Overlooking the lava spires of Lake Mývatn, inside an old white farmhouse, in a jar by the window, slumped in a cloudy green bath, Iceland’s last living lake ball is slipping away.

The fluffy green globes—supersize diatoms as large as a head of cabbage—are one of the planet’s most unusual plants. Vast colonies once teemed in this lake renowned for its dramatic aquascape and abundant wildlife. “Lake balls” is their polite name; in Icelandic, they’re called “Kúluskítur,” a colorful epithet uttered by fishermen when the balls got tangled in their nets.

This distinctive form of the freshwater alga Aegagropila linnaei is exceedingly rare. Mývatn, one of Europe’s most important waterbird habitats, used to host the largest lake ball colonies on Earth. This northern lake had all the right environmental factors: swirling currents to nudge the clusters so they grow outward in perfect, round shapes; consistent winds to stir the currents; a stable bottom for support; and most of all, clean, translucent water.

But like so many lakes worldwide these days, thick blooms of surface scum—the potentially toxic cyanobacteria, or blue-green algae—are increasingly turning Mývatn’s water opaque.

Beneath the blanket of slime, the lake balls are starved for sunlight. The chief culprit behind the algae blooms is pollution—fertilizers and sewage that feed them with nitrogen and phosphorus. Warming waters also may be amping up algae. Scientists predict climate change will make future outbreaks worse. (Read more about runaway growth of toxic blooms in ocean waters.)

Healthy, full-grown lake balls haven’t been seen here since 2013. Their disappearance signals an ecosystem collapse threatening one of the world’s most important breeding grounds for ducks.

“All of a sudden they were gone,” says Árni Einarsson, director of the Mývatn Research Station, who has been monitoring the balls for nearly four decades. Surface blooms had made the water unusually turbid for a number of years, so a diver went down to check on the colonies. “He could only find some dead or decayed. They were not healthy at all.”

A National Treasure

Mývatn, one of Iceland’s top tourist attractions, appears to be a victim of its own success. The number of visitors has nearly quintupled over the past two decades, according to local hotel owner Yngvi Ragnar Kristjánsson. Outdated septic systems haven’t been able to keep up with the growing load.

The soft, friendly-looking orbs had become practically a national pet for Iceland. Only one other lake—Akan, on Hokkaido, Japan—has all the right conditions for the balls to reach as large as six inches across. News of the balls’ demise hit folks in Iceland hard.

View Images The large balls of diatoms that once grew in Lake Mývatn have mostly vanished. They have been starved of light by slimy algae blooms fed by pollution. Photograph by Arctic Images, Alamy

“There was a huge public outcry,” says Einarsson, looking out at the misty lake from the old farmhouse, now converted to laboratories. “People were really sad.”

And while a finding of some pebble-size balls on the lakeshore early this summer sparked Icelanders’ elation, the biologist says the long-term outlook remains bleak. “It was a nice surprise,” he says. “But we shouldn’t make so much of it. They were just tiny things; they weren’t even real lake balls.”

For Einarsson, the saga of Mývatn’s lake balls has a personal significance. As a young man at the start of his career, he was the first to discover the immense colonies and bring them to international attention.

“What I saw on this fine day in 1978 when I leaned over the gunwale and looked through the viewer was just mind-blowing,” he writes in a nostalgic endnote to a scientific paper on the lake balls’ disappearance. “A huge field of dark green lake balls, like all the tennis balls in the world had gathered for their annual meeting.”

At the time, no one in Iceland knew much about the odd organisms, which had never before been seen in such numbers. He posted a crude drawing of one on the research station’s then primitive website. To his surprise, he was contacted by a Japanese scientist who had made Lake Akan’s lake balls—called marimo there—his life’s work. The two have collaborated ever since.

