Limnology

1. Nature of Inland water
environment

2. Physical Characteristics
• Pressure
• Water is a heavy substance. Pure water weighs 62.4 lb (pounds) per cubic
feet at 4°C.
• This is a direct result of density. Since, density changes with differences
in temperature, compression, substances in solution and substances in
suspension; the weight of a cubic foot of natural water is not always the
same.
• The pressure at any subsurface position is the weight of the
superimposed column of water plus the atmospheric pressure at the
surface.
• As depth increases, the pressure in water is rapidly become great, so that
ultimately a crushing effect is imposed upon objects submerged to
considerable depths. This collapse under pressure is called implosion.
• The pressure change in lakes and reservoirs are very small than
compared to sea. In lake, having maximum depth of 100 ft., the pressure
in the deepest region is about 58 lb. per sq. in.

3. Density
• Some of the most remarkable phenomena in Limnology
are dependent upon density relations in water. The
density of water depends on the quantity of dissolved
substances, the temperature and the pressure. With
increasing amounts of dissolved solids the density
increases in a roughly linear fashion.
• The quantity of dissolved solids for inland waters is
usually below 1 g / l, except, for mineral waters (springs)
inland salt water bodies, and water bodies subjected to
marine influence.
• The density difference due to chemical factors is not
more than 0.85 g /l and the density differences occurring
in different zones of the same water body are usually an
order of magnitude less than this.

4. • i) Variations due to pressure
• Water at the surface, subject to a pressure of only 1
atmosphere, is considered as having a density of unity (1.0); at a
pressure of 10 atmospheres, the density is about 1.0005; at 20
atmospheres, the density is about 1.001; and at 30
atmospheres, it is about 1.0015.
• ii) Variations due to Temperature
• Pure water forms ice at 0°C, and steam at 100°C, but there is
change in the density of the liquid due to temperature. Water
possesses the unique quality of having its maximum density at
4°C and it becomes less dense when the temperature decreases
from 4°C to freezing point. Density of water will be less during
summer and it will be high during winter. Sea water becomes
heavier at 0°C. The temperature of maximum density of sea
water is 0°C, where as for fresh water it is 4°C.

5. Mobility (Viscosity)
• Water is an exceedingly mobile liquid. Nevertheless, it
has internal friction (viscosity). This viscosity varies with
the temperature. Water is distinctly more mobile at
ordinary summer temperatures than that are just before
it freezes.
• The viscosity changes with temperature. The response of
water to wind of fixed velocity would differ with different
temperature of the water. Pressure does not cause any
significant change in viscosity.

6. 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.

7. • 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.

8. 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.

9. • 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.

10. • 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.