Limnology (2+1)

3.4.2. Thermal stratification

Unit 3- Nature of Inland water environment

3.4.2. a) Thermal stratification
In tropical lake, heat intake at the surface leads to the formation of a vertical temperature gradient, within which the thermal resistance become too great for the existing winds to continue mixing the whole water masses. The upper warmer layer is called epilimnion and the lower cooler layer is called hypolimnion. In between the two distinct portions, a layer called thermocline.
Summer stratification
In summer, there are three distinct layers are called epilimnion (upper layer), a bottom layer called hypolimnion and the middle layer called thermocline or metalimnion.
Epilimnion
a) It is upper layer of water.
b) It is warmer layer.
c) The temperature of this layer fluctuates with the temperature of the atmosphere. It will be about 27°C to 21°C.
Hypolimnion
a) It is the bottom layer of water.
b) At this layer, water will be cool.
c) The temp is between 5°C and 7°C.
d) It is a stagnant column of water.
Thermocline (metalimnion)
a) It is the middle layer.
b) The temperature is in between the temp of the upper layer and that of the lower layer.
c) It is characterized by a gradation of temperature from top to bottom.
d) It is also called transition zone.
In deeper lakes, a seasonal, thermal phenomenon occur which is so profound and so far reaching in its influence that it forms, directly and indirectly the substructure upon which the whole biological framework rests, particularly in the temperature zone. Therefore, a clear understanding of the salient features of thermal stratification is a necessity.

Thermal relations during spring
Uniform temperature of 4°C prevails throughout the water column of the lake. Wind depresses water at windward side and drives towards leeward side (towards the sheltered side), sinks at this end and moves at the bottom. This results in through mixing which is known as isothermic or homothermic condition.
During summer
As spring advances warmer winds and sun’s radiation increases surface water temperature. Water expands above 4°C and thus water at the surface is lighter than underlying colder water. Upper layers become more warm and lighter and no mixing can takes place. Wind drives water towards leeward and it sinks at that side which will sink down but not reaching the bottom of the lake but will be stopped at some intermediary level above cooler (colder) bottom water (Hypolimnion).
Currents in the upper lake will induce a counter current which is of a lesser magnitude in the bottom lake. At this depth, the current direction will be towards the opposite side of the lake ie, windward side from leeward to wind ward, sinks at this end and returns as the counter current at this region of lake. Thus two distinct layers are seen at this time in the lake. Between these two layers, temperature drops suddenly, upper layer in contact with the warmer waters of upper lake which is mixing by warmer winds and conduction.
On the other hand, the lower layers of this region is in contact with the layer which is yet to gain heat through conduction and other processes which are themselves slow process. This separating zone between upper lake and bottom of lake is called as Thermocline region. It is defined as a region wherein the temperature drops by more than 1°C per meter of depth. The term of thermocline was proposed by Birge (1897). Thus, epilimnion / upper lake is above thermocline and bottom lake / hypolimnion is below region of thermocline.
During fall (autumn)
Cold wind blow over the lake surface which cools surface water which become denser at -4°C. These denser waters sink through lighter warmer waters to a level where it meets the waters of similar density ie. first it will be at thermocline. Thus epilimnion gradually cools and on the other hand the hypolimnion will maintain the same temperature. A stage will be reached when there will be no thermocline region, water freely mixes. This mixing is called fall overturn. Mixing continuous till the temperature throughout will be at 4°C.
During winter
Cooling below 4°C will make water lighter and thus the surface waters are lighter than the warmer but denser subsurface water. This water floats and no sinking, cooling continuous at surface till ice is formed at 0°C. Once ice is formed at the surface wind has no effect as far as mixing is concerned a period of stagnation sets in.

During spring
With the onset of spring, warmer sun rays and wind melt the ice cover. Now colder but lighter water will be above warmer but denser water below. Once it attains a temperature of 4°C, it sinks down and reaches a level of 1°C which being lighter ascends up and in turn warms up. Thus the layer of denser water increases until the whole lake is uniformly of a same temperature. Mixing takes place now by spring winds and this is called as spring overturn.