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| Titel |
Secchi depth in the Oslofjord–Skagerrak area: theory, experiments and relationships to other quantities |
| VerfasserIn |
E. Aas, J. Høkedal, K. Sørensen |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1812-0784
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| Digitales Dokument |
URL |
| Erschienen |
In: Ocean Science ; 10, no. 2 ; Nr. 10, no. 2 (2014-03-18), S.177-199 |
| Datensatznummer |
250116964
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| Publikation (Nr.) |
 copernicus.org/os-10-177-2014.pdf |
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| Zusammenfassung |
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. |
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