Abstract
We examined the separate and interactive effects of light, nutrients, and zooplanktivorous fish on phytoplankton nutrient stoichiometry, taxonomic richness, community composition, and the associations among these variables. We manipulated light supply, nutrient supply, and zooplanktivorous fish (presence or absence), in a factorial design using mesocosms containing regional phytoplankton and zooplankton assemblages. C : N and C : P were highest under high nutrients and high light. We observed interactive effects on the relative abundance of major phytoplankton taxa. Chlorophytes were most abundant in the majority of the treatments, although cyanobacteria dominated under high light, low nutrients, and with fish present. Cryptomonads were rare in all but the low‐light, high‐nutrient treatment. Relative abundance of diatoms was negatively affected by light, but only in the presence of fish. Chlorophyte relative abundance (% chlorophytes) was positively correlated with seston C : P, while relative abundances of diatoms and cryptomonads were negatively correlated with C : P. Phytoplankton taxon richness was positively associated with both % chlorophytes and seston C : P, and the number of chlorophyte taxa increased disproportionately with total taxon richness. Assemblage‐level stoichiometric responses may be driven not only by direct responses to resource supply ratios, but also by algal community response. In particular, factors that increase taxonomic richness and both the richness and biomass of chlorophytes also result in increased seston C : P. Our results suggest strong feedbacks between phytoplankton community composition and stoichiometry, induced by interactive effects of light, nutrients, and predators.