A relative estimate of the phosphate concentration of a lake can be made using a Secchi disk to determine water clarity. The disk is lowered into the water and the depth at which it is no longer visible is recorded. Light extinction depth is determined and helps to estimate the microscopic plant and animal life supported by the water. This standard 20 cm diameter limnological disk is black and white on one side, white on the other, and constructed of durable ABS plastic. Supplied with integral weight and 48' (15m) of 1/8" nylon cord.