The Bay of Bengal: An Enigmatic Diazotrophic Niche

Abstract

Biological dinitrogen (N2) fixation is meagerly explored in the Bay of Bengal (Bay). Stratified, warm, oligotrophic (but relatively high iron and phosphate) and oxygen minimum zone (OMZ) waters of the Bay might be a niche for diazotrophs. Therefore, we conducted N2 fixation rate measurements during the spring inter-monsoon in the euphotic zone, the OMZ and below the OMZ down to 1,500 m depth near the coastal and in the central Bay. We further assessed primary production and cyanobacterial community composition along with their potential environmental controlling parameters. N2 fixation rates in the euphotic zone were low (0.02–0.38 nmol N L−1 d−1) and their contribution to primary production was small (<2%). Despite conducive conditions for diazotrophy in the Bay, the reason for the relatively low euphotic zone and OMZ N2 fixation rates remained unclear and enigmatic. Interestingly, significantly higher N2 fixation rates occurred below the OMZ (>600 m depth), ranging from 0.06 to 0.11 nmol N L−1 d−1 where oxygen concentrations ranged between 0.5 and 1.6 mL L−1, rather than within the OMZ where rates ranged from 0.02 to 0.08 nmol N L−1 d−1 and oxygen concentrations were ≤0.5 mL L−1. Euphotic zone N2 fixation showed seasonality in the Bay with increasing rates from spring to summer season, perhaps owing to increasing Fe flux as the summer monsoon approaches its peak.