As the days grow colder, the lakes are getting ready to turn over. This phenomenon, which happens every fall, is related to the unique chemistry of water.
In summer, the water in a lake is typically separated into two distinct zones, separated by a boundary called the thermocline. Near the surface, closer to air and sunlight, the water is relatively warm and oxygenated. Deeper, below the thermocline, it’s colder and contains less oxygen. Because of the different densities of water at different temperatures, these two zones don’t mix much – think of oil and water. If you’ve ever gone swimming in a lake you might have even felt where the water abruptly becomes colder at a certain depth.
As winter approaches, the surface water cools along with the air. When it gets cold enough and dense enough, it sinks. The two distinct zones break up and all the water in the lake circulates and mixes freely. This is the “turnover.”
Here’s what’s really cool. Almost any other liquid becomes denser and denser as it cools and gets densest of all as a solid. Water is densest at 4°C or 39°F, several degrees above its freezing point. This is why ice floats – and why lakes freeze from the surface down than from the bottom up. The water under the ice, freshly re-oxygenated from the fall turnover, remains at 4°C all winter long, allowing the fish and other aquatic live to survive until the spring thaw.
There’s nothing like walking over the frozen surface of a lake in the dead of winter and imagining all the sleeping life sealed beneath your feet. This morning we woke up to another dusting of snow… it won’t be long.