Spring turnover in our lakes

We are now near to the summer season on our lakes again.  This marks the fourth installment of my lake stratification (layers) and mixing articles.  Once the ice melts off the lake, the lake goes through another period of mixing like it did back in the fall.

First, I'll recap the underlying cause of the lake mixing phenomenon.   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 winter, most of the water under the ice is 39 Fahrenheit; however, there is a thin layer of water under the ice that is colder than 39 and therefore less dense. This thin layer of water floats on top of the lake under the ice throughout the winter.

In the spring in Minnesota the ice melts off the lake, and the top layer of water on the lake gets warmed by the sun to 39 F, which matches the temperature of the rest of the lake water. Then the spring wind picks up and the lake mixes again.  This is called spring turnover.  Oxygen and nutrients get distributed throughout the water column as the water mixes.

As nutrients are available at the surface of the water again and the sun gets stronger, the first algae are able to grow.  In May the water may look somewhat brownish, which is due to the type of algae that grows at that time of year.  The earliest algae are called diatoms, and they have more of a brownish color than a green color.

In the winter, the fish go where there is oxygen in the lake.  That usually means the deeper spots.  As the lake turns over and plants start growing in the shallow areas again, they add oxygen to the water through photosynthesis and provide cover for fish from predators. The fish then begin returning to shallow water.

The timing and duration of spring turnover depends on the size and depth of the lake.  As the weather becomes warmer in late May, the surface water warms again and begins to float on top of the cold deeper water.  Once the lake begins to form into layers, summer stratification (layering) has begun.

You can track spring turnover in an individual lake with a Secchi disk.  A Secchi disk measures water clarity.  During spring 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.  Also, the algae start growing due to the available nutrients, which decreases water clarity.  Then, when turnover is complete, the clarity increases.  If you take Secchi disk readings and surface water temperature readings every few days in May and early June, you can track spring turnover. 

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.  

You can read my past articles on lake mixing at:

http://www.rmbel.info/Reports/Static/Publications.aspx

Until next week, enjoy the lakes!