Unusually cool summer caused turnover in some area lakes
Have you noticed that the shallow areas of lakes (20 ft deep or less) look murkier and/or greener this week compared to the past couple weeks? The nighttime temperatures in the low 50s last weekend combined with 30+ mph winds caused many of the small, shallow area lakes to turnover. This turnover is earlier than normal. Today I'll explain how and why our lakes behaved a bit differently this year.
Lakes go through a seasonal cycle where they mix in the spring, separate into layers in the summer and mix again in the fall. This cycle is based on 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, which causes it to become less dense. The bottom layer of the lake 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.
This year we've experienced unusually cool temperatures compared to the previous few years. The year started with a January and February that barely made it above zero. Then came our three April snow storms and later-than-average ice out.
Detroit Lake has an average ice-out date of April 20, while this year the ice went out on May 5. Any lakes who took water clarity readings with a secchi disk in May experienced near-record clarity. To name a few, Big Cormorant was 26 ft, Big Detroit was 20 ft, Ida was 19.5 ft, Otter Tail was 17 ft and Pelican was 20 ft. These lakes usually average 8-15 ft clarity throughout the summer.
Then with the continued cool weather, our lakes were slower to warm up. Just like our gardens were slower to get started this year, the plants and algae in our lakes were behind the usual schedule as well. These conditions contributed to good spring clarity in area lakes.
The slow start to the summer caused many lakes to separate into layers later than usual, and the layers were less distinct because the surface water may not have become as warm as previous years. This means that the temperature difference between the warm top layer and the cool bottom layer of the lake was not as substantial. With these conditions, a couple cold days and heavy winds can mix the water with relative ease.
The lakes that mixed may be now experiencing an algae bloom. The mixing of the lake brings up water from the bottom that contains nutrients. These nutrients then feed the algae at the top of the lake. The mixing may also have an effect on fishing because when the lake mixes, the oxygen levels become uniform throughout the lake so fish can roam anywhere instead of their favorite hole.
Taking weekly secchi disk readings to measure water clarity is a good way to track the mixing and algae blooms in your lake.
Enjoy the lakes!
(Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, firstname.lastname@example.org.)