Secchi disk

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Secchi disk description


caption Secchi disk ready to use. (Photograph by Leszek Bledzki)


The Secchi disk is a device used to measure water transparency in all kinds of open waters (ponds, lakes, reservoirs, bays, oceans). The Secchi disk is an 8-inch (20 centimeter) white disk or with alternating black and white equal quadrants. It is lowered into the water of a lake or other water body until it can be no longer seen by the observer. This depth of disappearance, called the Secchi depth, Secchi disk reading, Secchi disk visibility or Secchi disk transparency (SDT), is a conventional measure of the transparency of the water. For the measurement of turbidity, the disk is lowered into the open water by unwinding the waterproof tape or calibrated line to which the disk is attached and until the observer loses sight of the disk. The disk is then raised until it reappears. The depth, measured from the water surface to the level where the disk vanishes and reappears, is the Secchi disk reading. Also, important is the orientation to the sun, which should be on our back while taking the measurement. To achieve the best result, the measurement should be taken off the shady side of a boat in the shadow of our own head lowered on close to the water surface. The original Secchi's disk was all-white. Today most disks used in freshwater bodies have alternating black and white quadrants, while marine disks are usually all-white. The reasons for the difference is not clear and may be more historical than theoretical. Early disks of the 19th century were all white, but George Whipple (1899) stated that a disk with alternating black and white quadrants was more easily seen and "Whipple's" disk became the standard in freshwater situations. The newest trend in disk color is the all-black disk. Developed and used in New Zealand, the disk has the advantage that it can be used in shallow rivers and streams, because the black disk does not require much water depth.

Value in modern limnology


caption Secchi disk just below the surface. (Photograph by Leszek Bledzki)


The Secchi disk remains one of the most common, inexpensive and efficient tools in the study of limnology. Regarded as an accepted method of measuring the transparency for water quality studies to assess the trophic status of surface waters, especially for various models, it is used widely. Several factors influence on the Secchi reading. The most important are some personal features such as the eyesight and age of the viewer as well as his/her previous experience, the distance of the observer from the water surface, cloudiness and other weather conditions, the height of the sun on the horizon (the time of day the readings are taken, most preferred between 10 a.m. and 2 p.m.), the reflectance and the diameter of the disc, the color of the water and reflection of light (the optics) at the water surface, clay particles or other suspended (organic and inorganic) materials in the water which are related to water turbidity. Scientifically, the depth of transparency is the path length in the Beer-Lambert's law equation through which light is scattered and absorbed as a function of the concentration of the particles in the water. The Secchi disk transparency is also a function of the reflection of light from the water's surface. Although a general parabolic relationship exists between dissolved organic matter and transparency, theoretical analyses and empirical observations have shown that the Secchi disk depth is more associated with the particulate suspended matter than dissolved organics. The particulate suspended matter may be algae or other suspended solids. The Secchi disk depth is a noticable guide to the euphotic depth that marks the lower bounds of the layer in which net photosynthetic production is possible and where the growth of water plants becomes limited by lack of light. It is estimated, that euphotic depth corresponds with about 1% of full daylight. Euphotic depth is usually supposed to approximate to 2.0-2.5 meters (m) × Secchi disk depth, in the very clear oligortophic lakes, the euphotic zone may extended to 30-50 m depth. Transparency (measured also with Secchi disk) is the reciprocal to the underwater attenuation of photosynthetically active radiation (PAR). Secchi disk has been widely accepted as the efficient tool for water pollution studies.


Measurements of Secchi disk depth have been used in models of lake eutrophication and to estimate throphic state (TSI – trophic state index) of lakes and were also related to plankton density, biomass and algal production, distribution depth of aquatic macrophites (plants). A detailed description of the application of the Secchi depth to models, as well as the relation (regression equations) of the Secchi disk and several limnological parameters (i.e.: pH, total phosphorus concentration, lake area) are given by Håkanson & Peters (1995). Aarup (2002) retrieved 40 829 Secchi depth measurements from the North Sea and Baltic Sea collected since 1902 from existing international archives including the ICES Oceanographic Data Center in Denmark, the World Ocean Data Center in the USA, literature, and institutes that measure Secchi depth on a routine basis. He showed the gradual change in Secchi depth observed between the clear water in the northern North Sea and the more brackish northern Baltic Sea.


caption Secchi disk lowered into the water. (Photograph by Leszek Bledzki)


Secchi depth values can range anywhere from a few centimeters in a very turbid waters to over 40 m in a clear lakes, but mostly observed is the range of 2 to 10 m; seasonal fluctuations occur in response to seasonal fluctuation of concentration of algae, zooplankton and other suspended solids. Scientifically accurate measurements of turbidity are performed using a nephelometer.


