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| Titel |
Phytoplankton distribution and nitrogen dynamics in the southwest indian subtropical gyre and Southern Ocean waters |
| VerfasserIn |
S. J. Thomalla, H. N. Waldron, M. I. Lucas, J. F. Read, I. J. Ansorge, E. Pakhomov |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1812-0784
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| Digitales Dokument |
URL |
| Erschienen |
In: Ocean Science ; 7, no. 1 ; Nr. 7, no. 1 (2011-02-08), S.113-127 |
| Datensatznummer |
250004436
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| Publikation (Nr.) |
 copernicus.org/os-7-113-2011.pdf |
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| Zusammenfassung |
| During the 1999 Marion Island Oceanographic Survey (MIOS 4) in late austral
summer, a northbound and reciprocal southbound transect were taken along the
Southwest Indian and Madagascar Ridge, between the Prince Edward Islands and
31° S. The sections crossed a number of major fronts and smaller
mesoscale features and covered a wide productivity spectrum from subtropical
to subantarctic waters. Associated with the physical survey were measurements
of size fractionated chlorophyll, nutrients and nitrogen (NO3, NH4
and urea) uptake rates. Subtropical waters were characterised by low
chlorophyll concentrations (max = 0.27.3 mg m−3 dominated by
pico-phytoplankton cells (> 81%) and very low f-ratios (< 0.1), indicative
of productivity based almost entirely on recycled ammonium and urea.
Micro-phytoplankton growth was limited by the availability of NO3
(< 0.5 mmol m−3 and Si(OH)4 (< 1.5 mmol m−3
through strong vertical stratification preventing the upward
flux of nutrients into the euphotic zone. Biomass accumulation of small
cells was likely controlled by micro-zooplankton grazing. In subantarctic
waters, total chlorophyll concentrations increased (max = 0.74 mg m−3
relative to the subtropical waters and larger cells became more prevalent,
however smaller phytoplankton cells and low f-ratios (< 0.14) still
dominated, despite sufficient NO3 availability. The results from this
study favour Si(OH)4 limitation, light-limited deep mixing and likely
Fe deficiency as the dominant mechanisms controlling significant new
production by micro-phytoplankton. The percentage of micro-phytoplankton
cells and rates of new production did however increase at oceanic frontal
regions (58.6% and 11.22%, respectively), and in the region of the
Prince Edward archipelago (61.4% and 14.16%, respectively). Here,
water column stabilization and local Fe-enrichment are thought to stimulate
phytoplankton growth rates. Open ocean regions such as these provide
important areas for local but significant particulate organic carbon export
and biological CO2 draw-down in an overall high nutrient low
chlorophyll Southern Ocean. |
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