Limnology (2+1)

3.2.1. Physical Characteristics- Pressure, Compressibility, Density

Unit 3- Nature of Inland water environment

3.2.1. 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. (4 atmospheres).
Compressibility
Water is virtually incompressible. The coefficient of compressibility for each atmosphere of pressure is usually given as 52.5 x 10- 6 at 0°C for pressures of 1 to 25 atmospheres. Lake Superior waters, suddenly rendered absolutely incompressible, would rise in level about 23 cm and an ordinary inland lake with the maximum depth of 100 ft. under the same circumstances, would rise about 0.25 mm. Since, increasing pressure compresses the water, thereby increasing its density to the same slight extent, objects sink in water of uniform temperature at essentially the same rate at all levels.
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.
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.

iii) Changes due to dissolved substances
The total amount of dissolved substances in freshwater is less than that in sea water. Such substances usually increase the density of water, the amount of increase depending upon the concentration of dissolved materials and their specific gravity. Evaporation increases the density by concentrating the dissolved materials and the dilution reduces the density.
iv) Changes due to substances in suspension
All waters contain some suspended particulate matter. The quantity and quality of these substances vary greatly in different waters and at different times. Silt and certain other materials are heavier than water and thus increase its weight and other material may have a specific gravity similar to that of water and cause no significant change in weight. Density currents and related phenomena may be caused by substances in suspension.