Deepest Recorded Secchi Depth

  • 80 m on October 13, 1986 in the Weddell Sea, near Antarctica
  • 53 m in the eastern Mediterranean
  • 42.4 m and 52.8 m was obtained in Crater Lake (Oregon, USA), using a 20 cm and a 100 cm  diameter disks, respectively. The maximum Secchi readings (average between descending and ascending) between 1896 and 2008. On average the 100 cm disk was observed 7 m deeper than the 20 cm disk on the same dates. The Crater Lake disc visibility data were provided by Dr. Mark Buktenica, the Aquatic Ecologist from the [Crater Lake National Park]. 


A famous Italian astronomer and one of the first astrophysicists, Father Pietro Angelo Secchi (1818-1878), who was a scientific adviser to the Pope, used the Secchi disk in 1865. Father Secchi was asked by Commander Cialdi, head of the Papal Navy, to test a new transparency instrument. This instrument, now named the Secchi disk, was first lowered from the papal steam yacht, l'Immacolata Concezione (The Immaculate Conception) in the Mediterranean Sea on April 20, 1865.

Great American Secchi Dip-In Program

Secchi disk measurements as the lake water quality assessment program have been an integral component of the Great American Secchi Dip-In, the volunteer monitoring program (run by the Department of Biological Sciences, Kent State University, Ohio, USA). Since 1994, lake residents all over the United States and Canada make periodic (on one day during the weeks surrounding Canada Day and July Fourth) measurements and submit their readings to state and local agencies. The aggregated longitudinal data are used to reveal general trends in water quality.

