Water mass dynamics drive nitrogen fixation rates and diazotroph community composition in the coastal ocean

Abstract Although significant N 2 fixation rates (NFRs) have been observed throughout mid‐Atlantic coastal waters, studies have been limited in their temporal coverage, and the methods used to detect nitrogen fixers (diazotrophs) and their findings have varied. This study investigated NFRs and diazotroph communities in a dynamic coastal setting between Nags Head and Pea Island, North Carolina, USA, during a monthly time series from June 2019 to August 2020, and an August 2019 cruise. We observed movement of the Cape Hatteras Front and distinct NFRs and diazotroph communities in the Mid‐Atlantic Bight, South‐Atlantic Bight, Chesapeake Bay and Oregon Inlet plume waters, and mixing zones. The highest NFR (42.4 ± 3.0 nmol N L −1 d −1 ) coincided with the frontal mixing zone, where nifH of assayed targets was most abundant and the nifH community highly diverse. Observations revealed significant seasonal variability in the diazotroph community and NFRs (below detection to 26.1 ± 9.3 nmol N L −1 d −1 ), with higher NFRs in the late summer and early fall. Furthermore, our comparison of three diazotroph detection methods ( nifH gene amplicon sequencing, nifH quantitative PCR, and microscopy) confirms a combined approach is needed to thoroughly survey the diazotroph community, and corroborates previous suggestions that Richelia is selected against by the classical nifH sequencing primers. This study advances our understanding of coastal N 2 fixation by documenting how water mass dynamics affect biogeochemical regimes and diazotroph communities. Our findings have implications for predicting how climate‐induced changes in water mass characteristics may impact coastal diazotroph community composition and activity in this region.