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

Abstract Dinitrogen (N 2 ) 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 15 N 2 and 13 C incubations were used to evaluate photic and aphotic N 2 fixation rates and diazotrophic contribution to water column productivity. N 2 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 N 2 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 N 2 fixation rates (10–75 μ mol m −2 d −1 ) comprised ~ 1–40% of new production (NP). During the winter mixing period N 2 fixation rates were considerably higher (11–242 μ mol 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.