Secchi Depth

Secchi depth is the depth at which a white or black-and-white Secchi disk, lowered into the water, just disappears from view, a simple measure of water transparency in meters. It is inversely related to the diffuse attenuation coefficient and to concentrations of phytoplankton, suspended sediment, and dissolved color. Clear open-ocean gyre water can reach Secchi depths above 40 meters, while turbid coastal and estuarine water drops below 1 meter. The euphotic depth is roughly 2 to 3 times the Secchi depth. It is one of the longest oceanographic time series, used since 1865, and now complements satellite ocean-color retrievals.