Kuroshio flow – seamount interaction effects on the spatial and temporal variability of the microbial community structure

Seamount research has been growing as early as a few decades ago, particularly aimed at explaining its role in enhancing primary productivity in oligotrophic areas. This study targets to describe the microbial community structure over a seamount within the Kuroshio Current. Picoplankton (<2µm) were collected from the epipelagic layer (0-150m) from along-stream and cross-stream transects traversing the summit of the seamount. A time-series survey (6 casts) over the summit was also done over a period of 24 hours. Autotrophic and heterotrophic cells were counted using flow cytometry. Temperature profiling revealed isotherms doming on the leeward side of the seamount, presumably due to the vertical mixing of nutrients in the near surface in this area. The Kelvin-Helmholtz (KH) billows mediating the Kuroshio flow energy to microscale turbulent mixing were observed in 110-160m on the leeward side, further supporting our hypothesis. It is expected that the KH billows were induced by the flow-seamount interactions. On the other hand, chlorophyll was observed to be remarkably concentrated on the summit in the upper 50m depth layer (approximately 0.45 µg L^-1 in both transect and time-series surveys), possibly as a result of gyrotactic trapping of plankton. Picoeukaryotes and Synechococcus (ranges: 0.01 – 1.18 x10³ cells ml^-1; 0 – 2.53 x10⁴ cells ml^-1) were distributed similarly in the upper 60 m, where they seem to be advected from the western flank towards the summit of the seamount. Prochlorococcus (range: 0.06 – 2.65 x10⁴ cells ml^-1) was generally more concentrated in the deeper layer (60 – 100m) in stations adjacent to the seamount, but consistently had a shallower distribution over the summit (40 – 80m). Heterotrophic bacteria (HB) was notably more abundant in the along-stream (mean: 2.11 x10⁵ cells ml^-1) than the cross-stream transect (mean: 0.70 x10⁵ cells ml^-1). However, in the time-series survey, HB showed a 4-fold increase in abundance from the first three casts (mean: 1.13 x10⁵ cells ml^-1) to the latter casts deployed (mean: 4.55 x10⁵ cells ml^-1). Overall, our analyses suggest that the presence of a seamount affects the microbial community structure, however, the combining effect of the magnitude of the Kuroshio and the KH billows is likely to play a role in transporting primary production downstream.