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History of the Secchi Disk

A Secchi disk is a circular metal plate attached to a calibrated rope. It is probably the most inexpensive and easy to use tool in water quality monitoring. One of the best aspects of the Secchi disk is that the information provided by the Secchi disk is easily interpreted by volunteers and can be used to detect water quality trends in lakes.

The Secchi disk is named after Fr. Pietro Angelo Secchi (Figure 1), scientific advisor to the Pope and astrophysicist. Secchi was asked by Commander Cialdi, head of the Papal Navy, to measure the transparency in the Mediterranean Sea. The first disk was lowered from the papal yacht, l'immacolata Concezion (Immaculate Conception), on April 20, 1865 (Carlson and Simpson, 1996). There have been many revisions to the first disks used by Secchi in terms of size and color. The two most common colors variations in use today are the all-white disk and black and white quaderant version disk (Figure 2). In Minnesota, we use an all-white, 8 inch diameter metal disk with notched sides for rope storage when the disk is not in use (Figure 2).

Figure 2. Secchi Disk Color and Style Varieties

For additional information on Pietro Angelo Secchi, check out the following Web sites:

http://www.hao.ucar.edu/public/education/sp/images/secchi.html
http://www.newadvent.org/cathen/13669a.htm

Reference: Carlson, R.E. and Simpson, J. 1996. A Coordinators Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society, Madison, Wisconsin.