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| Titel |
Influence of anticyclonic eddies on the Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean (BOUM cruise) |
| VerfasserIn |
T. Moutin, L. Prieur |
| 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 ; 9, no. 10 ; Nr. 9, no. 10 (2012-10-08), S.3827-3855 |
| Datensatznummer |
250007323
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| Publikation (Nr.) |
 copernicus.org/bg-9-3827-2012.pdf |
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| Zusammenfassung |
| We studied a longitudinal transect in the Mediterranean Sea (MS) and along
this transect, the influence of anticyclonic eddies at three long duration (LD)
stations. The deep chlorophyll maximum depth, the euphotic layer depth and
the top of the nitracline depth are clearly correlated outside of the
eddies, and deepen from the oligotrophic western to the ultraoligotrophic
eastern MS. We provide evidence that the locations of the three LD stations
studied were near the axis of the eddies. Their diameters were close to 100
km and the studied areas were less than 10 km from the centre of the eddies.
The positions of the LD stations are marked by an increase in the flux
function and a decrease in apparent oxygen utilization (AOU) and in excess
density σ), as expected for anticyclonic eddies. Integrated mean
primary production measured in situ inside the three studied eddies confirms the
previous conclusion that integrated primary production (IPP) about 150 mgC m−2 d−1 may appear as a lower limit for IPP during strong
oligotrophic conditions. The mesoscale activity is strong enough to locally
modify the very well-documented western-to-eastern gradient of trophic
conditions in the MS. We proposed a new calculation for mixed layer depths
(MLDs) enabling the determination of MLD to take into consideration processes
occurring with time scales ranging from a few hours to several days, and
also the winter MLD. Studying the main physical, chemical and dynamical
characteristics of the three eddies enables us to consider that the vorticity
barrier prevents any strong mixing and advection of outer water inside the
eddy and explains why the depth range of eddies starts from the surface. As
a first approximation, the anticyclonic eddies could be considered as closed
systems dating back to the previous winter, making possible to draw first-order budgets.
The daily new N-input in the photic zone is virtually
identical to the N-export measured at 230 m by drifting traps. This means
that the eddies are close to an equilibrium state where input is equal to
loss. The annual N-input by winter convection, which is a fundamental
criterion for new nutrient availability, may be extremely variable inside
eddies, with W-MLD varying from 90.5 m at the western station to 396.5 m at
the eastern station. W-MLDs are always deeper inside the eddies than
outside where they are in keeping with climatological averages. AOU was low
inside the eddies; this together with the near-identical export measured at
230 and 460 m seems to indicate that eddy cores are areas where low
mineralisation of particulate organic matter occurs. "In" and "out" AOU
comparisons indicate lower mineralisation inside the eddies suggesting a
higher efficiency for CO2 sequestration via sedimentation of
particulate organic matter. The three eddies are enriched in dissolved
organic carbon (DOC). Sequestration of CO2 by vertical export of
accumulated DOC therefore seems to be higher inside eddies. The relative
importance of DOC transport in the biological pump is probably one of the
main characteristics of low-P low chlorophyll (LPLC) areas, and it is likely
to be reinforced inside anticyclonic eddies. The numerous anticyclonic
eddies in the MS are likely to influence the water masses and their
dispersion, and therefore have a strong impact on the biogeochemical
properties at the scale of the MS. |
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