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| Titel |
The most oligotrophic subtropical zones of the global ocean: similarities and differences in terms of chlorophyll and yellow substance |
| VerfasserIn |
A. Morel, H. Claustre, 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 ; 7, no. 10 ; Nr. 7, no. 10 (2010-10-14), S.3139-3151 |
| Datensatznummer |
250005014
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| Publikation (Nr.) |
 copernicus.org/bg-7-3139-2010.pdf |
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| Zusammenfassung |
| The cores of the subtropical anticyclonic gyres are characterized by their
oligotrophic status and minimal chlorophyll concentration, compared to that
of the whole ocean. These zones are unambiguously detected by space borne
ocean color sensors thanks to their typical spectral reflectance, which is
that of extremely clear and deep blue waters. Not only the low chlorophyll
(denoted [Chl]) level, but also a reduced amount of colored dissolved
organic matter (CDOM or "yellow substance") account for this clarity. The
oligotrophic waters of the North and South Pacific gyres, the North and
South Atlantic gyres, and the South Indian gyre have been comparatively
studied with respect to both [Chl] and CDOM contents, by using 10-year data
(1998–2007) of the Sea-viewing Wide field-of-view Sensor (SeaWiFS, NASA).
Albeit similar these oligotrophic zones are not identical regarding their
[Chl] and CDOM contents, as well as their seasonal cycles. According to the
zone, the averaged [Chl] value varies from 0.026 to 0.059 mg m−3,
whereas the ay(443) average (the absorption coefficient due to CDOM at
443 nm) is between 0.0033 and 0.0072 m−1. The CDOM-to-[Chl] relative
proportions also differ between the zones. The clearest waters,
corresponding to the lowest [Chl] and CDOM concentrations, are found near
Easter Island and near Mariana Islands in the western part of the North
Pacific Ocean. In spite of its low [Chl], the Sargasso Sea presents the
highest CDOM content amongst the six zones studied. Except in the North
Pacific gyre (near Mariana and south of Hawaii islands), a conspicuous
seasonality appears to be the rule in the other 4 gyres and affects both
[Chl] and CDOM; both quantities vary in a ratio of about 2
(maximum-to-minimum). Coinciding [Chl] and CDOM peaks occur just after the
local winter solstice, which is also the period of the maximal mixed layer
depth in these latitudes. It is hypothesized that the vertical transport of
unbleached CDOM from the subthermocline layers is the main process enhancing
the CDOM concentration within the upper layer in winter. In summer, the CDOM
experiences its minimum which is delayed with respect to the [Chl] minimum;
apparently, the solar photo-bleaching of CDOM is a slower process than the
post-bloom algal Chl decay. Where they exist, the seasonal cycles are
repeated without notable change from year to year. Long term (10 y) trends
have not been detected in these zones. These oligotrophic gyres can
conveniently be used for in-flight calibration and comparison of ocean color
sensors, provided that their marked seasonal variations are accounted for. |
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