Origin and fate of the secondary nitrite maximum in the Arabian Sea.

Lam, P, Jensen, M. M., Kock, Annette , Lettmann, K. A., Plancherel, Y., Lavik, G, Bange, Hermann W. and Kuypers, M. M. M. (2011) Origin and fate of the secondary nitrite maximum in the Arabian Sea. Open Access Biogeosciences (BG), 8 . pp. 1565-1577. DOI 10.5194/bg-8-1565-2011.

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Abstract

The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO2−) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO2− accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO2−-producing and consuming processes, and examining the relationship between the overall NO2− balance and active N-loss in the Arabian Sea. Based on a combination of 15N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO2− accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO3−) reduction and to a certain extent ammonia oxidation. Meanwhile, NO2− consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH4+) were hardly detectable in this region, though some loss to NO2− oxidation was predicted from modeled NO3− changes. No significant correlation was found between NO2− and N-loss rates (p>0.05). This discrepancy between NO2− accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO2− reduction to N2O, N2 and NH4+, and indirectly for the remineralized NH4+ required by anammox. Altogether, our data do not support the long-held view that NO2− accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO2− accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.

Document Type: Article
Keywords: Marine chemistry; Microbiology; Biogeochemistry; Arabian Sea, N2O, denitrification, anammox
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
OceanRep > The Future Ocean - Cluster of Excellence
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.5194/bg-8-1565-2011
ISSN: 1726-4170
Projects: Future Ocean
Date Deposited: 06 Dec 2011 09:16
Last Modified: 06 Jul 2012 15:02
URI: http://oceanrep.geomar.de/id/eprint/12708

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