Thermocline On Lake Ouachita
Most of the heat energy of sunlight is absorbed in the first few feet on Lake Ouachita’s’ surface, which heats up during the day, and cools at night (as heat energy is lost to space by radiation). Waves mix the water near the surface layer and distribute heat to deeper water, such that the temperature may be relatively uniform for up to 15 – 20 feet or so, depending on wave strength and the existence of surface turbulence caused by currents.
Below this mixed layer, however, the temperature remains relatively stable over day/night cycles.
The temperature of Lake Ouachita drops gradually with depth. Water temperatures tend to settle into horizontal layers of warm water and cold water that are separated by a moderating layer known as the “thermocline”. The thermocline will be the most active “feeding zone”. One scuba diver in the dam area reported that on a July 1st – 3 major temperature layer changes were observed – at 30′ (73°), 60′ (62°) & 100′ (49°)
The thermocline varies in depth. Often deepest during the summer, and shallow to nonexistent in the winter. One result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline, as the water below the thermocline never circulates to the surface, and organisms in the water deplete the available oxygen.
As winter approaches, the temperature of the surface water will drop as nighttime cooling dominates heat transfer.
A point is reached where the density of the cooling surface water becomes greater than the density of the deep water, and overturning begins as the dense surface water moves down under the influence of gravity.
This process is aided by wind or any other process (currents for example) that agitates the water. This effect brings water to the surface which, although low in oxygen, is higher in nutrients than the original surface water. This enriching of surface nutrients may produce blooms of phytoplankton, making these areas productive.
As the temperature continues to drop, a new thermocline develops where the densest water sinks to the bottom, and the less dense water rises to the top. Once this new stratification establishes itself, it lasts until the water warms enough for the ‘spring turnover,’ which occurs when the surface water temperature rises. During this transition, a thermal bar may develop.
Waves can occur in the thermocline, causing the depth of the thermocline as measured at a single location to oscillate. Alternately the waves may be induced by flow over a raised bottom, producing a thermocline wave which does not change with time, but varies in depth as one moves into or against the flow.
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