Spring is here and the most visible change on lakes and ponds is “ice out”. But there is a lot more is going on in the lake. Underneath the surface, immediately after the ice is gone, the water temperature is generally uniform (approximately 4°C or 39.2°F) from top to bottom. Without the icy surface, wind can begin to circulate the water. Surface water is pushed toward the bottom and lower water rises to the surface, in a phenomenon called spring turnover. This is very important process in that oxygen is brought to the lower regions of the lake.
As springtime air temperatures rise, the heat begins to warm the lake. This solar radiation warms the lake water from the surface downward. This warmer water becomes less dense than the cooler water, and thereby stays on the top. This warmer top layer of water is called the epilimnion. The colder water below it is called the hypolimnion. Between these two layers, is another layer called the thermocline. The thermocline has rapidly changing temperatures within its depth and forms a barrier between the other two layers. The depth of the thermocline depends on many factors, including the chemistry of the water and the amount of light reaching this layer. These three layers together cause the lake’s stratification.
Throughout spring and into summer, the top layer (the epilimnion) increases in depth, reaching a maximum for the season. The combination of warm water, solar radiation and nutrients from spring’s overturn will provide a good environment for various species, including algae.
The summer stratification will prevent complete lake mixing, even with the influences of wind action. Without thorough mixing to provide dissolved oxygen to the low spots, a deep lake bottom tends to have a limited supply of oxygen during the summer.
Meanwhile, dead algae will sink to the lake bottom and be decomposed by bacteria. This decomposition depletes the supply of dissolved oxygen in the hypolimnion as the aerobic bacteria use some of the available oxygen to decompose the organic material descending from the epilimnion. During the summer, the lake bottom can become anoxic (without oxygen) and anaerobic bacteria decompose organic material without the aid of dissolved oxygen. If dead algae accumulate at a faster rate than bacteria can work on decomposition, the resulting sediment deposited in the lake will be rich in organics.
As autumn approaches and temperatures decrease, the epilimnion cools and begins to decrease in depth. Eventually the epilimnion gets so shallow that it can no longer be maintained as a separate layer and the lake loses its stratification. Thus, as in the spring, the lake water in the autumn has generally uniform temperatures (about 4°C or 39.2F) and wind can once again mix the lake water. In addition, surface water, in direct contact with the cold air, gets cooled faster than the water below. This cold, dense water sinks and further helps to mix the lake, and once more oxygen and nutrients are replenished throughout the lake. This process is called autumn turnover. Ice may form during the winter and the cycle will begin again!
The vital springtime lake water circulation (turnover) brings nutrients and oxygen to the lake’s organisms such as fish, eggs, aquatic insects, tadpoles. Thus starts another year of the lake’s natural food web cycle and essential ecological processes.
Date published on web site:
03-29-2010
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