Secchi disk science : Visual optics of natural waters ]

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 zs,, (in meters) yields a quantitative estimate of a single apparent optical property (CX + K) (in meter-‘) of a natural hydrosol, where (Y 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,, or a! + 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. The Secchi disk is a device used to visually measure the clarity of natural waters. It is usually a circular white disk of 30-cm diameter that is lowered vertically into the water, disk plane horizontal, until it disappears from sight. The depth of disappearance of the disk is inversely proportional to the average amount of organic and inorganic materials along the path of sight in the water. This technique, systematically studied by the Italian physicist Angelo Secchi (1866) is, interestingly, still in use today. It is one of the few instruments remaining in the armory of modern science for which the visual sense of the human operator is an integral part of the measurement pro* Contribution 836 from NOAA/Pacific Marine Environmental Laboratory. cedure. Some historical background is given by Sauberer and Ruttner (1941). This review examines the physical and physiological basis of the Secchi disk procedure. The theory of the white disk is detailed and the various laws of its operation, informally discovered over the years, are given quantitative expression. A sensitivity analysis of Secchi depths as a function of environmental parameters is made. In this way a potential user of the technique can balance its various subjective disadvantages against its simple and inexpensive operation. The Secchi disk procedure is valued by many aquatic biologists as a useful and informal visual index of the trophic activity of a lake or oceanic region. The accumulated listings of the depth of disappearance of the