Abstract

not-yet-known not-yet-known not-yet-known unknown A substantial community variation of free-living bacterioplankton and their main predator heterotrophic nanoflagellates (HNFs) is often unexplained because their mutual dependency through presumed trophic interactions is often overlooked. Here, we collected bacterioplankton and HNFs from the surface layer and depth of chlorophyll-a maximum (DCM) along 13 degrees of latitude in the Kuroshio from Taiwan to Japan. We performed three-way variation partitioning analyses to unravel their reciprocal influences, in addition to dispersal limitation and environmental dissimilarity. Our analyses reveal that bacterioplankton and HNFs mutually and uniquely explain over 10% of compositional variation when both layers are analyzed collectively, underscoring the significance of trophic interactions. The vertical compositional variations of bacterioplankton and HNFs also reciprocally explain each other. Whereas, considering the surface layer alone, bacterioplankton uniquely explains 12.9% of HNF composition, but HNF makes a non-significant unique contribution to bacterial composition. In the DCM layer, the two trophic levels do not mutually explain each other. Dispersal limitation uniquely accounts for more than 20% of the compositional variation in each layer but does not significantly explain vertical variations. Environmental dissimilarity makes a minor contribution to both trophic levels. Our results suggest that influences of trophic interactions are more pronounced when considering vertical rather than horizontal variations. The horizontal dispersal limitation dominates the compositional variation within a water layer, while trophic interactions between bacterioplankton and HNFs mediate their compositional variation across depths. By considering two trophic levels, this study advances our understanding of metacommunity dynamics of free-living bacterioplankton and HNFs.