Relationships between nutrient N:P ratio and P-limitation in phytoplankton are explored using a multi-nutrient photoacclimative quota-based model. The relationship depends on concentrations of input and residual nutrients, and also on variable phytoplankton C:N:P stoichiometry. In reality, usually only the residual nutrient concentrations and their ratios are known. However, the total amount of nutrient present in the system affects biomass growth potential through self-shading, and thence the potential for variation in organismal N:P. The critical external N:P resource ratio above which P becomes limiting increases as residual concentrations of nutrients increase to saturate transport kinetics; oligotrophic waters require a lower nutrient N:P to avoid P-limitation than do eutrophic waters. In eutrophic systems, which may support harmful algal blooms (HABs), and/or in systems in which light is rapidly attenuated (sediment loading, gelbstoff), P-limitation may not develop even in high resource N:P situations due to light limitation. This is more likely in high washout systems, where phytoplankton growth rates must remain elevated. The only diagnostics for nutrient stress are cellular functions (C-fixation, C:N:P), and the only nutrient parameters of consequence are concentrations and not ratios of them. Control of resource ratios alone should not be considered as a tool for mitigating HABs.