Solid Answer: 0
- jackie johnson
Thermal Stratification and lake turn over.
The loss of dissolved oxygen in lake water during the summer months is generally associated with a process that is called thermal stratification. This phenomenon involves the development of temperature “layers”, in which the water near the surface is uniformly warm to a depth that varies throughout the summer. Under the surface layer (referred to as the epilimnion) there is a zone of transition (thermocline) in which the water temperature drops rapidly. From the transition zone to the bottom of the lake is the layer of water that is the coldest (hypolimnion), and the most dense (heavy). During the summer months the coldest layer near the bottom is physically and chemically separated from the surface.
The bottom sediments in biologically productive lakes are rich in organic matter that has accumulated over time (algae and other plankton, and organic soil material). As this matter is degraded by microbes, oxygen is consumed from the overlying water. During the stratification period there may be no opportunity for oxygen to be replenished. Depending on the extent of the oxygen loss, coldwater fishery habitat may be reduced. Another possible result of oxygen depression or depletion is the potential release of biologically-available phosphorus from the bottom sediments.
Dissolved oxygen is replenished during the fall, when short days and cool air temperatures cause the lake to de-stratify or “mix”. This process is also referred to as “turning over”. Most lakes turn over twice each year - once in the fall and again in the spring. The degree to which individual lakes experience thermal stratification depends on the depth and volume of the lake and the orientation of the lake basin to prevailing winds. Annual weather patterns and individual weather events strongly influence the degree and duration of thermal stratification.