A simple instrument used to measure the transparency of surface water bodies such as lakes and reservoirs. A Secchi disk is a metal, plastic, or wooden disk that is 8–12 in (20–30 cm) in diameter. It may be white or have alternating black and white quadrants. A long calibrated rope is attached to the center of the disk. Researchers use the disk by lowering it into the water, usually from the shaded side of a boat, and measuring the depth at which it just disappears from sight. The disk is then lowered a little more and then raised until it reappears. The average of the depths at which the disk disappears and then reappears is known as the Secchi depth. The amount of sediment, algae, and other solids in the water affects clarity and depth of light penetration and thus, Secchi depth. Scientists use this measure of transparency as a simple indication of water quality, often by comparing Secchi depth measurements over a period of time to track changes in the clarity of a particular water body.

Testing water clarity with a secchi disk. (Photograph by W.A. Banaszewski. Visuals Unlimited. Reproduced by permission.)