FY 1986

Eyeball optics of natural waters: Secchi disk science

Preisendorfer, R.W.

NOAA Tech. Memo. ERL PMEL-67, NTIS: PB86-224060/226602, 90 pp (1986)

The Secchi disk is used to visually measure the clarity of natural waters such as lakes and seas. It is usually a white disk of 30 cm diameter that is lowered into the water until it disappears from sight. The depth of disappearance is called the Secchi depth of the medium, and is a measure of the amount of organic and inorganic materials in the water along the path of sight. This is a technique that was systematically studied among others by the Italian physicist Angelo Secchi in 1865. It is, amazingly, still in use today by environmentalists and laid-back professors of marine and lacustrine biology, wishing to give an easily made and visualized index of the trophic state of natural hydrosols. 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 loop. This review examines the physiological and physical basis of the Secchi disk procedure. The theory of the white disk is detailed in the hope that the truth about its subjective shortcomings, once revealed, will set the inveterate Secchi disker free. On the other hand, for those die-hard Secchi opticians who will persist in its use, the subjective disadvantages of the technique are balanced against the advantages of its simple and inexpensive operation. In particular, the bases for updating the Secchi disk parameters are derived. However, this is not to be construed as a blanket endorsement of the use of the disk. The only legitimate use of the disk is as a visual gauge of the clarity of natural waters. It is not to be unthinkingly linked with the apparent and inherent optical properties of such media; it is not to be used as a substitute for objective measurements of the optical properties of natural hydrosols. Were the great physicist Secchi a witness today, he would probably be delighted by the great advances in the spectroscopic study of natural hydrosols, using modern electronic devices. One can then imagine his reaction to the incongruity of the sight of his disk in all solemnity being lowered into a sea or a lake by someone wearing a quartz-driven, radio/television, computer-chip wristwatch, while speaking into the watch's microphone in satellite communication with his general purpose real-time data-processing computer at his home laboratory half a planet away.