Flohr, A., van der Plas, A. K., Emeis, K.-C., Mohrholz, V. and Rixen, Tim (2013) Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling regime. Biogeosciences Discussions, 10 (6). pp. 10459-10489. DOI https://doi.org/10.5194/bgd-10-10459-2013.

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Abstract

On a global scale the ratio of fixed nitrogen (N)and phosphate (P) is characterized by a deficit of N with re-gard to the classical Redfield ratio of N : P=16 : 1 reflect-ing the impact of N loss occurring in the oceanic oxygenminimum zones. The northern Benguela upwelling system(NBUS) is known for losses of N and the accumulation ofP in sub- and anoxic bottom waters and sediments of theNamibian shelf resulting in low N : P ratios in the water col-umn. To study the impact of the N : P anomalies on the re-gional carbon cycle and their consequences for the exportof nutrients from the NBUS into the oligotrophic subtrop-ical gyre of the South Atlantic, we measured dissolved in-organic carbon (CT), total alkalinity (AT), oxygen (O2)andnutrient concentrations in February 2011. The results indi-cate increased P concentrations over the Namibian shelf dueto P efflux from sediments resulting in a C : N : P : -O2ratioof 106 : 16 : 1.6 : 138. N reduction further increase C : N andreduce N : P ratios in those regions where O2concentrationsin bottom waters are < 20 μmol kg−1. However, off the shelfalong the continental margin, the mean C : N : P : -O2ratio isagain close to the Redfield stoichiometry. Additional nutrientdata measured during two cruises in 2008 and 2009 implythat the amount of excess P, which is created in the bottomwaters on the shelf, and its export into the subtropical gyreafter upwelling varies through time. The results further revealan inter-annual variability of excess N within the South At-lantic Central Water (SACW) that flows from the north intothe NBUS, with highest N values observed in 2008. It is pos-tulated that the N excess in SACW occurred due to the impactof remineralized organic matter produced by N2fixation andthat the magnitude of excess P formation and its export isgoverned by inputs of excess N along with SACW flowinginto the NBUS. Factors controlling N2fixation north of theBUS need to be addressed in future studies to better under-stand the role of the NBUS as a P source and N sink in thecoupled C : N : P cycles.

Document Type:	Article
Programme Area:	UNSPECIFIED
Research affiliation:	Biogeochemistry and Geology > Carbon and Nutrient Cycling
Refereed:	Yes
Open Access Journal?:	Yes
DOI:	https://doi.org/10.5194/bgd-10-10459-2013
ISSN:	1810-6285
Date Deposited:	16 Aug 2019 12:15
Last Modified:	01 Oct 2020 12:59
URI:	http://cris.leibniz-zmt.de/id/eprint/2636

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