By Kelli Saunders (with minor editing for style)
Last November, I talked about “fall turnover”, a phenomenon that happens on our lakes here in northwestern Ontario when things cool off and the temperature and density of water changes, resulting in the mixing of top and bottom water layers that have developed over the summer (thermal stratification). As soon as the ice goes out, “spring turnover” takes place, once again mixing the waters of the lake from top to bottom.
During the winter, ice cover on a lake eliminates the impact wind has on the water’s surface. This essentially puts a lid on the chances for any further oxygen to be mixed into the water, which is critical for living organisms. So, all winter, organisms have a set amount of oxygen to survive on under the ice. Spring turnover is a welcome relief! Once the ice is gone, wind can blow across the water surface again and start to reoxygenate the lake. After ice out, the water will reach a point where the temperature (and, therefore density) is the exact same throughout the entire lake. Because of this, a persistent wind can mix oxygen into the whole lake – right down to the bottom and this is a crucial mechanism for replenishing dissolved oxygen levels in the lake. When the lakes are a uniform temperature and density, it takes relatively little wind energy to mix water deep into the lake. As new life emerges in our lakes in the spring, there is an increased need for oxygen and this process really does provide what is needed to support that new life and rejuvenate the organisms that survived over winter.
As the season progresses and surface waters warm, thermal stratification sets in with warm, less dense waters floating on top of the deep cold waters, with little mixing between the layers. Whatever dissolved oxygen there is in the deepest parts of the lake is all that is available until lake turnover once again, in the fall. Nature is truly amazing.
This series is provided as part of the International Watershed Coordination Program of the Lake of the Woods Water Sustainability Foundation (www.lowwsf.com).
Kelli Saunders, M.Sc., is the International Watershed Coordinator with the Lake of the Woods Water Sustainability Foundation.