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Secchi Depth in the Oslofjord–skagerrak Area: Theory, Experiments and Relationships to Other Quantities : Volume 10, Issue 2 (18/03/2014)

By Aas, E.

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Book Id: WPLBN0004020182
Format Type: PDF Article :
File Size: Pages 23
Reproduction Date: 2015

Title: Secchi Depth in the Oslofjord–skagerrak Area: Theory, Experiments and Relationships to Other Quantities : Volume 10, Issue 2 (18/03/2014)  
Author: Aas, E.
Volume: Vol. 10, Issue 2
Language: English
Subject: Science, Ocean, Science
Collections: Periodicals: Journal and Magazine Collection, Copernicus GmbH
Historic
Publication Date:
2014
Publisher: Copernicus Gmbh, Göttingen, Germany
Member Page: Copernicus Publications

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Sørensen, K., Høkedal, J., & Aas, E. (2014). Secchi Depth in the Oslofjord–skagerrak Area: Theory, Experiments and Relationships to Other Quantities : Volume 10, Issue 2 (18/03/2014). Retrieved from http://worldlibrary.net/


Description
Description: Department of Geosciences, University of Oslo, Gaustadalleen 21, 0349 Oslo, Norway. The Secchi depth and its relationships to other properties of the sea water in the Oslofjord–Skagerrak area have been investigated. White and black disks of different sizes have been applied, and the Secchi depth has been observed with the naked eye, through colour filters and with a water telescope. Spectral luminances and illuminances have been calculated from recordings of radiance and irradiance, and attenuation coefficients have been determined. A theoretical expression for the Secchi depth based on luminances has been tested against field observations, and it is found that the field results for the product of Secchi depth and attenuation coefficients are on average only 4% less than the predicted value for the white disk. For the Secchi depths observed through colour filters or for the black disk, the average field results are more than 30% smaller than the theoretical estimates. The reduction in the disk diameter from 30 to 10 cm should theoretically reduce the Secchi depths by 13–22%, while the field observations show an average reduction of 10–20%. Similarly we find from theory that the removal of sun glitter should increase the Secchi depth by 12%, while the observed increase is 14% on average for the white disk. Our overall conclusion is that the theoretical expression works well for the white disk, but less so for the colour filter observations and the black disk.

Statistical relationships between Secchi depths and attenuation coefficients have been determined, and it is found that the root-mean-square errors relative to the mean value are smaller for the beam attenuation coefficients (12–24%, white disk) than for the vertical attenuation coefficients (16–65%, white disk). The depth of the 1% level of surface quanta irradiance (PAR) can be estimated with a relative root-mean-square error of 23% from observations of the white Secchi depth. Similar estimates of chlorophyll a and total suspended material will have rms errors in the range 40–90%. Our conclusion becomes that the Secchi depth observation is a very useful tool for checking the value and order of magnitude of other related quantities in the Oslofjord–Skagerrak area.


Summary
Secchi depth in the Oslofjord–Skagerrak area: theory, experiments and relationships to other quantities

