Virio- and picoplankton mediate important biogeochemical processes and the environmental factors that regulate their dynamics, and the virus-host interactions are incompletely known, especially in the deep sea. Here we report on their distributions and relationships with environmental factors at 21 stations covering a latitudinal range (2-23° N) in the Western Pacific Ocean. This region is characterized by a complex western boundary current system. Synechococcus, autotrophic picoeukaryotes, heterotrophic prokaryotes and virus-like particles (VLPs) were high (<2.4 × 102-6.3 × 104, <34-2.8 × 103, 3.9 × 104-1.3 × 106 cells mL-1 and 5.1 × 105-2.7 × 107 mL-1, respectively), and Prochlorococcus were low (<2.3 × 102-1.0 × 105 cells mL-1) in the Luzon Strait and the four most southerly stations, where upwelling occurs. Covariations in the abundances of VLPs with heterotrophic and autotrophic picoplankton, and their correlation (i.e. r2 = 0.63 and 0.52, respectively) suggested a strong host dependence in the epi- and mesopelagic zones. In the bathypelagic zone, only abiotic factors significantly influenced VLPs abundance variation (r2 = 0.12). This study shows that the dynamics of virio- and picoplankton in this Western Pacific are controlled by suite of complex and depth-dependent relationship among physical and biological factors that in turn link the physical hydrography of the western boundary current system with microbial-mediated biogeochemical processes.
Keywords: Western Pacific Ocean; bathypelagic; flow cytometry; picoplankton; upwelling; virioplankton.
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