Indirect bottom‐up control of consumer‐resource dynamics: Resource‐driven algal quality alters grazer numerical response

The nature of the growth‐rate response of grazing zooplankton to algal concentration depends critically upon food quality, which is often a plastic function of environmental conditions. Phytoplankton quality depends upon nutrient and light availability, which together affect cell C : P and C : N, so that the links from external environment to phytoplankton and then to zooplankton can vary in ways that influence consumer‐resource interaction strengths, and ultimately, food web structure. We measured the effects of resource‐limitation on the population growth rate of rotifers, Brachionus calyciflorus, feeding on the alga Chlamydomonas reinhardtii, and grown in nitrogen‐limiting, phosphorus‐limiting, or nutrient‐sufficient conditions. Consumption of nutrient‐sufficient algae produced a Type II numerical response with a low half saturation constant (K s ) and high asymptotic growth rate ( λ max ) consistent with high food quality; P‐limited algae also yielded a Type II response but with high K s and low λ max , meaning poor food quality. In contrast, N‐limited Chlamydomonas resulted in a novel Type IV numerical response in which rotifer growth rate rose to an intermediate λ max and then declined at the highest algal densities, suggesting the production of toxic compounds whose negative effect on growth increases as a function of cell density. Experiments with mixed diets confirmed that N‐limited food was indeed toxic, whereas P‐limited food was simply nutritionally deficient compared with light‐limited algae. Additional experiments showed that even the filtrate of N‐limited algae can be detrimental to rotifer growth.