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| Titel |
Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system |
| VerfasserIn |
A. Flohr, A. K. van der Plas, K.-C. Emeis, V. Mohrholz, T. Rixen |
| 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 ; 11, no. 3 ; Nr. 11, no. 3 (2014-02-14), S.885-897 |
| Datensatznummer |
250117203
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| Publikation (Nr.) |
 copernicus.org/bg-11-885-2014.pdf |
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| Zusammenfassung |
| On a global scale the ratio of fixed nitrogen (N) and phosphate (P) is
characterized by a deficit of N with regard to the classical Redfield ratio
of N : P = 16 : 1 reflecting the impact of N loss occurring in the oceanic
oxygen minimum zones. The northern Benguela upwelling system (NBUS) is known
for losses of N and the accumulation of P in sub- and anoxic bottom waters
and sediments of the Namibian shelf resulting in low N : P ratios in the
water column. To study the impact of the N : P anomalies on the regional
carbon cycle and their consequences for the export of nutrients from the NBUS
into the oligotrophic subtropical gyre of the South Atlantic, we measured
dissolved inorganic carbon (CT), total alkalinity (AT), oxygen
(O2) and nutrient concentrations in February 2011. The results indicate
increased P concentrations over the Namibian shelf due to P efflux from
sediments resulting in a C : N : P : -O2 ratio of 106 : 16 : 1.6 : 138.
N reduction further increase C : N and reduce N : P ratios in those regions
where O2 concentrations in bottom waters are < 20 μmol kg−1.
However, off the shelf along the continental margin, the mean C : N : P : -O2 ratio is again close to the Redfield stoichiometry.
Additional nutrient data measured during two cruises in 2008 and 2009 imply
that the amount of excess P, which is created in the bottom waters on the
shelf, and its export into the subtropical gyre after upwelling varies
through time. The results further reveal an inter-annual variability of
excess N within the South Atlantic Central Water (SACW) that flows from the
north into the NBUS, with highest N values observed in 2008. It is postulated
that the N excess in SACW occurred due to the impact of remineralized
organic matter produced by N2 fixation and that the magnitude of excess
P formation and its export is governed by inputs of excess N along with SACW
flowing into the NBUS. Factors controlling N2 fixation north of the BUS
need to be addressed in future studies to better understand the role of the
NBUS as a P source and N sink in the coupled C : N : P cycles. |
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