AbstractThe mechanism that determines the dynamics of subsurface chlorophyll maximum depth (z_SCM) has long been debated. Although a coupling between z_SCM and the top of nitracline (z_nit) has been widely observed in the open ocean, a co‐location of z_SCM and an isolume depth (z_iso) has often been reported in oligotrophic waters. In this study, based on continuous observations of ten BGC‐Argo floats, we found that the seasonal displacement of z_SCM in all subtropical gyres was driven mainly by light, but z_SCM displayed a nutrient‐driven pattern occasionally when z_nit became shallower than z_iso. We therefore proposed a “two‐group competition framework”: z_SCM in subtropical gyres is determined by the competition of two phytoplankton groups, nutrient‐sensitive picoeukaryotes and light‐sensitive Prochlorococcus. When z_nit (z_iso) is shallower than z_iso (z_nit), picoeukaryotes (Prochlorococcus) dominate the chlorophyll biomass at the SCM, and thus z_SCM follows z_nit (z_iso). This paradigm reconciles the inconsistent conclusions drawn from earlier studies.