Light climate under water

• turbidity describes the clarity of water and is measured using Secchi depths (round disk dropped in water until just visible giving the depth of turbidity)
• turbidity is due to resuspension of particles that scatter light caused by wind and currents and biota (benthivorous fish)
• light intensity decreases with depth (light attenuation)
• light attenuation occurs due to scattering, absorption, and reflection of light and depends on the wavelength of light used by photosynthesis (photosynthetically active radiation - PAR)
• Law of Lambert-Beer: Iz = I0 * e ^(-E*z)

  Iz = radiation at depth z

  I0 = radiation at surface

  z = depth

  E = extinction coefficient
• extinction coefficient (E) for pure water is 0.1-0.2/m, for a clear lake is 0.5-1/m and for algal bloom it is 3-10/m
• a high extinction coefficient results in a low secchi depth