English 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 NO 2- 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 (NO 3-) 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 NO 2- oxidation was predicted from modeled NO 3- 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.
