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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&minus;) 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&minus; accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO2&minus;-producing and consuming processes, and examining the relationship between the overall NO2&minus; 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&minus; accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO3&minus;) reduction and to a certain extent ammonia oxidation. Meanwhile, NO2&minus; consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH4+) were hardly detectable in this region, though some loss to NO2&minus; oxidation was predicted from modeled NO3&minus; changes. No significant correlation was found between NO2&minus; and N-loss rates (p>0.05). This discrepancy between NO2&minus; 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&minus; 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&minus; accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO2&minus; accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities

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