References and further reading

  • Aarup, T., 2002. Transparency of the North Sea and Baltic Sea - a Secchi depth data mining study., Oceanologia 44: 323-337
  • Almazan, G., Boyd, C.E. 1978. An evaluation of Secchi disc visibility for estimating plankton density in fish ponds. Hydrobiologia, 61, 205-208.
  • Arnone R.A., 1985, Coastal Secchi Depth Atlas, Naval Ocean Research and Development Activity, NSTL, Mississippi 39529, NORDA Rep. No. 83.
  • Berman, T., P.D. Walline, A. Schneller, J. Rothenberg, and D.W. Townsend. 1985. Secchi disk depth record: a claim for the eastern Mediterranean. Limnol. Oceanogr. 30: 447-448.
  • Canfield, D.E. Jr. and L.M. Hodgson. 1983. Prediction of Secchi disc depths in Florida Lakes: impact of algal biomass and organic color. Hydrobiologia. 99: 51-60.
  • Carlson, R.E. 1977. A trophic state index for lakes. Limnol. Oceanogr., 22, 361-369.
  • Carlson R. 1995. The Secchi disc and the volunteer monitor. LakeLine, 15,28-29, 35-37.
  • Carlson, R.E. 1997. The Secchi disk in black and white. LakeLine. 17: 14-15, 58-59.
  • Carlson, R., Lee, J. 1994. "The Great American Secchi Dip-In" Volunteers gather transparency information across the Midwest. Lake Resen'oir Manage., 9, 62.
  • Carlson, R.E. and J. Simpson. 1996. A Coordinator’s Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society. 96 pp.
  • Chambers, P. A, Kalff, J. 1985. Depth distribution and biomass of submersed aquatic macrophyte communities in relation to Secchi depth. Can. J. Fish. Aquat. Sci., 42, 701 709.
  • Cole, G.A., 1994. Textbook of Limnology. 4th ed. Prospect Heights: Waveland Press Inc. ISBN: 0881338001.
  • Cooke, D., et. al. Restoration and Management of Lakes and Reservoirs. 2nd ed. Boca Raton: Lewis Publishers, 1993. ISBN: 0873713974.
  • Davies-Colley, R.J. 1988. Measuring water clarity with a black disk. Limnol. And Oceanogr. 33: 616-623.
  • Davies-Colley, R.J, W.N. Vant, and D.G. Smith. 1993. Colour and Clarity of Natural Waters. Ellis Horwood.
  • Edmondson, W.T. 1972. The present condition of Lake Washington. Verh.Internat. Verein. Limnol., 18,284-291.
  • Gieskes, W.W.C., C. Veth, A. Woehrmann, and M.Graefe 1987. Secchi disc visibility world record Shattered. EOS, Transactions, American Geophysical Union. 68:123.
  • Håkanson, L.& R. H. Peters, 1995. Predictive limnology. SPB Academic Publishing bv, Amsterdam. ISBN: 9051031041.
  • Højerslev N.K., 1986, Visibility of the sea with special reference to the Secchi disc,Society of Photo-Optical Instrumentation Engineers (SPIE), Ocean Optics VIII, 637, 294–305.
  • Holmes, R.W., 1970. The Secchi disk in turbid coastal waters. Limnol. Oceangr., 15:688-694.
  • Horne, A., and C. Goldman. 1994. Limnology. 2nd ed. New York: McGraw-Hill Inc. ISBN: 0070236739.
  • Hou, Weilin, et al (2007). "Why does the Secchi disk disappear? An imaging perspective", Opt. Express, 15, 2791-2802.
  • Hutchinson, G.E. 1957. A Treatise on Limnology. Vol. 1. Geography, Physics, and Chemistry. John Wiley & Sons. ISBN: 0471425702.
  • Krümmel O., 1886, Der Ozean, G. Frentag, Leipzig and Prag, 1–242.
  • Kufel, L. 1998. Secchi disc measurements: personal aspects. Pol. Arch. Hydrobiol. 45 (1):3-9.
  • Kufel, L., Królikowska, J. 1996. Structure of submerged littoral vegetation in relation to pelagic trophic state indices. Ekol. pol., 44, 299-310.
  • Larson, G.L. and M. W. Buktenica, 1998. Variability of Secchi disk readings in an exceptionally clear and deep caldera lake.  Archive fur Hydrobiologie 141:377-388.
  • Larson, G.L., C. D. McIntire, M.W Buktenica, and S.F. Girdner, 2007. Thermal, chemical, and optical properties of Crater Lake, Oregon. Hydrobiologia 574:69-84.
  • Larson, D.W. 1972. Temperature, transparency, and phytoplankton productivity in Crater Lake, Oregon. Limnol. Oceanogr. 17: 410-417.
  • Lathrop, R.C. 1992. Nutrient loadings, lake nutrients, and water clarity. In: J.F. Kitchell. Food web management: A case study of Lake Mendota . Springer-Verlag. ISBN: 0387977422.
  • Lind, O.T. (1979). Handbook of Common Methods in Limnology. C.V. Mosby Co., St. Louis. ISBN: 0840337531.
  • Lorenzen, M.W. 1980. Use of chlorophyll-Secchi disc relationships. LimnoJ. Oceanogr., 25, 371-372.
  • Megard, R.0., Settles, J.C., Boyer, H.A, Combs Jr., W.S. 1980. Light, Secchi disc, and trophic states. Limnol. Oceanogr., 25, 373-377.
  • O'Sullivan, P.E.& C.S. Reynolds, 2004. The Lakes Handbook. Limnology and limnetic ecology. Blackwell Publishing, Malden, MA, USA. ISBN: 0632047976.
  • Preisendorfer, R. W. 1976. Hydrological Optics, United States Separtment of Commerce. Washington, DC, 382 pp.
  • Preisendorfer, R.W. 1986. "Secchi disk science: Visual optics of natural waters," Limnol. Oceanogr. 31, 909-926.
  • Preisendorfer, R.W. 1986. Eyeball optic of natural waters: Secchi disk science. NOAA Tech. Memo. ERL PMEL 67. 90 p. NTIS PB86 224060/AS.
  • Sanden, P., Haakansson, B. 1996. Long-term trends in Secchi depth in the Baltic Sea. Limnol. Oceanogr., 41, 346-351.
  • Secchi, P.A 1866. Rc1azione delle esperienze fatte a bordo dena pontificia pirocorvetta "I'Immacolata Concezione" per determinare la transparenza del mare [Report on experiments on board the papal steam corvette Immacolata Concezione to determine the transparency of the sea]. In: Cialdi, A. Sui moto oOOoso del mare e su Ie correnti di esso specialmente su queUe littorali. 258-288 Roma, Tipografia delle belle Arti. 258-288.
  • Smith, V.H. 1986. Light and nutrient effects on the relative biomass of blue-green algae in lake phytoplankton. Can. J. Fish. Aquat. Sci., 43, 148-153. J Water Quality Assessment. 1992. Deborah Chapman [Ed.] London, Chapman & Hall.
  • Taber, R.W. and H.W. Dubach. 1972. 1001 Questions Answered about the Oceans and Oceanography. Dodd, Mead. ISBN: 0396064965.
  • Tyler, F.E. 1968. The Secchi disc. Limnol. Oceanogr. 13: 1-6.
  • Verschuur, G.L. 1997. Transparency measurements in Garner lake, Tennessee; the relationship between Secchi depth and solar altitude and a suggestion for normalization of Secchi depth data. J. Lake and Reserv. Manage. 13 (2): 142-153.
  • Wetzel, R.G., 2001. Limnology: Lake and river ecosystems. Academic Press, San Diego. ISBN: 0127447601.
  • Wetzel, R.G.& G. E. Likens, 1990. Limnological analyses. Springer, New York. ISBN: 0387989285.
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Bledzki, L. (2013). Secchi disk. Retrieved from