What Does a Secchi Disk Depth Tell Us?
Secchi Disc Depths: The Secchi disc is a simple but robust method to measure water transparency (shown left). The disc is attached to a rope and slowly lowered through the water until it just disappears from view. This depth is noted. It is lowered some more, then slowly raised until the disc is just observed. This depth is also noted. The Secchi depth is the average of these two depths. The depth is proportional to water transparency and inversely proportional to water turbidity, but it is also influenced by cloudiness, position of the sun, roughness of the water, and observer bias. Plankton biomass and suspended mud particles are the main contributors to water turbidity. In Seneca Lake, the depth is primarily a function of plankton biomass. Chlorophyll a is an indicator of plankton biomass and productivity in a lake. Chart 1 below shows the inverse relationship between secchi disc depths and chlorophyll a concentrations in Seneca Lake. As the amount of chlorophyll declines, the secch
