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Turnover time comes regularly for local lakes/Biological Bits

Biological Bits

August 24, 2012
By Tom Rozich - For the Gazette , The Daily Mining Gazette

The Copper Country has just experienced the hottest July on record, which led to the surface water temperature of Lake Superior exceeding 70 degrees Fahrenheit. This has folks speculating that we are going to receive more than average lake effect snowfall this winter. Fact or Myth?

Myth, in my opinion, and if we do, it will not be as a result of our warmest-ever July or warm summer Lake Superior surface temperatures. Why not, you wonder? Fact is, Lake Superior will have cooled and mixed with the annual fall "turnover" long before our winter winds come howling.

Let me explain...

In looking at the National Weather Service data of Lake Superior surface temperatures from 2007 to present, lake turnover, which will be fully explained later, occurred in early to mid-December during this time frame. Right on schedule, regardless of the summer's surface water temperatures. These data also showed that in August 2011 the peak surface temperature exceeded this July's maximum, and look at the very mild winter we had last year. The track of the jet stream winds, in my opinion, has much more to do with the amount of lake effect snow the Keweenaw receives than water temperatures.

Prior to an explanation of "turnover," a brief explanation of the properties of water is necessary, which is truly an amazing substance and probably the most fascinating compound on Earth. Humans require a lot of water, needing to consume large amounts daily, but so do our finny fish friends, which also require fresh, clean, well oxygenated water to grow and survive and in which lake "turnover" plays a critical role. Water, unlike most compounds, is less dense (lighter) as a solid than as a liquid. That is a good thing; otherwise we would have flat fish! Come to think of it, flounder are very tasty!

Water is most dense (heaviest) at 39.2 degrees, lightest in its solid form of ice, and continues to get less dense (lighter) at temperatures above 40 degrees.

So, what happens? During the summer, the warmest and lightest water is at the surface of the lake and is very resistant to mixing. This is similar to oil floating atop water. You can blow on it and even tilt it back and forth and they do not mix much, because of the difference in density, one being much lighter than the other. As the weather cools during autumn, the surface water also cools, reducing the difference in density between the top and bottom layers. Eventually, the surface temperature reaches 39 degrees and that water begins to "sink", being the heaviest. Also, at this point in time, with the entire lake being about the same temperature, autumn winds actually mix the entire lake and the lake is said to "turnover," completely mixing from top to bottom. This same phenomenon occurs in the spring of the year in most lakes and is Mother Nature's way of replenishing the oxygen supply to the deepest portion of the lakes.

In my former life as a fisheries biologist, we knew exactly when the local lakes were turning over, as newbie lake residents would call to report "something" was wrong with their lake because it was dirty looking and smelled terrible. They were absolutely correct in their observations, but in reality all was well and Mother Nature was just doing her thing.

During the winter, in ice-covered lakes, the warmest water is on the bottom, being about 39 degrees. Veteran ice fishermen know this and generally fish on the bottom of the lake, at least during early winter. Later in the winter, fish move up and suspend, due to oxygen levels being lower near the bottom, the result of fish consuming oxygen and plants decomposing, while oxygen production during ice cover is a very low level.

However, not all lakes are created equal and do not turnover or mix twice a year. While most lakes in Michigan mix twice a year, others mix only once, and others mix many times a year. Most of our area lakes mix twice a year, in the spring and fall.

In general, lakes that are 10 feet deep or less mix many times a year. These water bodies during the summer months are about the same temperature from top to bottom, thus during windy days they are "mixed" by the wind.

Examples in our local area are Eleven Mile Lake, Lake Bailey, Thayer Lake and Schlatter Lake. These lakes typically do not have good fish populations, as they either winter kill, summer kill, or both, but that is another column.

The lakes that mix only once a year: these would be lakes that are relatively deep, being 100 feet or greater in depth, reasonably large, in excess of 1,000 acres and remain ice free during the winter. These lakes, by remaining open during the winter are about the same temperature from top to bottom and mix constantly during the winter months. Our Great Lakes are excellent examples of this type of lake, although Lake Superior does, rarely, completely freeze over, at which time our lake effect snow ceases.

During milder winters, Crystal Lake in Benzie County and Torch Lake in Antrim County also remain open and mix during the winter months. To my knowledge, we do not have any such lakes in our area.

The "reel" question to fisher-persons is, "How does all this affect fishing?" Once things have settled down after turnover, fishing is improved. In addition to oxygen supplies being replenished, bait-fish activity is stimulated as a result of nutrient or plankton movement and many fish will be in shallower water. This fact, along with the need for fish to bulk up for the winter season, makes for great fishing.

Go Fish!



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