Here in Wisconsin, like a lot of places, we are waiting for the reliably warm days of Spring. In the meantime, we’re seeing a lot of days with persistent, chilly winds. Stowing the winter gear in the attic might have to wait for another day, but those chilly winds play a big role in the life of a lake.
Each year, after any frozen lake thaws, an important phenomenon occurs beneath the surface of the frigid waters. It’s called “Spring turnover,” and it means the lake is, literally, all mixed up.
You see, during the winter, that lid of ice on a lake keeps any wind from troubling the surface. One main problem with this situation, if you’re an organism living in the lake, is that no wind means no oxygen.
During the ice-free months of the year, winds blowing across a lake help mix oxygen into the water. But, once ice forms, no new oxygen gets in. That means that all of the organisms that need oxygen to survive (which is a lot of them!) are stuck with whatever amount of the precious gas has mixed into the lake during the fall.
But, right about this time each spring, an event crucial to the life in a lake is taking place. Wind once again blows across the water. And, even better for the lake, that water is reaching a point where the temperature (and, therefore density) is the exact same throughout the entire lake.
That means a persistent wind can mix oxygen into the entire lake – all the way down to the bottom. According our friends at the International Institute for Sustainable Development, who oversee Canada’s Experimental Lakes Area, this Spring mixing “is extremely important in freshwater lakes, as it is the event that is responsible for replenishing dissolved oxygen levels in the deepest lake waters.”
You can see this phenomenon in action in the first half (non-stratified) demonstration of the video below:
Here in Madison, Spring mixing is well underway, if not already completed. And that means we’re now entering the season of lake stratification – but that’s a lesson for another day.
For now, just know that these pesky spring winds are an important part of a year in the life of a lake and, thanks to spring turnover, a whole lot of the fish and bugs and other critters that call our lakes home are getting a much needed breath of fresh air!