Secchi disk

Secchi disk

The Secchi disk, created in 1865 by Pietro Angelo Secchi, is a circular disk used to measure water transparency in oceans and lakes. 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.

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,:{I_{z}over I_{0 = 10^{-kz}to estimate "I" z, the intensity of light at depth z from "I" 0, the intensity of light at the ocean surface. [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.]

Secchi disk readings do not provide an exact measure of transparency, as there can be errors due to the sun's glare on the water, or one person may see the disk at one depth, but another, with better eyesight, may see it at a greater depth. However a Secchi disk 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.

A Secchi disk measurement should always be taken off the shady side of a boat or dock between 9 a.m. and 3 p.m. (Lind, 1979). According to Cole (1994), 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.

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.

Secchi disk measurements give no indication how the attenuation changes with depth or specific wavelengths of light. A submarine photometer can be used to depths of about 150 metres and can directly record the infrared, visible, and ultraviolet portions of the spectrum. Scientifically accurate measurements of turbidity are performed using a turbidimeter. This instrument measures finer gradations of transparency, and has a self-contained light source which can be used at any time of night or day. [ [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.]

Notes

References

*Cole, Gerald A. (1994). Textbook of Limnology. 4th ed. Waveland Press Inc., Prospect Heights
*Lind, Owen, T. (1979). Handbook of Common Methods in Limnology. C.V. Mosby Co., St. Louis.
*Preisendorfer, R. W. (1986). "Secchi disk science: Visual optics of natural waters," "Limnol. Oceanogr." 31, 909-926
*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

External links

* [http://www.algalpyramid.com/secchi/ Electronic Secchi handheld]
* [http://ewr.cee.vt.edu/environmental/teach/smprimer/secchi/secchi.html Secchi Disk Transparency]
* [http://www.epa.gov/volunteer/stream/155.html EPA OWOW]
* [http://www.mainevolunteerlakemonitors.org/recertify/disk.php Secchi disk simulator]
* [http://www.mlswa.org/secchi.htm The Secchi disk – what is it?]
* [http://dipin.kent.edu/secchi.htm The Secchi Dip-In]


Wikimedia Foundation. 2010.

Игры ⚽ Нужно сделать НИР?

Look at other dictionaries:

  • Secchi disk — or Secchi disc [sek′ē] n. a circular, white or colored disk lowered into a body of water to estimate the clarity of the water by measuring the depth at which it disappears * * * …   Universalium

  • Secchi disk — or Secchi disc [sek′ē] n. a circular, white or colored disk lowered into a body of water to estimate the clarity of the water by measuring the depth at which it disappears …   English World dictionary

  • Secchi — may refer to:* Angelo Secchi, an Italian astronomer * Secchi (lunar crater) * Secchi (crater on Mars) * Secchi disk, an oceanographic/limnological instrument * SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation), an instrument… …   Wikipedia

  • Secchi disc — /ˈsɛki dɪsk/ (say sekee disk) noun a disc used to gauge water clarity by measuring the depth at which it is no longer visible from the surface. {created in 1865 by Pietro Angelo Secchi, Italian astronomer, 1818–78} …  

  • Angelo Secchi — Infobox Scientist name = Angelo Secchi box width = image width =150px caption = Angelo Secchi birth date = 29 June 1818 birth place = Reggio Emilia death date = 26 February 1878 death place = residence = Rome citizenship = nationality = Italian… …   Wikipedia

  • Secchidisk — Secchi disk n. A disk, divided into black and white quarters, used to gauge water clarity by measuring the depth at which it is no longer visible from the surface.   [After Pietro AngeloSecchi (1818 1878), Italian astronomer hired to test such a… …   Universalium

  • Turbidity — standards of 5, 50, and 500 NTU Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key… …   Wikipedia

  • Trophic state index — The quantities of nitrogen, phosphorus, and other biologically useful nutrients are the primary determinants of a body of water s trophic state index (TSI). Nutrients such as nitrogen and phosphorus tend to be limiting resources in standing water …   Wikipedia

  • Lake — For other uses, see Lake (disambiguation). Oeschinen Lake in the Swiss Alps A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the… …   Wikipedia

  • undersea exploration — Introduction       the investigation and description of the ocean waters and the seafloor and of the Earth beneath. Primary objectives and accomplishments       Included in the scope of undersea exploration are the physical and chemical… …   Universalium

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”