A Secchi disk is a circular plate divided into quarters painted alternately black and white. The disk is attached to a rope and lowered into the water until it is no longer visible. Secchi disk depth, then, is a measure of water clarity. Higher Secchi readings mean more rope was let out before the disk disappeared from sight and indicates clearer water. Lower readings indicate turbid or colored water.
Clear water lets light penetrate more deeply into the lake than does murky water.
This light allows photosynthesis to occur and oxygen to be produced. The rule of thumb is that light can penetrate to a depth of about 2 - 3 times the Secchi disk depth. Clarity is affected by algae, soil particles, and other materials suspended in the water. Although it is only an indicator, Secchi disk depth is the simplest and one of the most effective tools for estimating a lake's productivity. Reasons for Natural Variation Secchi disk readings vary seasonally with changes in photosynthesis and therefore, algal growth.