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Epilimnion - Rough Draft/Plan

CURRENT:

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"The epilimnion or surface layer is the top-most layer in a thermally stratified lake, occurring above the deeper hypolimnion. It is warmer and typically has a higher pH and higher dissolved oxygen concentration than the hypolimnion.

Being exposed at the surface, it typically becomes turbulently mixed as a result of surface wind-mixing. It is also free to exchange dissolved gases such as O2 and CO2 with the atmosphere. Because this layer receives the most sunlight it contains the most phytoplankton. As they grow and reproduce they absorb nutrients from the water; when they die they sink into the hypolimnion resulting in the epilimnion becoming depleted of nutrients."

Physical Structure:

Properties

In the water column, the epilimnion sits above all other layers. The epilimnion is only present in stratified lakes, but is always present in those types of lakes. On the topside of the epilimnion it is in contact with air, which leaves it open to wind action, which can make waves and increase aeration. On the bottom side of the epilimnion is the thermocline. The termocline is created because of the difference in temperature between the epilimnion and the hypolimnion. This is due to the fact that since the epilimnion is in contact with air and is above everything, it interacts with the sun and heat more, making it warmer than the layers below. In certain areas during the winter, the epilimnion will freeze over, cutting off the lake from being aerated directly.

Lake Turnover and Mixing

In most stratified lakes, seasonal changes in the spring and fall air temperature cause the epilimnion to warm up or cool down. During these seasonal changes stratified lakes may experience a lake turnover. During this, the epilimnion and hypolimnion mix together and the lake generally becomes un-stratified, meaning it has a constant temperature throughout, and the nutrients are even throughout the lake. There are different names for these turnovers based on how many times the lake does it in a year. Monomictic lakes flip only once, dimictic flip twice, and polymictic lakes flip more than twice. These turnovers can be based on seasonal differences, or can even happen daily.

Chemistry:

With the layer being open to air, the epilimnion usually has high amounts of dissolved O2 and CO2. This is due also to the fact that the water in this area is typically warmer than other areas in the lake. Warm lake water is able to hold on to more dissolved nutrients than cold water is. The epilimnion is also an area of concern for algal blooms due to phosphorous and nitrogen runoff from terrestrial sources. Wind erosion carrying soil particles can also introduce many different nutrients into the water as well, and those particles will enter the lake system through the epilimnion.

Biology:

Because of it's closeness to the surface, and being the area that receives the most sunlight, the epilimnion is a great home for phytoplankton, and other primary producers. Algal blooms are common in this layer as a result of large accumulations of nutrients. In response to large amounts of algae and phytoplankton being present, many fish species are common in this layer as they look for their source of food. Birds will often use the epilimnion as an area for rest and/or fishing. Many insects also make various uses of the epilimnion when it comes to nest making and habitat. Human interactions are also an important part of the biological part of the epilimnion. Some direct human interactions are recreational uses such as swimming, boating, or other activities. Other indirect interactions may come from sewage, runoff of agricultural fields, or land development, will all affect properties of the epilimnion.