Background/Question/Methods: Phytoplankton compete for nutrients and light in a spatially-complex environment determined by the vertical distribution of turbulent mixing. We analyzed a model of competition between phytoplankton species in a stratified water column. We assume the surface layer is uniformly mixed and deep layer is poorly mixed, as is commonly observed in lakes and the ocean. We employed two analytical techniques, Iout-R* theory in the surface mixed layer and the ESS theory in the deep layer. We verified the robustness of these analytical results with numerical simulations. Results/Conclusions: At equilibrium, each species is either absent or resides in a benthic layer, a deep chlorophyll maximum (DCM), or in the surface mixed layer. For two species with a trade-off between light and nutrient competitive ability, five cases of coexistence and the corresponding parameter regions were analytically obtained. Good light competitors show two distinct ecological niches: in mesotrophic conditions, they live as understory species in a DCM below a well-mixed layer of good nutrient competitors; in eutrophic conditions, they live as competitive dominants in light-limited mixed layers. Multiple regions of multiple stable states are possible. This work synthesizes the competition theory of phytoplankton in a well-mixed water column developed by Huisman and Weissing (1995) and the game theoretical approach in a poorly-mixed water column by Klausmeier and Litchman (2001).