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

• Main Authors: P. Lam, M. M. Jensen, A. Kock, K. A. Lettmann, Y. Plancherel, G. Lavik, H. W. Bange, M. M. M. Kuypers
• Format: Article
• Language: English
• Published: Copernicus Publications 2011-06-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/8/1565/2011/bg-8-1565-2011.pdf

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 (NO<sub>2</sub><sup>−</sup>) 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 NO<sub>2</sub><sup>−</sup> accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO<sub>2</sub><sup>−</sup>-producing and consuming processes, and examining the relationship between the overall NO<sub>2</sub><sup>−</sup> balance and active N-loss in the Arabian Sea. Based on a combination of <sup>15</sup>N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO<sub>2</sub><sup>−</sup> accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO<sub>3</sub><sup>−</sup>) reduction and to a certain extent ammonia oxidation. Meanwhile, NO<sub>2</sub><sup>−</sup> consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH<sub>4</sub><sup>+</sup>) were hardly detectable in this region, though some loss to NO<sub>2</sub><sup>−</sup> oxidation was predicted from modeled NO<sub>3</sub><sup>−</sup> changes. No significant correlation was found between NO<sub>2</sub><sup>−</sup> and N-loss rates (<i>p</i>>0.05). This discrepancy between NO<sub>2</sub><sup>−</sup> 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 NO<sub>2</sub><sup>−</sup> reduction to N<sub>2</sub>O, N<sub>2</sub> and NH<sub>4</sub><sup>+</sup>, and indirectly for the remineralized NH<sub>4</sub><sup>+</sup> required by anammox. Altogether, our data do not support the long-held view that NO<sub>2</sub><sup>−</sup> accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO<sub>2</sub><sup>−</sup> accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.