Secchi disk science: Visual optics of natural waters1

The Secchi disk is a circular white disk that is lowered into a natural body of water by a human observer until it disappears from view. The depth of disappearance is a visual measure of the clarity of the water. This review examines the physical and physiological basis of the Secchi disk procedure. The theory of the white disk is detailed to show the underlying assumptions and the consequent strengths and limitations of the procedure. The theory shows how to use a calibrated Secchi disk to predict illuminance levels as a function of depth. In particular it is shown how to predict the euphotic depth of a medium. Ten laws of the Secchi disk are stated verbally and in mathematical form. The laws show how variations in properties of the disk and the surrounding light field affect the depth of disappearance of the disk. Theory and examples lead to the following three main conclusions of this paper: (i) the Secchi disk reading z sd (in meters) yields a quantitative estimate of a single apparent optical property (a + K ) (in meter −1 ) of a natural hydrosol, where a is the (photopic) beam attenuation coefficient and K the (photopic) diffuse attenuation coefficient of the medium; (ii) the primary function of a Secchi disk is to provide a simple visual index of water clarity via z sd or α + K ; (iii) to extend the use of the Secchi disk by auxiliary objective electronic measurements of a or of K, or both, is to risk obviating or abusing this primary function.