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What dramatic year-to-year swings in ice off dates mean for lake ecosystems


Ice covered many Northwoods lakes through the first week of May this year. But last year, lake ice melted in early April.

That’s at least a 30-day difference.

According to researchers from Trout Lake Station, the difference in ice off from one year to the next has become more extreme in the past ten years.

“We are seeing within the last ten years, that year-to-year variation in ice off date is almost five times higher than it’s been in the last century,” says Gretchen Gerrish, the station’s director.

Last year’s ice off dates for lakes in Minocqua were the fourth earliest on record. Gerrish expects this year’s dates to be among the latest.

She attributes this increase in yearly variation to changes in climate and weather patterns across the world.

“Climate models are definitely complex,” she says. “Weather patterns and localized temperature patterns are also dependent on things like global thermohaline circulation in the ocean, wind and current events.”

Those changes lead to more extreme local weather events.

In the Northwoods, one way that plays out is dramatic swings in ice off dates from year to year.

This has consequences for lake ecosystems.

“The idea is if you think about a 30-day change for something like algae or the little plankton in the water, they often will have a whole generation in a day or three days,” Gerrish says. “When you contrast that with something like a long-lived fish, they’re going to be feeding on those plankton and depending on those resources for their life cycles and for their growth. What can happen is what’s called a trophic mismatch where you have things that are feeding on other things and the timing of those peaks and needs becomes offset.”

Gerrish says this can ultimately lead to unhealthy aquatic populations and slower growth and reproductive rates.

One recent study showed walleye were less successful spawning in years with both really early and really late ice offs.

“The walleye are spawning and succeeding into the next generation much better if you have an average ice off year,” she says.

Researchers like Gerrish are still trying to figure out more about how the timing of ice off affects fish like walleye.

But one thing seems likely – there will continue to be an increase in ice off and weather variability in coming years and that will impact lake ecosystems as we know them today.

Erin Gottsacker worked at WXPR as a Morning Edition host and reporter from December 2020 to January 2023. During her time at the station, Erin reported on the issues that matter most in the Northwoods.
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