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MEPS 256:1-11 (2003)
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Abstract
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UV (280 to 400 nm) optical properties in a Norwegian fjord system and an intercomparison of underwater radiometers
Berit Kjeldstad1,*, Øyvind Frette2, Svein Rune Erga3, Howard I. Browman4, Penny Kuhn5, Richard Davis5, William Miller5, Jakob J. Stamnes2
1Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway
2Department of Physics, and
3Department of Microbiology, University of Bergen, 5020 Bergen, Norway
4Institute of Marine Research, Austevoll Aquaculture Research Station, 5392 Storebø, Norway
5Department of Oceanography, Dalhousie University, Halifax B3H 4J1, Canada
*Email: berit.kjeldstad@phys.ntnu.no
ABSTRACT: The depth to which solar ultraviolet (UV) radiation penetrates water columns is highly variable. The range in diffuse attenuation coefficients observed in the clearest ocean waters versus small oligotrophic lakes can be as much as 3 orders of
magnitude. In this study, we investigated the variability of UV penetration (and its sources) in a typical Norwegian fjord system (Samnanger fjord, 60°N, 5°E) after the main spring diatom bloom had passed, so that the primary production in the fjord was
low, with chl a concentrations of 1 to 2 mg m-3 at 10 m depth (April 1999). At 320 nm, diffuse attenuation coefficients varied between 1.2 and 5.0 m-1 from the outer to the inner part of the fjord. At 305 nm, the variation was
between 2.3 and 5.4 m-1. Less variability was found at longer wavelengths (340 and 380 nm). River run off, containing high concentrations of colored dissolved organic matter (CDOM), provided a low salinity layer of highly UV-absorbing water at
the surface. There was a strong correlation between diffuse attenuation coefficients in the UV and CDOM, but only a weak correlation with chl a. Uncertainty in measuring diffuse attenuation coefficients are seldom addressed. Thus, we undertook an
intercomparison of diffuse attenuation coefficients calculated from data produced using 3 different UV radiometers: a spectroradiometer, a narrowband filter radiometer and 2 moderate-bandwidth filter radiometers. The deviations in attenuation ranged from
20 to 40%, being most pronounced at the shortest wavelengths (305 and 320 nm) at 3 different stations. This intercomparison illustrates that there has not been significant improvement in the uncertainties of measuring diffuse attenuation coefficients
since a similar intercomparison was performed in 1994.
KEY WORDS: Ultraviolet radiation · Fjord · Marine · Penetration · Attenuation · Intercomparison
Full text in pdf format
Published in MEPS Vol.
256
(2003) on July 17
Print ISSN: 0171-8630; Online ISSN: 1616-1599.
Copyright © Inter-Research, Oldendorf/Luhe, 2003
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