Why do lakes turn over in the spring?

Why do lakes turn over in the spring?

When and how does lake turnover occur? The surface water temperature begins to shift in the spring and fall due to seasonal variations in sunshine intensity. The lake surface heats up in the spring. This causes the temperature of the lake's top and bottom layers to equalize. As a result, the weight of the water above the thermocline becomes evenly distributed between the hot upper layer and the cold lower layer. This eliminates any potential energy that could cause turbulence or other activity at the surface of the lake.

The warm temperatures of the spring continue to rise throughout the summer. When the daytime temperatures are high, more heat is absorbed from the sun than is lost to the atmosphere, so the average temperature of the lake continues to rise. By contrast, in the winter the colder temperatures of the night drop rapidly toward freezing, so most of the water beneath the ice remains frozen until spring thaws out the remaining pockets of open water on the lake.

The rising temperatures of the spring also cause an increase in the amount of sunlight that reaches the lake's deep waters. More intense sunlight means higher levels of oxygen consumption by organisms living in the lake's deeper regions. To compensate for these losses, springtime oxygen concentrations increase in depth, allowing more aerobic life to develop in the warmer depths of the lake.

As the oxygen levels rise in the spring, so do the levels of carbon dioxide (CO2) in the water.

How long does it take a lake to turn over?

Lakes may shift over in a matter of days or overnight. A cold front, chilly rain, or strong wind can all speed up the turnover rate. When a lake rotates, the top water lowers and the bottom water, which is now warmer, rises. The new surface layer contains much of the same material as the old one -- in fact, it's usually just a thin film of oxygen-deficient anoxic (without oxygen) sediment at the bottom -- but some materials are lost during the rotation and not recovered when the next high water comes around.

The length of time for a lake to rotate depends on how large it is. Smaller lakes may switch back and forth several times a day, while larger ones like Lake Superior will probably take more than a year to complete a single spin.

Lakes can also be affected by tides. In fact, most lakes that are situated near the coast have extensive shoreline deposits called clays because they look like wet clay. These clays contain many nutrients that support plant growth. The plants in turn consume much of the nitrogen and phosphorus in the lake, preventing them from reaching the water we drink. The nutrients are instead removed from the lake bed through weathering of rocks at the bottom of the lake into a soluble form that can be taken up by plants.

Clays also help protect against other types of damage to lakes.

Why do lakes and ponds turn over?

The slow, spontaneous mixing of pond and lake waters is known as turnover. It is induced by changes in surface water temperatures caused by the passage of the seasons. Turnover happens in the fall when surface waters cool and again in the spring when they reheat. The process spreads nutrients throughout the water column and removes some harmful substances such as toxins stored by algae. Without turnover, these bodies of water would become depleted of oxygen and polluted with toxic chemicals.

Lakes and ponds can also turn over due to human activity. For example, a fisher who casts his line into a lake will likely not get it back until after it has turned over because fishing boats move through the water and disturb the sediment at the bottom of the lake. This movement spreads out nutrients that have been buried in the sediment and gives fish an opportunity to eat them.

Lakes and ponds can also turn over naturally. Rainfall and snowmelt feed streams that flow into lakes and ponds. This adds more water to the body of water and increases its depth. The increased weight of the water above itself causes friction between the water and the surrounding land, which forces it to break up and mix with other nearby lakes or ponds. This process is called lateral migration and it helps lakes and ponds remain stable despite being surrounded by land.

Lakes and ponds can also turn over due to geology.

Why is it cooler by the lake in the spring?

On a sunny spring day, the land may quickly heat up, but a lake has virtually little day-to-day variation. The air moves up and down due to the temperature differential. Warm air rises, but cold air sinks. Because air has a continuous mass (it acts like a fluid), it must be restored if it leaves one point. So, the warm air over the lake will cause the nearby cooler air below it to rise, while the cold air above the lake will cause the nearby warmer air below it to sink.

This process takes place very rapidly, so by the time the warm air reaches the top of the atmosphere it is replaced by cool air. This cool air then descends back to the surface of the lake, keeping the water at a constant temperature.

This is why lakes tend to be cooler in the summer than in the winter. There are two things that can change this: solar radiation and wind speed. Solar radiation increases with more sunlight, so you would expect the lake to get hotter as well. But because clouds block out part of the sun's energy, there is less solar radiation at night when there is no direct sunlight. This means that the lake will be colder at night than during the day. Wind also affects temperature differentials between land and lake. If the wind is from the south or west, for example, then it will blow hot air over the lake and give it the appearance of being warmer than it actually is.

About Article Author

Alisa Wagner

Alisa Wagner is a biologist who has been conducting research for over two decades. Alisa loves to teach others about the biology of living creatures and enjoys sharing her knowledge with those around her. She started out as an undergraduate student studying zoology at Cornell University before going on to receive a PhD in developmental biology from the University of Michigan.

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