Cyanobacterial distributions along a physico‐chemical gradient in the N ortheastern P acific O cean

Summary The cyanobacteria P rochlorococcus and S ynechococcus are important marine primary producers. We explored their distributions and covariance along a physico‐chemical gradient from coastal to open ocean waters in the N ortheastern P acific O cean. An inter‐annual pattern was delineated in the dynamic transition zone where upwelled and eastern boundary current waters mix, and two new S ynechococcus clades, Eastern Pacific Clade ( EPC ) 1 and EPC2 , were identified. By applying state‐of‐the‐art phylogenetic analysis tools to bar‐coded 16S amplicon datasets, we observed higher abundance of P rochlorococcus high‐light I ( HLI ) and low‐light I ( LLI ) in years when more oligotrophic water intruded farther inshore, while under stronger upwelling S ynechococcus   I and IV dominated. However, contributions of some cyanobacterial clades were proportionally relatively constant, e.g. S ynechococcus   EPC2 . In addition to supporting observations that P rochlorococcus   LLI thrive at higher irradiances than other LL taxa, the results suggest LLI tolerate lower temperatures than previously reported. The phylogenetic precision of our 16S rRNA gene analytical approach and depth of bar‐coded sequencing also facilitated detection of clades at low abundance in unexpected places. These include P rochlorococcus at the coast and C yanobium‐ related sequences offshore, although it remains unclear whether these came from resident or potentially advected cells. Our study enhances understanding of cyanobacterial distributions in an ecologically important eastern boundary system.