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

Abstract

AbstractDinitrogen (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.