By Jeff Gillies
Dec. 3, 2009
Lake Erie’s pollution in the ’60s and ’70s killed off its mayflies, insects that spend most of their lives underwater before flying off in huge hatches that carpet coastal towns.
But the bugs have returned in a big way.
“I’ve seen people out there with snow blowers, blowing them around,” said Justin Chaffin, a doctoral student in the University of Toledo’s biology department. “If you walk down the sidewalk or a parking lot it’s like you’re walking on bubble wrap.”
Cleaner water is responsible for the resurgence. But in a weird twist, the thriving insects may be a new cause of old environmental problems.
Before mayflies turn into flying bags of bug guts, they live a wingless life burrowed in lake sediment. Scientists now 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 published in the Journal of Great Lakes Research.
In the 1960s, phosphorus from farm fields, city streets and wastewater treatment plants fueled massive algae blooms in Lake Erie. Those blooms would die and 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.
In the 1970s and 1980s, the U.S. and Canada spent $8 billion to control phosphorus, spurring Lake Erie into recovery. By 1997, as many mayflies were burrowed in the lake’s western basin as there were before the species collapsed.
But now Lake Erie’s algae blooms and low oxygen are back, and scientists aren’t sure why. More phosphorus isn’t 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 William Edwards, an associate professor of biology at Niagara University in New York.
Since more phosphorus doesn’t seem to be coming from outside, Edwards and his colleagues turned to the lake bottom, where as many as 1,500 mayflies can burrow in a square yard of sediment.
Though fish and aquatic animals breathe some of the oxygen dissolved in water, most is sponged up by the mud at the bottom of the lake, Edwards said.
“The use of oxygen in a lake is really determined by how much is going into the sediment,” he said “That’s 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.
It’s not that the insects use up 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 that they do 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 that’s available for the sediment to use up oxygen,” he said.
And that’s 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 don’t know if it’s just a drop in a bucket, or if it is a main reason why we’d 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, it’s not a sign that Lake Erie managers need to kick the bugs back out.
“It’s not necessarily the mayflies’ fault that there’s so much phosphorus in the sediment,” Chaffin said. “The mayflies are going to do their thing if there’s 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’ll need to consider the effects of animals on the lake, not just the effect of the lake on animals, Edwards said.
“What we’re arguing is that these things need to be considered when we’re planning for the health of the lake,” he said. “We can’t 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 I’m wiping mayfly guts off my feet, I don’t get too upset about it.”