Hypolimnion:
The
hypolimnion is the dense, bottom layer of water in a thermally-
stratified lake. It is the layer that lies below the
thermocline.
Typically the hypolimnion is the coldest layer of a lake in summer, and the warmest layer during winter. Being at depth, it is isolated from surface
wind-mixing during summer, and usually receives insufficient irradiance (light) for
photosynthesis to occur.
In deep,
temperate lakes, the bottom-most waters of the hypolimnion are typically close to 4 °C throughout the year. The hypolimnion may be much warmer in lakes at warmer latitudes.
Thermoclines:
Thermoclines can also be observed in lakes. In colder climates, this leads to a phenomenon called stratification. During the summer, warm water, which is less dense, will sit on top of colder, denser, deeper water with a thermocline separating them. The warm layer is called the
epilimnion and the cold layer is called the
hypolimnion. Because the warm water is exposed to the sun during the day, a stable system exists and very little mixing of warm water and cold water occurs, particularly in calm weather.
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 also occurs in Arctic and Antarctic waters, bringing 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, the water on the surface may get cold enough to freeze and the lake/ocean begins to ice over. A new thermocline develops where the densest water (4 °C) sinks to the bottom, and the less dense water (water that is approaching the freezing point) rises to the top. Once this new stratification establishes itself, it lasts until the water warms enough for the 'spring turnover,' which occurs after the ice melts and the surface water temperature rises to 4 °C. During this transition, a
thermal bar may develop.
Waves can occur on the thermocline, causing the depth of the thermocline as measured at a single location to oscillate (usually as a form of
seiche). 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.
Epilimnion:
The
epilimnion 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.
Destratification:
Destratification is a process in which the air or water is mixed in order to eliminate
stratified layers of temperature, plant, or animal life. The first example of destratification was in 1919, in a small resorvoir.
[1]
A
pond's condition deteriorates when the
bottom environment cannot support animal life. The bottom is the area that runs out of oxygen first, it is where the most oxygen is used, and it is the farthest from the surface where it is replenished. Without oxygen a lake or pond's self-purification capability is not only reduced, it is reversed. The small animals, snails, worms, bacteria, etc., which help keep a pond clean cannot live, and the pond's nutrients are then recycled from the
sediment. This forms a layer of
muck at the bottom which serves as a fertilizer for weed and excessive
algae growth. It can also cause large
fish kills.
