While they are a nuisance for vacationers and residents, piles of dead mayflies on streets and sidewalks around Lake Erie are a good thing. An abundance of the insect shows that Lake Erie is healthy. But this year, locals and several charter boat captains have noticed a shortage of mayflies.
The mayfly population acts as a barometer for water oxygen levels. "Mayflies are the canary in the coal mine," says John Hageman, Stone Laboratory Co-Manager. "The lower numbers of emerging mayflies this year match up with the low oxygen levels seen in Lake Erie’s western basin last summer."
Lake Erie’s mayflies live for approximately two years in the sediment when oxygen levels remain adequate for their survival. Then they emerge, come to the surface, mate, and die 24 to 36 hours later, piling up under lights and causing slippery roads and docks. In recent summers, mayflies were so numerous that residents were cleaning them up twice a week. Mid-June through mid-July is the timespan to see the most mayflies, with the emergence peaking June 20 through 30. But this year, that 10-day stretch was basically the only time Hageman saw them.
"In those ten days, there were mayflies on every blade of grass, but we haven’t seen the long-term emergence that we used to and we’ve seen virtually none in July," he says.
Last July, students at Ohio State University’s Stone Laboratory measured parts of the lake’s western basin and found areas with low oxygen. Later that month, Ohio Division of Wildlife biologists cited low oxygen as a possible reason for low walleye catches in their July trawls.
Areas of low oxygen occur in the summer as animals and decaying algal blooms use up the water’s limited oxygen. Toward the summer’s end, there isn’t enough oxygen left in some areas to support fish and other animals, including mayflies, which serve as food for important sport fish, like walleye and yellow perch. These areas of low oxygen are called "Dead Zones."
Starting in the 1960s, the annual mayfly emergence was barely noticeable for 30 years. Then, around the mid-1990s, mayflies returned in large populations as Lake Erie’s water quality improved. Now, Lake Erie phosphorus is climbing to record levels, and continuing to trigger larger harmful algae blooms. As these algal blooms cause oxygen levels to drop, we may be seeing the beginning of another mayfly decline.
Ohio State University’s Ohio Sea Grant program is part of NOAA Sea Grant, a network of 32 Sea Grant programs dedicated to the protection and sustainable use of marine and Great Lakes resources. For information on Ohio Sea Grant and Stone Laboratory, visit ohioseagrant.osu.edu .
John Hageman, Stone Laboratory Co-Manager, Ohio Sea Grant Extension Educator, 614.247.6500, firstname.lastname@example.org