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“Interwoven tapestry” of lakes and land: Iceland

Swarms of midges rise out of a lake in northern Iceland in such enormous numbers every spring and summer that they can impair breathing and darken the sky, giving the lake its name—Myvatn, or “midge lake.”

CALS entomology professor Claudio Gratton and other ecologists are trying to understand why the midge population can fluctuate by 100,000-fold across a decade, and what impact these massive swarms have on the surrounding landscape. It’s becoming clear that the billions of midges falling on land fertilize and alter the vegetation on the lakeside, but the causes behind such large fluctuations in the insects’ population remain a mystery.

Gratton’s research aims to better understand lake-dominated environments, including those of Wisconsin.

Lake Myvatn sits at the edge of the Arctic Circle, where the sun barely sets from May to August. The ecosystem is extreme yet simple; a relatively small number of species, like the midges, dominate. This bare-bones environment is perfect for exploring complex interactions within ecosystems.

In 2006, when Gratton first saw the huge numbers of midges rising out of the lake and dying on land, he thought of them as a living transfer of nutrients from water to shore. Gratton calculated that the midges were the nutritional equivalent of scattering a half-million Big Macs around the edge of the lake, which is about the size of Lake Mendota in Madison. He wondered how the lakeside responded to this nutritional glut.

To test how the midges alter the landscape, Gratton’s laboratory set up experimental plots in the vegetation around the lake. In some, they added dead midges; in others, they used netting to exclude them.

Over the years, Gratton’s team saw that where they added midges, grasses flourished. Normally starved of nutrients in the poor soil and outcompeted by heartier plants, the grasses took off in response to the influx of rotting- midge fertilizer. The research explained why grass grew in some areas and withered in others.

“Only by understanding the linkage between midges and grass can you explain this pattern in nature,” says Gratton. “The lake is causing that to happen.” Gratton was originally introduced to Lake Myvatn by colleague Tony Ives, a professor of zoology who has a lifelong connection to the island and researches fluctuations in the midge population.

Local shepherds have long called the grass in midge-infested areas “midge grass”—they
harvest the grass and feed it to their flocks. Gratton’s work suggests that the shepherds’
folklore contained a kernel of truth, and that midges might indirectly nourish the sheep by encouraging more grass growth.

Gratton and colleagues are extending these studies to the lake-filled Wisconsin landscape. Gratton and postdoctoral researcher Mireia Bartrons, now at the University of Vic in Spain, developed a model of how insect emergencies from Wisconsin lakes affect lakeside ecosystems. With more than 15,000 lakes and 34 percent of the state lying within 200 meters of a lake or stream, the scientists expect aquatic insects to affect a large share of the state.

Gratton sees ecosystems, whether in Iceland or the American Midwest, as an interwoven tapestry of interactions rather than isolated patches of land or water.

“The character of the land would change without these lakes,” says Gratton. “Our landscapes are completely interconnected.”