Limnology (2+1)

Unit 3- Nature of Inland water environment

3.4.1. Physical characteristics- Surface film, Temperature
Surface film
When water is exposed to air, it acts as if it were encased within an extremely thin elastic, surface membrane. This boundary is commonly known as the surface film and is interpreted as a manifestation of unbalanced molecular action. However, at surface film, there is a surface tension due to unbalanced attractions between water molecules at surface on one side only and upward attraction is lacking because there are no water molecules above them.
Surface tension is maximum in pure water than in any other liquid except mercury. Surface film provides support for organisms and miscellaneous particulate material, upper as well as under surface of surface film offers mechanized support.
Plants are pleuston whereas animals which are associated with the surface film are termed as neuston (minute and big).
Effects of surface film
a)Beneficial effects are (i) mechanical support and (ii) respiration mainly air breathing aquatic insects.
b)Harmful effects are (i) reduction of light penetration thereby it will have effects on photosynthesis and (ii) traps the minute organisms thereby fall easy prey to big animals.
Temperature
Temperature is one of the most important factors in an aquatic environment. In fact, it is possible that no other single factor has so many profound influences and so many direct and indirect effects.
Diurnal and seasonal variations are very much common in freshwater environments than in marine environment. A diurnal variation range of 4.8 to 5°C has been recorded in a tropical pond with an average depth of 3.0 m. In shallow water bodies within an average depth of 1.5 m, the lowest night temperature was 26.6°C. The highest day time temperature was 32°C with a variation of 5.4°C. In flowing water bodies like streams and rivers there is no such wide fluctuations in temperature.
Lentic waters of lakes and ponds undergo thermal stratification phenomenon according to seasons. Thermal stratification has been reported most frequently in the lakes of tropical countries such as Java, Sumatra and India.
According to temperature relations lakes have been classified into three types
1)Tropical lakes : In which surface temperature are always above 4°C.
2)Temperate lakes : In which surface temperature vary above and below 4°C.
3)Polar lakes : In which surface temperature never goes above 4°C.
Decrease in temperature cause reduction in metabolism resulting in lower rate of food consumption. Extreme higher or lower temperature has lethal effects on the aquatic organisms. Fluctuation in temperature of water regulates the breeding periods, gonodal activation and thermal induced migration. On the basis of their ability to tolerate thermal variations, most fresh water organisms are classified into stenotherm and eurytherm. Stenothermic are the organisms with a narrow range of temperature tolerance while the eurythermic are those organisms with a wide range of temperature tolerance.
Source of heat for evaporation
a)Sun
b)Water
c)Surroundings
Inland waters are subjected to very extreme variation of temperature due to small expanse and shallow areas and get heated rapidly during day and are cooled at night.
Rate of evaporation is determined by several factors such as
a)Temperature
b)Relative amount of free surface area of the water
c)Vapour pressure
d)Barometric pressure
e)Amount of wind action
f)Quality of water ie. fresh or salt
e) Thermal conductivity
The thermal conductivity of water is very low. Heat coming to a lake from the sun as partially absorbed and to some extent conducted, but the really effective heat distribution is due to wind action in agitating the water and to a much more limited extent, to convection currents.
f) Convection
Convection is the process of the transfer of heat by the movement of heated particles themselves. For eg, when water in a beaker is heated by a flame placed below it, that portion of water first heated, expand and rise while the upper, colder, denser portion sink. If the heat supply continues for some time, there are thus set up ascending and descending currents by means of which heat is carried all through the total water mass. This form of heat distribution is known as convection. Most forms of artificial heating of water are of this type.

Convection does occur under the following conditions:
Cooling and sinking of surface water as when the sun sets and under conditions of falling air temperature
a) Entry of colder water from a tributary
b) Cooling of surface water with the passage of autumn into winter
c) Alterations of winds and calm conditions
d) Entry of cooler subterranean water at a high level in the basin
e) Advent of rain in temperate region
f) Cooling of the surface water by evaporation

All the plants and animals have an adaptation to certain range of temperature ie. - 200°C to the boiling point of +100°C. Some can withstand very low temperature for a short duration in an active state and some blue green algae and bacteria living in hot spring (mineral) condition exist at temperature up to 90°C, however they reproduce at a slightly lower temperature.