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

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 similar to 10-20% of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO2-) co-inciding 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 N-15-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO2- accumulated in the centralNE 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 centralNE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO2- reduction to N2O, N-2 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.