a jetty very near to Lough Neagh. “It’s an indication of very high amounts of material,” he says.

Around 40 percent of all Northern Ireland’s drinking water is sourced from Lough Neagh. NI Water, the public body responsible for drinking water, says it uses methods known to remove cyanotoxins. Chlorination alone is not enough, notes Paerl. In 2007, a blue-green algal bloom at Lake Taihu in China was so severe that 2 million people were forced to go without drinking water for at least a week.

A spokeswoman for NI Water says that drinking water is treated using granular activated carbon, a kind of filtration that removes certain chemicals, including cyanotoxins. Tests for one particular cyanotoxin, microcystin-LR, in drinking water post-treatment have consistently shown extremely low levels throughout 2023, well below World Health Organization guidelines, she adds.

However, NI Water does not test for cyanotoxins in the source water. “To the best of my knowledge, no one has yet tested for toxins either in water or fish,” says Matt Service at Northern Ireland’s Agri-Food and Biosciences Institute. Some local scientists are concerned that our understanding of how abundant these toxins are in places like Lough Neagh remains very murky.

“I was interested in whether I could get some funding to specifically study the toxicology of the blue-green algae,” says Neil Reid, a senior lecturer in conservation biology at Queen’s University Belfast. He has collected multiple samples of surface water but hasn’t yet been able to secure the funding needed to conduct research on them.

Reid points out that quite a lot of the visible sludge could be a harmless species of algae and not the dreaded cyanobacteria. It would help local people understand the risk when fishing on the lough, for example, if they knew more about its toxicity, he suggests. But, for now, the samples will remain frozen in a laboratory freezer.

Besides nutrients entering lakes and rivers, which can spur the proliferation of algae and cyanobacteria, there are other factors that can trigger major blooms. Northern Ireland just had its wettest July on record—potentially accelerating the runoff of nutrients into bodies of water including Lough Neagh, says Reid. The lough is also 1 degree Celsius warmer today than it was just 30 years ago. That could benefit cyanobacteria over competing species, including algae, says Don Anderson, a senior scientist in the biology department at Woods Hole Oceanographic Institution in Massachusetts.

“When it gets too hot, other species don’t grow, or grow slowly,” he explains. “Cyanobacteria are extraordinarily flexible in terms of their tolerance.”

Then there’s the zebra mussels. These invasive mollusks have been resident in Lough Neagh since at least 2005. Here, as in other lakes in Europe and the United States, they appear to have consumed large quantities of algae, clarifying the water in the process. That might sound good, but the problem, Reid explains, is that this then allows more light into the lake, potentially giving the cyanobacteria a chance to thrive while their competitors get gobbled up by the mussels.

“I think it’s a very reasonable hypothesis,” says Robin Rohwer at the University of Texas at Austin, who has studied the prevalence of cyanotoxins in Lake Mendota in Wisconsin. Data collected across two decades suggests that, following zebra mussel invasion, the “toxic season” on the lake during the summer lengthened dramatically—lasting more than 50 days longer, on average. There are plenty of mysteries, though. Rohwer says she didn’t detect a boom in the cyanobacteria itself, just an increase in the volume of toxins present in the lake. What’s driving that remains unclear.

Rohwer adds that, as someone who enjoys sailing on the lake herself, she avoids boating whenever algae buildup is visible. In unpublished results, she says she has found that toxin levels out in the middle of the lake aren’t usually a serious concern—though she has detected “extremely toxic” scum washed up at the shoreline.

There’s little that humans can do to stymie blue-green algal blooms, says Paerl. And Rohwer notes that it is practically impossible to eradicate zebra mussels once they have become established. The only tactic available, really, is reducing nutrient runoff into lakes and rivers, for example by lowering fertilizer use on farms and building buffer zones or artificial wetlands around the edges of large bodies of water to try to soak up the nutrients. Paerl says such efforts have been reasonably successful in North Carolina, for example.

For Rob Skelly, the damage, sadly, is already done. He says he has spent recent months chasing public bodies over the cyanobacteria problem. “Nobody will take responsibility,” he alleges, adding that the sudden closure of his business feels like the end of an era. Many former customers have been in touch, he says, to express their regret at what has happened.

“I have loved every day of my working life because I’ve had the river. It’s just been part of my DNA,” adds Skelly. “I never thought it would be the river that would come back and bite me.”

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