Contribution of photic and aphotic N₂ fixation to production in an oligotrophic sea

Dinitrogen (N₂) fixation was investigated at a pelagic station in the oligotrophic waters of the northern Gulf of Aqaba, Red Sea, between February 2016 and December 2018. In situ ¹⁵N₂ and ¹³C incubations were used to evaluate photic and aphotic N₂ fixation rates and diazotrophic contribution to water column productivity. N₂ fixation rates were typically low (below detection to <0.5 nmol N L⁻¹ d⁻¹). Maximal rates of 3.1 nmol L⁻¹ d⁻¹, measured at 100 m when conspicuous slicks of the cyanobacterium Trichodesmium appeared on the surface water. Amplicon sequencing of nifH demonstrated that non-cyanobacterial diazotrophs, mostly α- and γ-proteobacteria comprised the majority (82–100%) of amplicon sequence variants retrieved from photic and aphotic depths when low N₂ fixation rates were measured, while amplicons representing cyanobacteria were nearly absent, but appeared in low abundance (~ 2%) when maximal rates were measured. During the stratified summer-period, water-column N₂ fixation rates (10–75 μmol m⁻² d⁻¹) comprised ~ 1–40% of new production (NP). During the winter mixing period N₂ fixation rates were considerably higher (11–242 μmol m⁻² d⁻¹) but made up only <1% of NP. This is because, during this period, nitrate supplied to the photic zone by the vertical mixing becomes the major N source for NP. We conclude that on an annual average, diazotrophy plays a minor role in the NP of the Gulf. The major “new” nitrogen sources are cross-thermocline turbulent diffusion of nitrate during summer stratification and vertical mixing during the fall–winter.