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| Titel |
A seasonal study of dissolved cobalt in the Ross Sea, Antarctica: micronutrient behavior, absence of scavenging, and relationships with Zn, Cd, and P |
| VerfasserIn |
M. A. Saito, T. J. Goepfert, A. E. Noble, E. M. Bertrand, P. N. Sedwick, G. R. DiTullio |
| 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. 12 ; Nr. 7, no. 12 (2010-12-22), S.4059-4082 |
| Datensatznummer |
250005109
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| Publikation (Nr.) |
 copernicus.org/bg-7-4059-2010.pdf |
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| Zusammenfassung |
| We report the distribution of cobalt (Co) in the Ross Sea polynya during
austral summer 2005–2006 and the following austral spring 2006. The vertical
distribution of total dissolved Co (dCo) was similar to soluble reactive
phosphate (PO43−), with dCo and PO43− showing a
significant correlation throughout the water column (r2 = 0.87, 164
samples). A strong seasonal signal for dCo was observed, with most spring
samples having concentrations ranging from ~45–85 pM, whereas summer
dCo values were depleted below these levels by biological activity. Surface
transect data from the summer cruise revealed concentrations at the low
range of this seasonal variability (~30 pM dCo), with concentrations
as low as 20 pM observed in some regions where PO43− was depleted
to ~0.1 μM. Both complexed Co, defined as the fraction of dCo
bound by strong organic ligands, and labile Co, defined as the fraction of
dCo not bound by these ligands, were typically observed in significant
concentrations throughout the water column. This contrasts the depletion of
labile Co observed in the euphotic zone of other ocean regions, suggesting a
much higher bioavailability for Co in the Ross Sea. An ecological
stoichiometry of 37.6 μmol Co:mol−1 PO43− calculated
from dissolved concentrations was similar to values observed in the
subarctic Pacific, but approximately tenfold lower than values in the
Eastern Tropical Pacific and Equatorial Atlantic. The ecological
stoichiometries for dissolved Co and Zn suggest a greater overall use of Zn
relative to Co in the shallow waters of the Ross Sea, with a
Co:PO43−/Zn:PO43− ratio of 1:17. Comparison of these
observed stoichiometries with values estimated in culture studies suggests
that Zn is a key micronutrient that likely influences phytoplankton
diversity in the Ross Sea. In contrast, the observed ecological
stoichiometries for Co were below values necessary for the growth of
eukaryotic phytoplankton in laboratory culture experiments conducted in the
absence of added zinc, implying the need for significant Zn nutrition in the
Zn-Co cambialistic enzymes. The lack of an obvious kink in the dissolved
Co:PO43− relationship was in contrast to Zn:PO43− and
Cd:PO43− kinks previously observed in the Ross Sea. An excess
uptake mechanism for kink formation is proposed as a major driver of
Cd:PO43− kinks, where Zn and Cd uptake in excess of that needed
for optimal growth occurs at the base of the euphotic zone, and no clear Co
kink occurs because its abundances are too low for excess uptake. An unusual
characteristic of Co geochemistry in the Ross Sea is an apparent lack of Co
scavenging processes, as inferred from the absence of dCo removal below the
euphotic zone. We hypothesize that this vertical distribution reflects a low
rate of Co scavenging by Mn oxidizing bacteria, perhaps due to Mn scarcity,
relative to the timescale of the annual deep winter mixing in the Ross Sea.
Thus Co exhibits nutrient-like behavior in the Ross Sea, in contrast to its
hybrid-type behavior in other ocean regions, with implications for the
possibility of increased marine Co inventories and utility as a
paleooceanographic proxy. |
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