Lake Is a Rainbow: Weird, Warm Fall Makes “Three-Layer Lake Cake”

by Luke Loken

Autumn is typically associated with fall colors, pumpkin spice lattes, and cool temperatures. However this year, much of the Midwest has been stuck in summer-like conditions. And these record temperatures come with unexpected consequences.

Welcome to the new normal? Lake Mendota blowing in the wind – in March. Photo: A. Hinterthuer

Usually, as the air cools in autumn so do the surfaces of our lakes. When a lake’s surface water nears the same temperature as the colder, bottom layer of water, the entire lake mixes. We limnologists call this ‘fall turnover’. This is a very exciting time of the year from the lake’s perspective. Nutrients from the bottom of the lake can cause blooms of algae. Oxygen near the bottom of the lake is replenished, offering a breather for cold-water fishes that may have been gasping for a little O2.
But, this year hasn’t been like most years. As so many climate models have forecasted, we have seen an increased number of days of extreme rain events and we’ve sweated through several September days that set record highs.
This obviously means a prolonged summer season and later fall mixing of the lake, but what is incredibly odd, is that the lake had already started its yearly mixing cycle before our September heat wave hit. Thanks to that heat wave, the lake surface started warming up again and separating, or stratifying, from the cooler waters deeper down. So, not only did Lake Mendota’s mixing stop, a late spring phenomenon started back up.
This is typically what happens after winter in May when the lake surface warms and separates into its two distinct vertical zones of warm and cold water for the summer. But the heat this month caused the very top of the lake to warm when the lake was already stratified and not yet mixed. Add into the equation calm winds, and you have the recipe for three layer lake cake.
Earlier this week, the surface water of Lake was near 22 C (71 F) the middle of the lake 18 C (64 F) and bottom of the lake 14 C (57 F). (Fig 2). Add in some fun color ramps and you can make the lake a rainbow!
How long will this last? Two climate factors will decide. First, the air will get colder. Cold air temps will cool the top of the lake, eventually making it the same temperature as the middle layer and breaking the thermocline, or line that separates the two. Second, wind will play a role. When it gets windy, wave action mixes water from the surface down to deeper levels breaking up the layers. Think about a nice balsamic vinaigrette. Left alone, the oil and vinegar separate into two layers. But shaking the bottle (or, in this case, being the wind) distributes the oil and vinegar throughout the bottle.
Luke Loken. Photo: J. Crawford

These colder and windier days, like yesterday, should mix the top to layers in the coming weeks. But the bottom layer is still very deep and cold. Fall turnover typically happens in late October on Lake Mendota, but this gradual process has been delayed, so watch out for turnover as late as November. And that means a few things. One of which is not that appealing – blooms of algae are common following turnover as nutrients stored in the bottom of the lake become available for algae at the lake surface.
But turnover also is good. Oxygen from the top of the lake and the atmosphere can replenish oxygen to the lake bottom. For the last three months, oxygen levels at the bottom of the lake have been near zero. Cool-water fishes like walleye and cisco need that cold water, but they also need oxygen, so they are left with a tough decision and usually spend summers in less-than-optimal habitat. The annual cool down lets them head back to a life where they can breathe and swim wherever they want. Which, for a year this unusual at least, is happening on a timeframe of “better late than never.”

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