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Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system

Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system
Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system
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 ?mol 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.
1726-4170
885-897
Flohr, A.
1e293a22-bdba-408e-9608-fed8b65e4e79
van der Plas, A.K.
45d2c17b-5905-4f91-affa-b0ec365937de
Emeis, K-C.
b41a29b3-0112-4600-bded-6470815c5f6f
Mohrholz, V.
41f1128a-0eff-4571-a94a-5ec7485fd7c2
Rixen, T.
dc65e23d-a115-4dbd-8aa8-7b1d402d6c66
Flohr, A.
1e293a22-bdba-408e-9608-fed8b65e4e79
van der Plas, A.K.
45d2c17b-5905-4f91-affa-b0ec365937de
Emeis, K-C.
b41a29b3-0112-4600-bded-6470815c5f6f
Mohrholz, V.
41f1128a-0eff-4571-a94a-5ec7485fd7c2
Rixen, T.
dc65e23d-a115-4dbd-8aa8-7b1d402d6c66

Flohr, A., van der Plas, A.K., Emeis, K-C., Mohrholz, V. and Rixen, T. (2014) Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system. Biogeosciences, 11 (3), 885-897. (doi:10.5194/bg-11-885-2014).

Record type: Article

Abstract

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 ?mol 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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Published date: 14 February 2014
Organisations: Geochemistry

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Local EPrints ID: 400922
URI: http://eprints.soton.ac.uk/id/eprint/400922
ISSN: 1726-4170
PURE UUID: 976d3bfa-b12e-4afd-b34e-6d997460db78
ORCID for A. Flohr: ORCID iD orcid.org/0000-0002-5018-5379

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Date deposited: 29 Sep 2016 16:16
Last modified: 08 Oct 2019 00:36

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Author: A. Flohr ORCID iD
Author: A.K. van der Plas
Author: K-C. Emeis
Author: V. Mohrholz
Author: T. Rixen

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