Substantial community variation in free‐living bacterioplankton and their main predators—heterotrophic and mixotrophic nanoflagellates (HNFs)—often remains unexplained due to the frequent oversight of their mutual dependency through reciprocal influences. Here, we sampled bacterioplankton and HNFs from both the surface layer and the deep chlorophyll maximum (DCM) across a 13‐degree latitudinal transect in the Kuroshio, spannin...