“We're pretty sure warming has continued, because global air temperature has continued to rise.” The average rate of warming lakes across the globe is .34 degrees Celsius per decade. Lake Superior is heating up at three times that pace.
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Sun warms the surface waters of a lake. Winds die down and are no longer strong enough to mix the whole water column, or depth of water. Surface water becomes very warm, but the bottom water remains cold.
A lake has turned over when water temperatures are the same from the surface to the bottom. The process can take days or even months to complete, depending on lake shape and depth, and air and water temperatures.
Because water warmer than 40oF is more buoyant than cooler water, spring warming creates a shallow, warmer layer of water late in the day on the downwind side of a lake. Other factors being the same, the north side will warm more quickly than the south side of the lake.
If you are getting 50-55 degrees Fahrenheit, turnover may be nearing or just starting. By 45-50 degrees Fahrenheit the lake is making the change rapidly, and fall turnover is complete at around 40.
During the early morning hours, the land and the water start out at roughly the same temperature. On a calm morning, a given pressure surface will be at the same height above both the land and water. A few hours later, the sun's energy begins to warm the land more rapidly than the water.
During rainy seasons, oxygen concentrations tend to rise in most surface waters because rain saturates with oxygen as it falls. More sunlight and warmer temperatures also increase plant growth and animal activity, which may increase or decrease DO concentrations and increase diurnal fluctuation.
Farm ponds and small lakes less than 10 acres are the perfect antidote for this situation. Their small size and relatively shallow waters warm up quicker in spring than large reservoirs sprawling over thousands of acres or state-owned lakes several hundred acres in size.