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| Titel |
Distribution of inorganic and organic nutrients in the South Pacific Ocean - evidence for long-term accumulation of organic matter in nitrogen-depleted waters |
| VerfasserIn |
P. Raimbault, N. Garcia, F. Cerutti |
| 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 ; 5, no. 2 ; Nr. 5, no. 2 (2008-03-03), S.281-298 |
| Datensatznummer |
250002366
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| Publikation (Nr.) |
 copernicus.org/bg-5-281-2008.pdf |
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| Zusammenfassung |
| During the BIOSOPE cruise the RV Atalante was dedicated to study the
biogeochemical properties in the South Pacific between the Marquesas Islands
(141° W–8° S) and the Chilean upwelling (73° W–34° S).
Over the 8000 km covered by the cruise, several different trophic situations
were encountered, in particular strong oligotrophic conditions in the South
Pacific Gyre (SPG, between 123° W and 101° W). In this isolated
region, nitrate was undetectable between the surface and 160–180 m and only
trace quantities (<20 nmoles l−1) of regenerated nitrogen (nitrite
and ammonium) were detected, even in the subsurface maximum. Integrated
nitrate over the photic layer, which reached 165 m, was close to zero.
Despite this severe nitrogen-depletion, phosphate was always present in
significant concentrations (≈0.1 μmoles l−1), while
silicic acid was maintained at low but classical oceanic levels (≈1 μmoles l−1).
In contrast, the Marquesas region (MAR) to the west
and Chilean upwelling (UPW) to the east were characterized by high nutrient
concentrations, one hundred to one thousand fold higher than in the SPG. The
distribution of surface chlorophyll reflected the nitrate gradient, the
lowest concentrations (0.023 nmoles l−1) being measured at the centre
of the SPG, where integrated value throughout the photic layer was very low
(≈ 10 mg m−2). However, due to the relatively high
concentrations of chlorophyll-a encountered in the DCM (0.2 μg l−1),
chlorophyll-a concentrations throughout the photic layer were
less variable than nitrate concentrations (by a factor 2 to 5). In contrast
to chlorophyll-a, integrated particulate organic matter (POM) remained more
or less constant along the study area (500 mmoles m−2, 60 mmoles m−2
and 3.5 mmoles m−2 for particulate organic carbon,
particulate organic nitrogen and particulate organic phosphorus,
respectively), with the exception of the upwelling, where values were two
fold higher. The residence time of particulate carbon in the surface water
was only 4–5 days in the upwelling, but up to 30 days in the SPG, where
light isotopic δ15N signal noted in the suspended POM suggests
that N2-fixation provides a dominant supply of nitrogen to
phytoplankton. The most striking feature was the large accumulation of
dissolved organic matter (DOM) in the SPG compared to the surrounding
waters, in particular dissolved organic carbon (DOC) where concentrations
were at levels rarely measured in oceanic waters (>100 μmoles l−1).
Due to this large pool of DOM in the SPG photic layer,
integrated values followed a converse geographical pattern to that of
inorganic nutrients with a large accumulation in the centre of the SPG.
Whereas suspended particulate matter in the mixed layer had a C/N ratio
largely conforming to the Redfield stochiometry (C/N≈6.6), marked
deviations were observed in this excess DOM (C/N≈16 to 23). The
marked geographical trend suggests that a net in situ source exists, mainly
due to biological processes. Thus, in spite of strong nitrate-depletion
leading to low chlorophyll biomass, the closed ecosystem of the SPG can
accumulate large amounts of C-rich dissolved organic matter. The
implications of this finding are examined, the conclusion being that, due to
weak lateral advection, the biologically produced dissolved organic carbon
can be accumulated and stored in the photic layer for very long periods. In
spite of the lack of seasonal vertical mixing, a significant part of new
production (up to 34%), which was mainly supported by dinitrogen
fixation, can be exported to deep waters by turbulent diffusion in terms of
DOC. The diffusive rate estimated in the SPG (134 μmolesC m−2 d−1),
was quite equivalent to the particles flux measured by sediments traps. |
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