The population of Trichodesmium cyanobacteria in the northern Pacific Ocean associated with Kuroshio Extension

The cyanobacteria Trichodesmium are broadly distributed in the photic zone of oligotrophic tropical and subtropical waters. Mesoscale physical processes associated with Kuroshio Extension (KE) can be important regulators of variability in Trichodesmium diversity and distribution. We investigated the spatial patterns, temporal variability, and regulations of Trichodesmium population across the KE in the northern Pacific Ocean (23°–40° N, 145°–160° E). Repeated surveys were conducted in May and September 2017. Spatial–temporal variability in the physicochemical variables between the two voyages was substantially attributed to the intensity and structure of KE and its recirculation gyres. The low temperature/high salinity conditions in May were reversed in September. Nutrients, DO, and suspended particulate matter (SPM) in May were higher than in September, while the opposite pattern was observed for pH. The spatial pattern of Trichodesmium volumetric abundance and calculated biomass was patchy, rather than varying consistently with latitude or longitude. T . thiebautii was more abundant in September, whereas T. erythraeum and T. hildebrandtii were similarly abundant in May and September. Temporal variability in Trichodesmium population abundance suggested the most suitable conditions of Trichodesmium distribution were a temperature of 26–30°C in a low Chl a environment with deep nutricline at 100–150 m. Specifically to different Trichodesmium species, salinity was a critical factor for abundant T . hildebrandtii , however, low N/P ratio and phosphate concentration facilitated higher abundance of T . thiebatuii and T . erythraeum . Finally, we confirmed anticyclonic and cyclonic eddies generated by the detachment of the Kuroshio had positive and negative impacts on Trichodesmium diversity and abundance, respectively.