Will Erie’s Mayflies Survive a Warming World?

After decades of regulatory change and cleanup efforts, mayflies returned to the lake. Measuring their resilience to the climate crisis presents new challenges.

By: Anika Jane Beamer

Photo taken by Marcel Karssies / Wikimedia Commons / CC-BY-SA 3.0

This June, residents on Lake Erie’s shoreline had to bust out leaf blowers to clean up the insect corpses that carpeted their yards. Swarms of inch-long mayflies had emerged from the lake to mate, lay eggs, and — having completed their reproductive mission — die. To humans, it seemed like an invasion, but behind this semblance of abundance, the mayfly population is in jeopardy and scientists struggle to measure the extent of their peril.

Exquisitely sensitive to changes in water quality and predator populations, mayflies are powerful bioindicators that reflect the overall health of freshwater ecosystems. In 1960, at the peak of Lake Erie’s pollution crisis,%252520without%252520much%252520government%252520oversight.) local mayfly populations fell to practically zero. Populations have since rebounded, but researchers say they may be on the precipice of another catastrophe — one that Dr. Luke Jacobus, a freshwater entomologist at Indiana University Columbus, describes as “death by a thousand cuts.”

Over the last few years, Jacobus has witnessed a worldwide decline in freshwater mayfly populations. Nearly a quarter of Indiana’s mayfly species are considered threatened or endangered by the state Department of Natural Resources, a label shared with iconic creatures like the trumpeter swan and the lake sturgeon. In Canada and some European countries, more than one-third of mayfly species may be threatened, as nutrient runoff and high levels of sediments decrease water quality. In Lake Erie, scientists like Jacobus are holding their breath. Soon, the mayflies may be holding their breath too: polluted waters and rising temperatures are endangering their oxygen supplies.

Lake Erie’s Hexagenia mayflies begin their lives in the sediments along the lake’s floor, where they burrow as nymphs for up to two years. Undulating their segmented bodies and feathery gills, the nymphs draw water through their burrows to absorb organic materials and oxygen, but increasingly, there’s less and less oxygen to go around. Agriculture and industry leach nutrients into the water, fueling massive algal blooms, and the bacteria that decompose these blooms consume a hefty portion of the available oxygen. Rising global temperatures further shrink the limited supply by concentrating oxygen at the water’s surface.

“A period of hot, calm weather can pretty quickly deplete the oxygen in the bottom layer of Lake Erie, creating hypoxia, and the mayflies don’t survive that,” explains Dr. Kristen Fussell, a mayfly ecologist and director of the Ohio Sea Grant. In the summer, warm water at the surface naturally becomes less dense than the colder water along the lake’s floor, creating a gradient of oxygen availability. But the warmer the weather, the stronger this gradient, and the harder it becomes for wind and currents to re-disperse oxygen amongst the layers. If oxygen can’t reach the lake’s depths, the mayflies will suffocate, Fussell says.

As Earth continues warming, more oxygen-depleting events are on the horizon, but quantifying just how much danger the mayflies are in is challenging. Scientists struggle to reach consensus about mayfly population numbers, in part because where, when, and how populations are counted can paint different pictures of change.

For the last decade, Ohio EPA samples of sediments have indicated healthy levels of nymphs in the lake’s Western basin, but a 2020 study that used weather radar data to track adult swarms suggested that summer swarm sizes on the lake are shrinking. The discrepancy is puzzling, scientists say. It could indicate that nymphs are dying before they become adults, or that existing methods for measuring mayfly populations fail to capture the full picture.

Lake Erie is bordered by four U.S. states and Canada, and nymph sampling is currently carried out independently by state EPA agencies. Getting a lake-side sense of mayfly health would require cross-state data exchanges or, at minimum, for states to measure mayflies the same way. Radar measurements could help fill this gap, providing a cohesive understanding of the lake’s adult mayfly population, but only if the method proves reliable.

“People have historically used radar for different applications, but not to understand large-scale species population changes from environmental change,” explains Dr. Sally Entrekin, an aquatic entomologist at Virginia Tech who co-authored the weather radar study. Her team is now trying to understand the gap between radar measurements and data gathered via manual nymph sampling.

Entrekin thinks that radar could potentially change how people see freshwater ecosystems, but she’s frustrated by the pace of progress. “It’s really hard to find funding for monitoring,” Fussell explains. “There’s a reason why the published data kind of stops. It’s because we don’t have it.”

Mayflies live so much of their lives underwater and out of sight, given the challenges of tracking their populations and limited funding to do so, Entrekin, Fussell, and Jacobus fear that their suffering will go unnoticed. Says Fussell, “If something is happening right now, we might miss it.”

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