lightdependent grazing can drive formation and deepening of deep chlorophyll maxima deep chlorophyll maxima dcms are subsurface peaks in chlorophylla concentration that may coincide with peaks in phytoplankton abundance and primary productivity work on the mechanisms underlying dcm formation has historically focused on phytoplankton physiology eg photoacclimation and behavior eg taxis while these mechanisms can drive dcm formation they do not account for topdown controls such as predation by grazers here we propose a new mechanism for dcm formation lightdependent grazing by microzooplankton reduces phytoplankton biomass near the surface but allows accumulation at depth using mathematical models informed by grazing studies we demonstrate that lightdependent grazing is sufficient to drive dcm formation further when acting in concert with other mechanisms lightdependent grazing deepens the dcm improving the fit of a global model with observational data our findings thus reveal another mechanism by which microzooplankton may regulate primary production and impact our understanding of biogeochemical cycling at and above the dcm