Light penetration in a pond is a) an indicator of pond trophic state, b) the principal origin of heat acquisition and c) a critical resource determining the growth potential of primary producers. As indicated earlier in Figure 9, light is also closely linked to algal biomass, and thus responds indirectly to nutrient supply.
       A portion of the light entering the water column is back-scattered and leaves the pond as light. Most of the light, however, is absorbed by water molecules, particles and dissolved materials and converted to heat. The color of a pond is determined by which wavelengths of light are scattered most and absorbed least. For example, high phytoplankton densities in ponds absorb most red and blue wavelengths for use in photosynthesis, but do not use green light, which is back-scattered to the observer (pond owners in Chester County need to like the color green...)
       Light decreases exponentially with depth as shown in Figure 17. Light penetration is greatly reduced in ponds with abundant algae, suspended sediments or high amounts of dissolved organic substances. The depth to which 1% of light entering the pond penetrates is termed the “compensation depth”. The upper portion of the water column above this depth is considered to have sufficient light to support phytoplankton and aquatic plants. Light levels below the compensation depth are inadequate for most photosynthetic organisms, although tolerance of low light varies with species.

Fig. 17. Diagram of light penetration with depth. High light intensities at the surface (upper right) decline exponentially with increasing depth (shown as an inverted vertical axis). Secchi depth is described below.

       Light penetration can be measured in several ways, but a commonly used approach is to measure the “Secchi depth”. A Secchi disk (Fig. 18) is lowered into the water using a calibrated line to the point where it is just visible from the surface. Normally this Secchi depth receives approximately 15% of incident light (the compensation depth is sometimes assumed to be roughly twice the Secchi depth). A Secchi disk provides less information about light penetration than do more expensive methods, and cannot be used in ponds where Secchi depth exceeds the maximum depth of the pond, but is a convenient, and widely used means of monitoring changes in water quality by homeowners.

Fig. 18. Diagram of a standard Secchi disk, with calibrated line, for measuring light penetration in ponds.

      Percent light depletion (often measured with a quantum meter) is negatively related to Secchi depth (high rates of depletion are associated with shallow Secchi depths). As shown in Figure 19, although light penetration varied widely among the 13 ponds studied during March and May 2002, the ponds consistently experienced more rapid light depletion during July, with Secchi depths often < 1 m. The more rapid depletion of light later in the growing season is related to increased phytoplankton abundance (see Section O).

Fig. 19. Relationship of light depletion (as % decline in light per meter) to Secchi depth in 13 ponds visited in March, May and July 2002. Most ponds fell close to the line of best fit, indicating that Secchi depth could be used effectively to estimate light availability in the water column. Note that in July 80-100% of surface light was typically scattered or absorbed per meter, corresponding to much shallower Secchi depths.