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| [[Image:Secchi disk pattern.svg|thumb|right|Secchi disk pattern]]
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| The '''Secchi disk''', created in 1865 by [[Angelo Secchi|Pietro Angelo Secchi SJ]], is a circular disk used to measure water [[transparency (optics)|transparency]] in [[ocean]]s and [[lake]]s. The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the pattern on the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the '''Secchi depth''' and is related to water [[turbidity]].
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| ==History==
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| [[George C. Whipple]] modified the original all-white Secchi disk<ref> Cialdi, M. and Secchi, P. A. (1865). “Sur la Transparence de la Mer.” ''Comptes Rendu de l'Acadamie des Sciences.'' 61: 100–104.</ref> to “…a disc about 8 inches in diameter, divided into quadrants painted alternately black and white like the target of a level-rod…” <ref name=Microscopy>Whipple, George C. (1899). ''The Microscopy of Drinking-Water.'' New York: John Wiley & Sons, 73–5.</ref> The black and white Secchi disk is the standard disk currently used in [[limnology]] investigations and marine water quality studies. <ref>[http://www.mlswa.org/secchi.htm The Secchi Disk—What Is It?] Accessed 2012-07-05.</ref><ref>[http://www.secchidipin.org/black_and_white.htm Why a Black and White Secchi Disk?] Accessed 2012-07-05.</ref>
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| ==Secchi depth==
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| [[Image:Secchi disks.svg|thumb|right|Different kinds of Secchi disks]]
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| The Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water. This depth in metres divided into 1.7 yields an '''attenuation coefficient''' (also called an '''extinction coefficient'''), for the available light averaged over the Secchi disk depth. The light attenuation coefficient, k, can then be used in a form of the [[Beer–Lambert law]],
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| :<math>{I_z \over I_0} = e^{-kz}</math>
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| to estimate ''I''<sub>''z''</sub>, the intensity of light at depth ''z'' from ''I''<sub>0</sub>, the intensity of light at the ocean surface.<ref>[[Sherwood B. Idso|Idso, Sherwood B.]] and Gilbert, R. Gene (1974) [http://www.jstor.org/pss/2402029 ''On the Universality of the Poole and Atkins Secchi Disk: Light Extinction Equation''] British Ecological Society.</ref>
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| The Secchi disk readings do not provide an exact measure of transparency, as there can be errors because of the sun's glare on the water, or one person may see the disk at one depth, but another person with better eyesight may see it at a greater depth. However, it is an inexpensive and straightforward method of measuring water clarity. Because of the potential for variation between practitioners, methods should be standardized as much as possible.
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| A Secchi disk measurement should always be taken off the shady side of a boat or dock between 9 am and 3 pm<ref>Lind, Owen, T. (1979). ''Handbook of Common Methods in Limnology'' St. Louis:C.V. Mosby Co.</ref> The period for best results is between 10 am and 2 pm. The same observer should take Secchi depth measurements in the same manner every time. One can approach the measurement by lowering the disk beyond a point of disappearance, then raising it and lowering it slightly to set the Secchi depth. Another method is to record the depth at which the disk disappears, lower another few feet, then record the depth at which the disk reappears as it is slowly brought up. The Secchi depth is taken as the average of the two values.<ref>Cole, Gerald A. (1994). Textbook of Limnology. 4th ed. Prospect Heights:Waveland Press Inc.</ref>
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| Secchi disk measurements have been an integral component of [[Minnesota]]'s lake water quality assessment programs for some time; lake residents make periodic measurements and submit their readings to state and local agencies. The aggregated longitudinal data are used to reveal general trends in water quality. Similarly, the Indiana Clean Lakes Program trains and relies on volunteers to monitor turbidity in over 80 Indiana lakes using secchi disks, and uses data submitted by volunteers to monitor lake quality in the state.<ref>{{cite web|title=Indiana Clean Lakes Program – Volunteer Monitoring|url=http://www.indiana.edu/~clp/volunteermonitoring.php|accessdate=9 February 2011}}</ref>
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| Secchi disk measurements do not indicate how attenuation changes with depth or particular wavelengths of light. Submarine [[photometer]]s can operate at depths of 150 m and can record visible, ultraviolet and infrared parts of the spectrum. [[Nephelometer|Turbidimeter]]s have their own light source and can measure transparency with scientific accuracy.<ref>[http://www.britannica.com/EBchecked/topic/614246/undersea-exploration/57193/Collection-of-biological-samples#ref520244 ''Undersea exploration.''] (2008). In Encyclopædia Britannica. Retrieved October 01, 2008, from Encyclopædia Britannica Online.</ref>
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| ==See also==
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| * [[Trophic state index]]
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| ==References==
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| {{reflist}}
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| ==Other readings==
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| * Preisendorfer, R. W. (1986). "Secchi disk science: Visual optics of natural waters," ''Limnol. Oceanogr.'' 31: 909–26
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| * Hou, Weilin, et al. (2007). [http://www.opticsexpress.org/viewmedia.cfm?id=131137&seq=0 "Why does the Secchi disk disappear? An imaging perspective"], ''Opt. Express.'' 15: 2791–2802.
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| ==External links==
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| * [http://www.eoearth.org/article/Secchi_disk Leszek Bledzki (Lead Author);Nidhi Nagabhatla (Topic Editor) "Secchi disk". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth October 15, 2009; Last revised Date October 5, 2010; Retrieved January 6, 2012 <http://www.eoearth.org/article/Secchi_disk>]
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| * [http://www.webapps.cee.vt.edu/ewr/environmental/teach/smprimer/secchi/secchi.html Secchi Disk Transparency]
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| * [http://www.epa.gov/volunteer/stream/155.html EPA OWOW]
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| * [http://www.mainevolunteerlakemonitors.org/recertify/disk.php Secchi disk simulator]
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| * [http://www.mlswa.org/secchi.htm The Secchi disk – what is it?]
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| * [http://www.secchidipin.org/secchi.htm The Secchi Dip-In]
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| * [http://www.secchidisk.org Secchi App – The global scientific experiment by seafarers to study the marine phytoplankton]
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| {{DEFAULTSORT:Secchi Disk}}
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| [[Category:Oceanographic instrumentation]]
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| [[Category:Fisheries science]]
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