OUTDOOR CORNER: LAKES TAKE A DEEP BREATH OF FALL AIR

Has anyone ever told you that their lake has turned over but couldn't explain what that means? Has any one ever told you that their lake turned over and the fish died?

In reality, ponds and lakes turn over every year in the spring and fall, with little harm to the fish.

During the seasonal changes of the year, lakes will also have changes. Turnover is mainly caused by temperature changes in the lake's water surface. Turnover occurs in the spring when the water begins to warm and again in the fall when water cools.

Water in lakes forms layers throughout the year. During winter, under the ice, the warmest water is near the bottom. When surface water reaches 39 degrees in the spring, it becomes heavier than the water immediately below it and therefore sinks. The downward movement of surface water forces water in the deeper parts of the lake upward. The resulting circulation of water is called spring turnover.

Spring winds also help promote water circulation. Spring turnover may last several weeks if the weather is cool and calm, or a matter of days if the weather suddenly turns hot and windy.

During spring turnover, oxygen is continually absorbed at the surface as the water circulates. This, plus the fact that water is cold, results in high oxygen levels. Theoretically, fish can inhabit all areas of the lake, but they tend to lie near the bottom and around submerged structures such as old stream channels, trees, and brush. Since most Missouri sport fish spawn during the spring in relatively shallow water, fishing these waters can be productive.

Water temperature continues to rise through spring into summer, and distinct temperature layers form- a phenomenon called "thermal stratification." Surface water, which is lighter because it's warmer, eventually forms a layer above a zone of colder water. In deep, clear lakes, this layer may extend 20 feet down.

This upper layer also tends to have higher oxygen levels due to aeration at the surface and oxygen production by aquatic plants live in this zone. The cold, bottom layer of water loses most of its oxygen due to the decomposition of organic materials and the consumption of oxygen by aquatic animals.

The density differences between the cold and warm layers prevents mixing. Little or no sunlight penetrates to the bottom, so oxygen-producing plants cannot survive in this zone.

As summer progresses, oxygen levels get low, so fish and most other aquatic animals must move to shallower waters which have adequate oxygen. Between the warm, upper water and cold, bottom water, a narrow layer called the thermocline forms. Water temperature and oxygen within this layer change dramatically from its upper to its lower levels.

During the hot summer months, many sport fish prefer the cool water in the thermocline, but they may not be able to live in this zone if oxygen levels are low.

When surface water cools in the fall, it becomes as dense as deeper water and sinks. The downward movement of the surface water forces deeper water upward. As happens in spring, the water begins to circulate, and this is called fall turnover.

Circulation is also enhanced by fall winds. Since the circulating water continually comes into contact with the surface, oxygen levels become high throughout the lake. The onset of fall turnover sometimes produces a sulfurous or rotten-egg odor. It comes from sulfur dioxide gas produced by decomposition during the summer and trapped within the bottom layer of water. When the deep water reaches the surface during fall turnover, the sulfur dioxide is released into the atmosphere.

As in spring, fish can be found throughout the lake. Sport fish will tend to gather near the bottom and around submerged structures. Some degree of water circulation can continue until ice covers the lake. When ice forms, wind can no longer provide the driving force to circulate the water. Minor stratification then sets in, with the dense, 39-degree water at the bottom and water as cold as 32 degrees at the surface. This condition remains until spring turnover.

Water temperature, dissolved oxygen, and submerged structures are the three most important factors affecting the location of sport fish. Your ability to find good structures in water which contain preferred temperatures and adequate oxygen will improve your odds of catching a fish.

If you want to be a good fisherman during all times of the year, turnover is an important concept to understand. For more information about turnover or questions about your lake, contact the Missouri Department of Conservation in Cape Girardeau at 290-5730.

Christopher Kennedy is a fisheries management specialist for the Missouri Department of Conservation.