LIGHT IN LAKES

LIGHT IN LAKES

• Source of energy for biota of lakes• Influences physical properties of lakes– Thermal structure– Density– Water movement

• Influences water chemistry of lakes– Reaction rates– Photodegradation– Biotic activity related to nutrient cycling

• Electromagnetic spectrum (based on frequency and wavelength)

• In aquatic ecosystems, solar radiation ranges from infrared radiation (>3000 nm) to ultraviolet radiation (<100 nm)

• Plants absorb light energy in the range from 400 nm -700 nm (this range is known as Photosynthetically Active Radiation - PAR)

• Light is absorbed and scattered as it passes through the atmosphere– UV radiation is absorbed by O3and O2

– IR radiation is absorbed by H2O, O3, and CO2

• Light is really waves and particles– Packets of energy - quanta or photons– Light is a transverse wave of energy– Photon carries energy

Two important factors influencing light impinging on lake

• Distance from sun• The angle of light rays reaching the water

surface

• Equator– Perpendicular– Constant input throughout the year

• Polar region/Temperate– Varies according to season– Time of the day– 1/3 of the year 0 input

• Radiation comes in 2 forms– Direct– Indirect

• Direct radiation influenced by– Latitude– Season– Time of the day– Altitude– Weather conditions

• Indirect radiation:– Scattering when passes through atmosphere– Scattering is high for radiation with short

wavelength (high frequency)– Blue is highest– Depends on distance from sun (distance travelled)

• Distribution of radiation on lake surface– Not all penetrate– A significant portion is reflected

• Amount reflected depends on:– Angle of incidence– Surface characteristics of water– Surrounding topography– Meteorological conditions

• In the water, light entering the lake will be absorbed by:– Dissolved substances in the water– Suspensoids: organic and inorganic materials in

the form of suspended particles– A significant portion is reflected

• Scattering depends on:– Depth– Season– Location in the lake

• Absorption in lake– Longer radiation (IR) will be absorbed more– Diminution of radiant energy with depth (by

scattering and absorption), is referred to as light attenuation

• To describe percentage of transmission or absorption, Birgean percentile absorption is used i.e. based on the expression:

= 100 (Io – Iz) Io

where Io = irradiance at the lake surface Iz = irradiance at depth z

• In general, 53 percent of light energy (mainly infrared) is absorbed in the first meter

• Organic matter, especially humus play a role in absorbing heat