Posted by Lem on March 4, 2005

Q: I saw a documentary a number of years ago. At one point in the show, the naturalist that was guiding and narrating it progressed to a small lake. He explained that while the lake was very cloudy at that time, it was usually very clear. He went on to explain that the lake was in the process of cleaning itself at that time, and that this was something that occurred every so many years, (I believe it was seven). I have never forgotten that wondrous bit of information. Could you please tell me if the phenomenon has a particular name and where I go to learn more about this fascinating process?

Thank you in advance, and warm regards.

A: I'm quite sure the naturalist was referring to seasonal lake turnover. I'm familiar with this process but couldn't find any "term" for it other than Lake Turnover - I was sure there was a specific term, but maybe not!

Many lakes have layers of water at considerably different temperatures (thermoclines) due to the effects of sunlight and wind action on the lake. In the spring and again in the fall, the surface water or layer in these lakes sinks while deeper water rises to takes its place (water at 4 degrees C or 39 degrees F is the most dense). In the spring, the frozen surface warms to 39 degrees and sinks to the bottom, forcing the bottom water up. In the fall, the warm surface water cools to 39 degrees and sinks, forcing the bottom water up. This turnover or mixing of water is extremely important to the life of these lakes as it mixes nutrients and oxygen more thoroughly than otherwise possible and makes it available for the animals and plants living there. After a lake turnover, there may be an algal bloom (great increase in plant growth) due to the available nutrients that have been brought to the sunlit surface. These algal blooms are often temporary and this may be what the naturalist was talking about when he said the lake was very cloudy. It also occurred to me that there could have been an especially turbulent lake turnover (maybe this occurs sporadically), at your particular lake, that caused lots of sediment to be brought up into the water column.

Seasonal lake turnover is, or at least can be, a rather complicated process and one that occurs in varying degrees in different bodies of water - "one size doesn't fit all". I did find a website

www.conservation.state.mo.us that has more detailed information on this process and hope it provides additional answers for you. Thanks for your question.

John Wiessinger ( March 5, 2005)