A model approach to planktonic stoichiometry and consumer-resource stability

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 ( Q _a).
4. Qualitatively deficient food will yield decreased growth efficiency in zooplankton. As Q _a 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 Q _a below which the grazer with high P : C suffers negative population growth regardless of algal biomass. In cases with low minimum Q _a 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 Q _a 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.