A Canary in the Duck Factory

While the lake balls themselves have relatively little importance to wildlife, their disappearance sounds an alarm for this critical habitat’s health. Known as the "Duck Factory,” the lake has among the greatest densities of breeding ducks in the world. Waterfowl of all types are abundant, as well as a wide variety of other birds. And Mývatn’s fish have fed the region since the first Norse settlers in the ninth century A.D.

Key to Mývatn’s productivity is its huge population of midges—the small flies that give the lake its name (mý means flies and vatn means lake in Icelandic). In peak years, the amount of midges that emerge every summer equals the biomass of roughly 10 humpback whales, says Anthony Ives, a University of Wisconsin, Madison ecologist who studies Mývatn midges.

The midges feed the fish and govern the lifecycle of the birds. But when the midge population is low, fish die and birds don’t reproduce.

“You have these major changes in the whole ecosystem driven by these midges,” Ives says.

Historically, midges and cyanobacteria waxed and waned in regular, natural cycles. When surface blooms were high, stifling bottom plants that feed midge larvae, fly populations tended to be low. The cycles generally reversed once or twice a decade. But over the past 13 or so years, Einarsson and his researchers saw the blooms growing longer and more intense. During the last two summers, they were so thick the water looked “like vegetable soup,” he says. “With cream.”

Such heavy, persistent blooms give the lake little chance to clear, so the cycle doesn’t reset like it used to. Bust years are becoming more extreme. The lake balls’ departure is the first sign of potentially catastrophic ecosystem-wide impacts.

View Images Known as the "Duck Factory,” Mývatn Lake has one of the greatest densities of breeding ducks in the world. This tufted duck and her ducklings are among its inhabitants. Photograph by Wolfgang Kaehle, LightRocket/Getty

“They’re the canary in the coal mine,” says Einarsson. “If they disappear, there is something wrong. And they have disappeared.” Not only the lake balls, he says, but most of the algal mat at the foundation of the lake food web—all victims of the encroaching scum.

Longtime residents say recent cyanobacteria blooms are far worse than anything in the past.

“It looks like green paint,” says Hjördís Finnbogadóttir, a high school teacher and naturalist who grew up near the lake. “Last year is the most I have seen, and the most my father’s ever seen. And he is in his 87th year.”

Sewage seeping into the lake from overloaded hotel septic tanks is one of the most obvious sources of nutrients feeding the blooms. Concealed pipes deliver an extra dose into the groundwater, and the porous lava bedrock holds nothing back. Cyanobacteria feast on the pollution, triggering a flush of scum over the lake that smothers the plants below.

Behind one of the large hotels along the shore, Einarsson tromps through a lush field of buttercups. The smell of sewage fills the air. He points to a dark, cloudy slick bubbling through the meadow, oozing into the lake. It trails from a leaking underground septic tank.

The Rest Is Up to Nature

A commission of scientists, farmers, and local leaders organized to study Mývatn’s problems and solutions recently issued a report calling for a new sewage treatment plant and improvements to existing facilities. The Icelandic government is considering the request.

As with lakes around the globe, farms, ranches, construction, and other human activities also add to Mývatn’s nutrient overload. The commission has recommended tighter regulations on agricultural runoff and fertilizer use.

“I hope it has some effect,” says Finnbogadóttir, a commission member. “But this is all taking a very long time. If we try from now on to do everything we can, I should hope we would see some recovery in 10 or 20 years, probably.”

Sewage, however, isn’t the only force driving Mývatn’s harmful blooms.

The lake’s troubles may have begun with a diatom mine that operated from the late 1960s until 2004. Einarsson and others believe the dredging for an ingredient that was used in beer destabilized the lake bed and set off a cascade of changes.

Shifting weather patterns may also play a role. And above it all lurks the unknown impacts of climate change.

Nonetheless, local leaders say, the community’s pollutants are the only factor they can control.

As hotel owner Kristjánsson, chairman of the village council, puts it, “The rest is up to nature.”

Back at the old farmhouse, Einarsson reaches into the jar and picks up the forlorn-looking clump. He’s surprised to find it has shrunken to the size of a baseball.