Limnology (2+1)

3.5.2 Physical characteristics- Colour and Tubidity

Unit 3- Nature of Inland water environment

3.5.2. Physical characteristics- Colour and Turbidity
Colour
Pure water bodies appear nearly black as they absorb all light components of the spectrum. The lake water containing suspended materials is seen blue in colour due to the scattering of light by water molecules. Natural waters differ greatly in colour, depending upon the materials dissolved and suspended in it.
It is a common misconception that in large water bodies, such as the oceans, the water color is blue due to the reflections from the sky on its surface. Reflection of light off the surface of water only contributes significantly when the water surface is extremely still, ie, mirror like, and the angle of incidence is high, as water's reflectivity rapidly approaches near total reflection under these circumstances. Some constituents of sea water can influence the shade of blue of the ocean and hence it can look greener or bluer in different areas.
Scattering from suspended particles also plays an important role in the color of lakes and oceans. A few tens of meters of water will absorb all light, so without scattering, all bodies of water would appear black. Because, most lakes and oceans contain suspended living matter and mineral particles as coloured dissolved organic matter (CDOM) and thus the light from above is reflected upwards. Scattering from suspended particles would normally give a white color, as with snow, but because the light first passes through many meters and the scattered light appears blue. In extremely pure water as is found in mountain lakes, where scattering from white coloured particles is missing, the scattering from water molecules themselves also contributes a blue color.

Turbidity
Degree of opaqueness developed in water by means of suspended water is known as turbidity. Turbidity producing substances may be divided into two groups.
i) Settling suspended matters – those substances which in motionless water, will settle to the bottom sooner or later.
ii)Non-settling suspended matters - Finely divided solids or those materials whose specific gravity is less than water which are in permanent.
The settling of particulate materials is by no means at a uniform rate, particularly in deeper lake having considerable difference in temperature between the surface and the bottom layers.
Effects of materials in suspension
a)Light reduction: Favourable for animals but unfavourable for plants (photosynthesis)
b)Effects of temperature: Turbid waters are warmer than clear waters. Suspended particles absorb heat more rapidly than water itself and then radiate the heat to the surrounding water, adding to the heat content of the water.