As air temperatures in fall fluctuate between freezing at night and sunny 70 degree days, many deeper lakes within the region experience a phenomenon known as turnover. Journeying into the science of fall turnover can lead you into a wide berth of topics including physics, chemistry, thermodynamics, and biology. While complex and varied across systems, fall turnover influences how we use and interact with our beloved regional lakes.
Waking to a sunny morning in September after our first overnight frost advisory the surfaces of many lakes are barely visible below the steamy mist rising from the water. At this time of year, the surface waters of lakes hold warmth that has been stored up over the sunny days throughout the summer. This warm surface layer is called the epilimnion. Most deep lakes have a cold deep layer of water called the hypolimnion that can be refreshing and a bit shocking if you dive too deep. As fall arrives, kiddos may not be too enthusiastic about jumping in for a swim, but they are willing to brave the cold air for a cruise on the kayak or a round of tubing in the surface waters which still feel pretty warm. As more cold events occur and periods of cold are prolonged, the air cools the surface waters of the lake. Cold water is more dense than warm water so the cold water sinks. This can buy a few extra weeks of warm surface temperatures but eventually sinking cold water and windy days will mix the water column of the lake bringing cold waters from the bottom up to mix with warmer surface waters. This is fall turnover. Very few would brave entering the waters after this point, and many folks start removing boats and docks around this time.
Fall turnover changes more than just temperature. All summer the cold waters trapped in the bottom layer of the lake have harbored unique communities of bacteria that help decompose algae, leaf litter, and all other types of organisms whose final resting place may be the bottom of a lake. As the bacteria eat away at these organisms, they can use up most of the oxygen in the deep layer, and they produce methane – that stinky gas released when food is broken is down! Some lakes will smell like methane during fall turnover and some even ‘burp’ or release big air bubbles of methane gas. The good news is that as the bacteria break down materials, they release many nutrients. These nutrients mix throughout the water column during turnover and support a fall pulse of algal growth that feeds the food web allowing fish and other organisms to store resources needed to make it through winter.
Many fishing enthusiasts consider fall turnover to be the time to take a break. The hot spots of summer disappear, and the ice will not be ready for at least a month. With oxygen level changes, a big shift in temperature, cloudy water, and numerous other chemical and biological shifts occurring, fish change their behaviors. Warmer water fishes like bass and sunfish can still be found feeding but they hang out in shallow refuges of clear, warmer waters very close to the shoreline. In deeper areas these species are adjusting to low visibility conditions, cold water, and lower oxygen so it is not surprising they stop biting down there! Pike and walleye head for the live weed beds and other panfish will group up to feed on aggregates of plankton. The fish shift their behaviors during fall turnover, and successful fall anglers know to do the same.
The signs of fall approaching are all around. Leaves are just beginning to change, the mushrooms are popping, and pesky mosquitoes are disappearing. Watch for more fish along the shorelines, cloudier deep water, and a lot fewer kiddos willing to brave a swim. These are sure signs that fall has arrived and that your lake might be turning.