Limnology

3.5.1 Physical characteristics- Light, Color, Turbidity

Unit 3- Nature of Inland water environment

3.5.1. Physical characteristics- Light, Colour, Turbidity
Light
Light influences freshwater ecosystems greatly. Fresh waters contain more of suspended materials. These suspended materials obstruct the light that penetration reaches the water. The degree of such obstruction of light influences the productivity of the freshwater ecosystem. A shallow lake receives light to its very bottom resulting in an abundant growth of vegetation both phytoplankton and rooted vascular plants. Light affect the orientation and changes in position of attached species and their nature of growth and it also causes the diurnal migration of planktonic organisms. The factors affecting the light penetration in natural waters are the intensity at the surface, angle of contact of light with surface, differences in latitude, seasonal differences, diurnal differences and suspended materials.
The light intensity at which oxygen production by photosynthesis and oxygen consumption by the respiration of the plants concerned are equal is known as the compensation point, and the depth at which the compensation point occurs is called the compensation depth.
Light exerts a great influence on many biological process of water. Most important future of water is its transparency. This fluctuates in different seasons and water bodies such as flooding livers, mountain streams etc. The source of light on the earth - a) Sun and b) Moon
Electromagnetic spectrum emitted by Sun (a) short gama rays (0.0001 mm) to (b) long Hertizan waves (several km long). The Hertizan waves are the electromagnetic waves used in radio and it is pronounced as Hertz.
Intensity of light is the number of quanta passing through on a unit area, ie, light energy and the unit of expression of light intensity is ‘Lux’
Wave length is the measure of light colour
nm = nanometer = mille micron 10-9
AO = 1/ton billionth of a meter
nm = 1/billionth of a meter or 10AO
Intense radiation is restricted to 300 to 1300 nm. Peak radiation distribution is in the blue green range.
Wave length heating water is 0.1 to 770 nm (infra red spectrum).
In a year the amount of radiant energy that reaches earth from the sun is 1.3x1021 k cal
Visible wave length/light : 400 to 770 nm; Ultraviolet light >286 to 400 nm
Light penetration in natural waters is affected by
a) Dissolved substances
b) Suspended substances
c) Planktonic organisms
d) Geographical features (latitude and longitude etc)
e) Meteorological conditions
f) Angle of light etc.
Methods for estimation
a) Secchi’s disc

Secchi (1865), an Italian professor employed a metallic disc for measuring the transparency of waters of Mediterranean sea. It considered in lowering into the water a white metallic disc of 20 cm in diameter, on a graduated rope, noting / recording the depth at which the disc disappeared then lifting the dosc and noting the depth at which it reappeared. The average of these two readings was considered the limit of viability or Secchi disc depth. This method was used subsequently by many investigators. Whipple modified this method by dividing the disc into four quadrants and paintings them in such a way that two of the quadrants which were directly opposite to each other, black and intervening ones white. He also increased the efficiency of the method by viewing the disc, as it sank in the water through a water telescope held under the sun shade.

This method is not actual measure of light penetration, but instead merely a useful rough index of visibility when used under standard conditions. They are (a) Clear sky (b) Sun above the head (preferably) (c) Shaded or protected side of the boat (d)) Under a sun shade. This method has come into a wide use as a means of comparing different waters.
Factors influencing the light penetration
1) Intensity of light at surface
This varies (a) degree of clarity of sky (b) presence of fog, dust, smoke etc and (c) time of the day/season of the year.
2) Angle of contact with surface
Light in contact with surface part of it is reflected rest enters water and becomes refracted. Penetration depends on angle of contact and maximum penetration when sun is at zenith.
3) Different in latitude
More remote the water mass is from equator, greater will be the departure of sun’s rays from vertical and hence penetration varies.
4) Seasonal differences
Closely associated with latitude are the seasonal changes in the position of the sun. Only locations at or between 23° 28i N and 23° 28i S (Tropic of cancer and Tropic of Capricorn) ever have a vertical sum. Beyond this zone, north or south not only do locations have on regular sun but the angle changes progressively with change of seasons.
5) Diurnal difference
Angle of light in contact with water is ever changing during day, reaches zenith at noon.
6) Dissolved materials
One of the important factors is absorbance of light which varies with chemical substances such as (a) chloride of Ca and Mg affect light penetration ie. Diminishes, (b) Traces of NH₃ proteins, nitrate, carbohydrates etc reduces the light penetration with respect to ultraviolet rays.
7) Suspended materials
Silt, clay etc. are effectively screen light and also the penetrations of light reduce by phytoplankton and zooplankton.
Penetration of light in pure water
When light penetrates or enters into pure water (a) certain portion of light is absorbed and (b) some of it is scattered in the form of deflection in all directions.
Absorption is selective in which certain wave lengths are absorbed more quickly than others.
Penetration of light in natural waters
Every quantitative determination records were only in marine waters probably because of more clarity. Here photographic plate method used by Forel (1865) in lake Geneva at about 200 m.