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| Titel |
The dissolved yellow substance and the shades of blue in the Mediterranean Sea |
| VerfasserIn |
A. Morel, B. Gentili |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 6, no. 11 ; Nr. 6, no. 11 (2009-11-24), S.2625-2636 |
| Datensatznummer |
250004097
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| Publikation (Nr.) |
 copernicus.org/bg-6-2625-2009.pdf |
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| Zusammenfassung |
| When the nominal algorithms commonly in use in Space Agencies are applied to
satellite Ocean Color data, the retrieved chlorophyll concentrations in the
Mediterranean Sea are recurrently notable overestimates of the field values.
Accordingly, several regionally tuned algorithms have been proposed in the
past to correct for this deviation. Actually, the blueness of the
Mediterranean waters is not as deep as expected from the actual (low)
chlorophyll content, and the modified algorithms account for this
peculiarity. Among the possible causes for such a deviation, an excessive
amount of yellow substance (or of chromophoric dissolved organic matter,
CDOM) has been frequently cited. This conjecture is presently tested, by
using a new technique simply based on the simultaneous consideration of
marine reflectance determined at four spectral bands, namely at 412, 443,
490, and 555 nm, available on the NASA-SeaWiFS sensor (Sea–viewing Wide
Field-of-view Sensor). It results from this test that the concentration in
yellow colored material (quantified as ay, the absorption coefficient
of this material at 443 nm) is about twice that one observed in the nearby
Atlantic Ocean at the same latitude. There is a strong seasonal signal, with
maximal ay values in late fall and winter, an abrupt decrease beginning
in spring, and then a flat minimum during the summer months, which plausibly
results from the intense photo-bleaching process favored by the high level
of sunshine in these areas. Systematically, the ay values, reproducible
from year to year, are higher in the western basin compared with those in
the eastern basin (by about 50%). The relative importance of the river
discharges into this semi-enclosed sea, as well as the winter deep vertical
mixing occurring in the northern parts of the basins may explain the high
yellow substance background. The regionally tuned [Chl] algorithms, actually
reflect the presence of an excess of CDOM with respect to its standard
(Chl-related) values. When corrected for the presence of the actual CDOM content,
the [Chl] values as derived via the nominal algorithms are restored to more
realistic values, i.e., approximately divided by about two; the strong
autumnal increase is smoothed whereas the spring bloom remains as an
isolated feature. |
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