https://cdn.earthdata.nasa.gov/conduit/upload/5182/KeywordsCommunityGuide_Baseline_v1_SIGNED_FINAL.pdf 23.7 2026-04-15T10:53:30.610Z https://gcmd.earthdata.nasa.gov/KeywordViewer/scheme/instruments/c5824666-8c74-404a-9303-c67fc0c4bd8f SECCHI DISKS Date=2021-08-05 User Id=sritz Entity=AltLabel Operation=INSERT System Note=insert AltLabel New Value=id: null category: primary text: SECCHI DISKS language code: en The Secchi (rhymes with etch-e) disk originated with Fr. Pietro AngeloSecchi, an astrophysicist, who was requested to measure transparencyin the Mediterranean Sea by Commander Cialdi, head of the Papal Navy. Secchiwas the scientific advisor to the Pope. Secchi used some white disksto measure the clarity of water in the Mediterranean in April of l865. Varioussizes of disks have been used since that time, but the most frequentlyused disk is an 8 inch diameter metal disk painted in alternate black and whitequadrants.The Secchi disk is used to measure how deep a person can see into thewater. It is lowered into a body of water until the observer losessight of it. The disk is then raised until it reappears. The depth ofthe water where the disk vanishes and reappears is the Secchi diskreading.Even though the Secchi disc measurement of water clarity is anapproximate evaluation of the transparency of water, it is usedprimarily for its simplicity. A more accurate measurement ofunderwater irradiance can be made by the use of photometer.A Secchi Disk measures water clarity. Water clarity may be affected bythree different factors -- algae, sediment and / or water color.One of the major reasons why Secchi disc measurements decrease fromspring to fall, is due to the increase of plankton-- bothphytoplankton and zooplankton. Since zooplankton graze onphytoplankton, (algae i.e.), the Secchi disc readings may increaseduring the summer by the reduction of algae inthe water. Algae blooms may occur when the amount of availablenutrients increases faster than the macrophytes (plants rooted in thebottom of a lake)can absorb them.This summary has been adapted from the MSLA page on'THE SECCHI DISK - WHAT IS IT?'Additional information on Secchi Disk available at'http://www.pollardwater.com/emarket/pages/L0921002disks.asp' 2021-08-05 17:06:46.0