Mayfly (Photograph courtesy of University of California at Berkeley) Lake Eries pollution in the 1960s and 1970s killed off its mayflies, insects that spend most of their lives underwater before hatching into huge flying swarms that carpet coastal towns.

But the pesky bugs have returned in a big way.

"Ive seen people out there with snow blowers, blowing them around," said Justin Chaffin, a doctoral student in biology at the University of Toledo. "If you walk down the sidewalk or a parking lot, its like youre walking on bubble wrap."

Cleaner water is responsible for the mayfly resurgence. But in a weird twist, the thriving insects may be a new cause of old environmental problems.

Before mayflies hatch , they live a wingless life burrowed in lake sediment. Scientists suspect those burrows might contribute to a resurgence of low oxygen in the water, a problem they once thought was gone for good, according to two new studies in the Journal of Great Lakes Research.

Heres how:

In the 1960s, phosphorus from farm fields, city streets, and wastewater treatment plants fueled massive algae blooms in Lake Erie. When those blooms died, the bacteria that ate them sucked oxygen out of the water.

Without that oxygen, fish and other aquatic animals like mayflies died. Fish kills fouled beaches and mayflies all but disappeared.

Birds, salamanders, mudpuppies, bluegill, smelt, and bass feed on mayflies, according to Michigan State Universitys Environmental Literacy Project. And Michigan State Extension says the large number that hatch provide "a buffet for fish and can be a problem for people in areas near water, where the carcasses of mayflies drawn to lights can pile up on the ground and make roads and bridges slippery."

The Department of Natural Resources has listed mayflies among the "species of greatest conservation need" in the Lake Huron, Lake Michigan, Lake Superior, and Lake Erie basins.

In the 1970s and 1980s, the U.S. and Canada spent $8 billion to control phosphorus, spurring the recovery of Lake Erie. By 1997, as many mayflies were burrowed in the lakes western basin as there had been before the species collapsed.

But now Lake Eries algae blooms and low oxygen are back, and scientists arent sure why. More phosphorus isnt going into the lake, yet the concentration of phosphorus in the water has increased.

"The lake that died was better, but now the harmful algae blooms are back to where they were before phosphorus control," said biologist William Edwards of Niagara University in New York.

Since more phosphorus doesnt seem to be coming from outside, Edwards and his colleagues studied the lake bottom, where as many as 1,500 mayflies can burrow in a square yard of sediment.

Although fish and aquatic animals breathe some of the oxygen dissolved in water, the mud at the bottom of the lake sponges up most of the oxygen, Edwards said.

"The use of oxygen in a lake is really determined by how much is going into the sediment," he said "Thats really the main sink of oxygen."

Edwards team filled containers with Lake Erie mud and water, dropping mayfly nymphs into some of them. The sediment with mayfly burrows consistently sucked up more oxygen than sediment without them.

Its not that the insects consume a lot of oxygen, but that their burrows help more of it go into the sediment.

If mayflies act the same way in Lake Erie as in the laboratory, their tube-shaped burrows likely turn the lake bottom into a porous oxygen sponge, Edwards said.

"If you put a tube through the sediment, you increase the amount of surface area thats available for the sediment to use up oxygen," he said.

And thats not all.

In a similar experiment, Chaffin found that a burrowing mayfly can kick up buried phosphorus. Once that phosphorus is back in the water, it can fuel more algae blooms.

"There is an effect," Chaffin said. "I dont know if its just a drop in a bucket or if it is a main reason why wed be seeing these blooms come back since mayflies have come back."

Even if the return of the mayflies has contributed to the resurgence of algae blooms and low oxygen, its not a sign that Lake Erie managers need to kick the bugs back out.

"Its not necessarily the mayflies fault that theres so much phosphorus in the sediment," Chaffin said. "The mayflies are going to do their thing if theres a lot of phosphorus or not."

But it is a sign of the complexity of environmental problems and solutions.

As scientists struggle to find the causes of low oxygen and the sources of phosphorus, they must consider the effects of animals on the lake, not just the effect of the lake on animals, Edwards said.

"What were arguing is that these things need to be considered when were planning for the health of the lake," he said. "We cant discount this and expect to get the results that we want in managing the lake."

Mayflies are sensitive to changes in water quality, so an abundance of mayflies is a symptom of a healthy lake, Edwards said.

"I grew up on the shores of Lake Erie, so I remember there being no mayflies," he said. "So every time Im wiping mayfly guts off my feet, I dont get too upset about it."