A Protocol for Standardizing Secchi Disk Measurements, Including Use of a Viewer Box

ABSTRACT Despite being used by limnologists and oceanographers for over a century, the use of the Secchi disk for estimating water transparency has no standardized protocol. Because of this lack of uniformity, and the inherent optical problems with most current Secchi depth procedures, an appeal is made for production of a standardized protocol for Secchi observations. A possible protocol, including the use of a closed-ended viewer box on the sunny side of the boat, is suggested here. To assist in protocol development, this paper briefly reviews some aspects of optical physics and examines some recent field measurements made by staff of New York City Department of Environmental Protection. These measurements were made using a closed-ended viewer box and are compared with simultaneous naked-eye measurements. The use of a viewer box increases Secchi depth measurement by removing the interfering effects of water surface glare and glitter. A viewer box also increases between-observer precision. Measurements made both with and without the viewer box are slightly greater on the shady side of the boat than on the sunny side; the latter is the preferred side if Secchi data are to be related to other optical properties. The difference between viewer box and naked eye measurements increases with increasing wave height but not with increasing illuminance.