N-cycling and balancing of the N-deficit generated in the oxygen minimum zone over the Namibian shelf-An isotope-based approach

Nagel, Birgit; Emeis, Kay-Christian; Flohr, Anita ORCID: https://orcid.org/0000-0002-5018-5379; Rixen, Tim; Schlarbaum, Tim; Mohrholz, Volker; van der Plas, Anja. 2013 N-cycling and balancing of the N-deficit generated in the oxygen minimum zone over the Namibian shelf-An isotope-based approach. Journal of Geophysical Research: Biogeosciences, 118 (1). 361-371. 10.1002/jgrg.20040

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Abstract/Summary

The northern Benguela upwelling system is a nutrient‐replete region with high plankton biomass production and a seasonally changing oxygen minimum zone. Nitrate:phosphate ratios in fresh upwelling water are low due to denitrification in the near‐seafloor oxygen minimum zone and phosphate efflux from sediments. This makes the region a candidate for substantial dinitrogen fixation, for which evidence is scarce. Nutrient and oxygen data, N isotope data of nitrate, nitrogen isotope ratios of particulate matter, particulate organic carbon content, and suspended matter concentrations on a transect across the shelf and upper slope at 23°S illustrate N‐cycling processes and are the basis for estimating the contribution of N‐sources and N‐sinks to the reactive nitrogen pool. It appears that N‐removal due to denitrification exceeds N gain by N2 fixation and physical mixing processes by a factor of >6, although inorganic N:P ratios again increase as surface water is advected offshore. Nitrate and ammonium regeneration, nutrient assimilation with N:P < 16, shelf break mixing, atmospheric input, and N2 fixation all contribute to the restoration of inorganic N:P ratios back to Redfield conditions, but in seasonally changing proportions. The Benguela upwelling system thus is a nutrient source for the oceanic‐mixed layer where N‐sources and N‐sinks are not in balance and Redfield conditions can only re‐adjust by advection and mixing processes integrated over time.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	10.1002/jgrg.20040
ISSN:	21698953
Date made live:	11 Mar 2020 13:51 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/527119

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