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1. The model explores stoichiometric feedback effects between an alga and a grazer (Daphnia) in a simplified chemostat‐type system in stagnant conditions or with fixed dilution rate. 2. When running the model with fixed stoichiometry and P‐sufficient food, the grazer with highest requirements for phosphorus (P) will exert the most efficient control of algal biomass owing to more P being allocated to zooplankton biomass and less P recycled. 3. When including potential P‐limitation of the grazer, the grazer with high P requirements (high P : C ratio) will be the least efficient grazer in a system with fluctuating and temporarily low P : C ratio in algae (Qa). 4. Qualitatively deficient food will yield decreased growth efficiency in zooplankton. As Qa decreases, the grazer isocline for zero net growth is shifted upwards, and the required algal biomass for positive growth increases. There may then be a critical level of Qa below which the grazer with high P : C suffers negative population growth regardless of algal biomass. In cases with low minimum Qa and a P‐demanding grazer, this may cause the system to enter an irreversible stage with high biomass of P‐deficient phytoplankton which do not support zooplankton growth. 5. Cumulative primary production for scenarios with continuous P input is, in general, higher the more Qa is allowed to drop below saturation values, and highest when this is combined with a grazer with a high P : C ratio. The lower growth rate of P‐deficient phytoplankton was compensated for by reduced success of the P‐limited grazer, yielding low grazing pressure and resulting in larger phytoplankton biomass.