Excerpt
Aarup, T., Holt, N., and Højerslev, N. K.: Optical measurements in the North Sea-Baltic Sea transition zone, III. Statistical analysis of bio-optical data from the Eastern North Sea, the Skagerrak and the Kattegat, Cont. Shelf Res., 16, 1355–1377, 1996b.; Aas, E.: Estimates of radiance reflected towards the zenith at the surface of the sea, Ocean Sci., 6, 861–876, doi:10.5194/os-6-861-2010, 2010.; Aas, E. and Højerslev, N. K.: Analysis of underwater radiance observations: Apparent optical properties and analytic functions describing the angular radiance distribution. J. Geophys. Res., 104, 8015–8024, 1999.; Aas, E. and Korsbø, B.: Self-shading effect by radiance meters on upward radiance observed in coastal waters, Limnol. Oceanogr., 42, 968–974, 1997.; Morel, A. and Gentili, B.: Diffuse reflection of oceanic waters, III. Implication of bi-directionality for the remote-sensing problem, Appl. Opt., 35, 4850–4862, 1996.; Aarup, T.: Transparency of the North Sea and Baltic Sea – a Secchi depth data mining study, Oceanologia, 44, 323–337, 2002.; Aarup, T., Holt, N., and Højerslev, N. K.:, Optical measurements in the North Sea-Baltic Sea transition zone, II. Water mass classification along the Jutland west coast from salinity and spectral irradiance measurements, Cont. Shelf Res., 16, 1343–1353, 1996a.; Aas, E. and Høkedal, J.: Reflection of spectral sky irradiance on the surface of the sea and related properties, Remote Sens. Environ., 70, 181–190, 1999.; Aas, E., Andresen, T., Løyning, T., and Sørgård, E.: Eutrofisituasjonen i Ytre Oslofjord. Delprosjekt 3.7: Optiske observasjoner – Overflatevannets kvalitet sett ut fra observasjoner i overflatelaget. SFT (Norw. Environ. Agency) Report 388/90, 50 pp., 1989.; Aas, E., Højerslev, N. K., and Høkedal, J.: Conversion of sub-surface reflectances to above-surface MERIS reflectance, Int. J. Remote Sens., 30, 5767–5791, 2009.; Andresen, A.: Siktedyputviklingen i Oslofjorden 1936–92, Master thesis, Dept. Geophys., Univ. Oslo, 228 pp., 1993.; Aure, J., Molvær, J., and Stigebrandt, A.: Observations of inshore water exchange forced by a fluctuating offshore density field, Mar. Poll. Bull., 33, 112–119, 1996.; Blackwell, H. R.: Contrast thresholds of the human eye, J. Opt. Soc. Am., 36, 624–632, 1946.; Boguslawski, G. V.: Handbuch der Ozeanographie, Band I. Räumliche, physikalische und chemische Beschaffenheit der Ozeane, Verlag von J. Engelhorn, Stuttgart, 400 pp., 1884.; Cox, C. and Munk, W.: Statistics of the sea surface derived from sun glitter, J. Mar. Res., 13, 198–227, 1954a.; Cox, C. and Munk, W.: The measurements of the roughness of the sea surface from photographs of the sun's glitter, J. Opt. Soc. Am., 44, 838–850, 1954b.; Davies-Colley, R. J.: Measuring water clarity with a black disc, Limnol. Oceanogr., 33, 616–623, 1988.; Duntley, S. Q.: The visibility of submerged objects. Final Rep., Visibility Lab., Mass. Inst. Tech., 74 pp., 1952.; Gade, H. G.: Some hydrographic observations of the Inner Oslofjord during 1959, Hvalrådets skrifter, Nr. 46, Universitetsforlaget, Oslo, 62 pp., 1963.; Fleming-Lehtinen, V. and Laamanen, M.: Long-term changes in Secchi depth and the role of phytoplankton in explaining light attenuation in the Baltic Sea, Est. Coast. Shelf Sci., 102–103, 1–10, 2012.; Gade, H. G.: Delrapport nr. 2, Hydrografi. Oslofjorden og dens forurensningsproblemer, I. Undersøkelsen 1962–1965, Norsk institutt for vannforskning, Oslo, 164 pp., 1967.; Gordon, H. R. and Ding, K.: Self-shading of in-water optical instruments. Limnol. Oceanogr., 37, 491–500, 1992.; Gordon, H. R. and Wouters, A. W.: Some relationships between Secchi depth and inherent optical properties of natural waters, Appl. Opt., 17, 3341–3343, 1978.; Gröen, P. and Dorrestein, R.: Zeegolven. KNMI Opstellen op Oceanografisch en Maritiem Meteorologisch Gebied, 11, 124 pp., 1976.; Haltrin, V. I.: Spectral relative clarity of Black and Aegan Seas. Geoscie

 

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