Fall turnover in our lakes

The temperature has cooled lately and the wind has picked up.  These are the ingredients needed for fall turnover in our lakes.  A couple months ago (July 29, DL Tribune) I wrote about how the lakes separated into layers in the summer, which is called stratification.  To recap, the layering of lakes has to do with the relationship between water density and temperature.

Water is most dense at 39 Fahrenheit (4 degrees Celsius), and as water warms or cools from that mark it gets less dense.  This has implications for a lake's structure because the denser water is heavier and will be at the bottom of a lake while the less dense water is lighter and will generally be at the top of the lake.

In the summer in Minnesota, the sun heats the top layer of a lake (called the epilimnion) which causes it to become less dense.  The bottom layer of the lake (called the hypolimnion) does not receive sunlight and therefore remains cold.  Since the top layer of the lake is less dense, it floats on top of the bottom layer and the two do not mix.  The metalimnion is the middle layer of water.  The metalimnion also encompasses the thermocline, the area of the most drastic change in water temperature.

In the fall in Minnesota, the sunlight is not as strong and the nights become cooler.  This change in season allows the top layer of the lake to cool off.  As the water cools, the density difference between the top layer of the lake and bottom layer is not as great.  Wind can then mix the layers.  Eventually the top layer fades and mixes with the middle layer.  The thermocline sinks in the water column as the heavier upper water layer forces it deeper.  Soon, though, the temperature in this mixed upper layer equals that of the bottom layer; wind easily mixes the entire water column because similar temperature and density exist throughout.  This mixing allows oxygen and nutrients to be distributed across the whole water column again, and is called fall turnover. 

The timing and duration of fall turnover depends on the size and depth of the lake.  Sometimes it can occur in a couple days and sometime it takes a week or more.  The deeper the lake, the longer the water column takes to become uniform in temperature since the bottom stays cooler.  Most lakes in Minnesota that are deeper than about 20 feet turnover in early to mid October. 

You can track fall turnover in an individual lake a couple different ways.  The easiest way is with a Secchi disk and a water thermometer.  A Secchi disk measures water clarity.  During fall turnover, the clarity of a lake usually decreases because mixing brings up nutrient rich water from the bottom of the lake and causes the lake to look cloudy.  Sometimes there is even a distinct smell as decomposing plants, algae and other matter surfaces.  Then, when turnover is complete, the clarity increases dramatically.  If you take Secchi disk readings and surface water temperature readings every day or every other day in late September and early October, you can track fall turnover.  Once the surface is 39 degrees F and the water clarity is deeper than it's been all summer, you will know that turnover is complete.

A more precise way to track fall turnover and the sinking of the thermocline is to take the temperature at 2-foot intervals from the surface of the lake to the bottom.  You can tie a rope to a thermometer and mark 2-foot intervals on the rope.  If you graph a line of temperature versus depth in the lake, you can see how the lake cools off and becomes uniform in temperature after turnover.

Some people report that fishing is not as good during fall turnover.  The main reason fishing is harder is because the fish move and are more dispersed.  In the summer, walleyes school in high oxygenated holes and structures.  When the lake mixes, the oxygen levels become uniform throughout the lake so fish can roam anywhere.  For information on how to fish during fall turnover, visit: http://fishingminnesota.com/fishinfo343.html.

Most lakes in northern Minnesota are considered dimictic, meaning they mix twice a year - spring and fall.  Shallow lakes, less than 15-20 feet, behave differently and can mix more often throughout the summer.  

Until next week, enjoy the lakes!

Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, lakes.rmbel@eot.com.