Light penetration in water

Light at sediment

The amount of light under water is an important ecological factor for the establishment and growth of macrophyte's. For the establishment by sprouting light at the sediment is an important stimulating factor. When 4% of the surface light reaches the sediment, this is considered sufficient for the germination of aquatic plants.

The percentage light remaining at the sediment after passing the water column can be calculated using the attenuation coefficient and the water depth according to the following formula for the determination of the radiation at depth z:

I_z = I₀ * e^(-K*d)

The percentage of light at the sediment is I_z / I₀ * 100%

where:

• I_z = light at depth z (W/m²)
• I₀ = light below the water surface (W/m²)
• K = attenuation coefficient (m^-1)
• d = water depth (m)

Measured attenuation is tuned to the phytoplankton and aquatic plants sensitive spectrum (PAR: Photosynthetic Active Radiation 400-700 nm). Therefore measured attenuation delivers a more reliable measure of the potential primary production than Secchi depth (visibility). Empirical relationships on primary production and germination of seeds, therefore, can better be based an attenuation coefficient.

That transparency is still widely used is partly due to the lack of measurements of attenuation or reliable calculations on one hand and on the other hand by the fact that Secchi disk measurements are more easily be performed.