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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.

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LIGHT IN LAKES

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  1. LIGHT IN LAKES

  2. 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

  3. Electromagnetic spectrum (based on frequency and wavelength)

  4. 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

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

  6. Two important factors influencing light impinging on lake • Distance from sun • The angle of light rays reaching the water surface

  7. 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

  8. Radiation comes in 2 forms • Direct • Indirect • Direct radiation influenced by • Latitude • Season • Time of the day • Altitude • Weather conditions

  9. 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)

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

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