The lakes of Minnesota are about to turn themselves inside out. 

All summer, the waters have been warming. You’ve felt it when swimming in a summery lake; the warm surface water and then, near your toes, deep colder water. Warmer water, like warmer air, is less dense and floats while colder, more dense water sinks. These summertime temperature layers in a lake- or summer strata- have fancy names; epilimnion (surface), metalimnion (mid-depths), and hypolimnion (deepest depths).

All summer, many fish species have lived in the hypolimnion stratum (at the bottom). It’s cool and dark down there and easier to hide. There’s a lot of good dead stuff to eat- or a lot of good hiding prey to eat- depending on the fish species. But, as summer continues, the hypolimnion stratum becomes more crowded and all those bottom-living creatures have a harder time breathing as dissolved oxygen becomes harder to find. The thermo-layers of lakes can act like dangerous traps towards the end of the warm season. 

But now it’s October and summer is over. So are the lakes’ strata. The sun’s intensity is waning, winds are getting colder, and the surface of the lake is cooling fast, faster than the lake as a whole. Degree by degree, the lake prepares to turn inside out. And it can’t happen soon enough for the oxygen-starved fish at the bottom.  

There’s something chemically magic about water at 39 degrees. For some complicated reason (aka, chemistry), water is the most dense at 39 degrees fahrenheit. Any water that is cooler or warmer than 39 degrees will be less dense, and will float on top of 39 degree water. (And thank heavens for that- if ice didn’t float, life as we know it perhaps wouldn’t have evolved, but that’s a whole other essay.) 

When the surface water of Minnesota’s lakes cools to 39 degrees (usually in October, but dependent on the season and lake), the surface will begin to sink, completely mixing up summer’s orderly strata. The warmer mid-layers (metalimnion) rise to the surface, the deepest depths (hypolimnion) swirl off the bottom as the dense, oxygen rich, ex-surface waters take its place. And the fish can breathe again.

It’s an essential but inconspicuous transformation. 

This inside-out mixing of lakes is called ‘turnover’ and it’s necessary to Minnesota’s lakes as we know them. Not only does turnover bring fresh oxygen to the bottom of lakes, it distributes nutrients that collect on the bottom, stirring it up like a healthy soup. 

You can tell a lake is turning over in the fall or spring when you can see dead leaves suspended in murky waters. Sometimes lakes smell sulfur-y during turnover. Turnover can last a few days to a few weeks depending on the lake or the season. Some very shallow lakes do not turn over ever and our beloved Lake Superior turns over just once a year, instead of the usual twice. 

While the lakes are mixing themselves, I find I’m preparing for winter too. I’ve replenished our shelf with hot cocoa mixes, pulled out last year’s sweatshirts for the kids to try on, and learned how to light the gas fireplace in our new house. What needs more oxygen around the house? How has the daily grind depleted my nutrients; mind, body, soul, and otherwise? What should I turnover? 

The lakes will be locked in ice soon. But today in St. Paul it’s 61 degrees and sunny while our lakes are doing the work of turning over. The surface waters are reflecting golden sun and whatever blazing tree leaves are stubbornly clinging. It’s a good day for turning inside out.