Grazers modify the dinoflagellate relationship between toxin production and cell growth

•Cellular toxin content of the dinoflagellate Alexandrium catenella during a laboratory-simulated bloom increased with time and plateaued after the stationary phase.•There was a significantly positive relationship between growth rate and constitutive toxin production, predominantly in the exponential phase.•Grazer-induced toxin production was evident throughout the bloom and greatest in the exponential phase.•Cell growth rate was negatively related to grazer-induced toxin production, consistent with a defense-growth trade-off.•The dynamics of cell growth vs toxin production are fundamentally different for constitutive and inducible defense. Although the typical framework for studies and models of bloom dynamics in toxigenic phytoplankton is predominantly based on abiotic determinants, there is mounting evidence of grazer control of toxin production. We tested for the effect of grazer control of toxin production and cell growth rate during a laboratory-simulated bloom of the dinoflagellate Alexandrium catenella. We measured cellular toxin content and net growth rate when cells were exposed to copepod grazers (direct exposure), copepod cues (indirect exposure), and no copepods (control) throughout the exponential, stationary, and declining phases of the bloom. During the simulated bloom, cellular toxin content plateaued after the stationary phase and there was a significantly positive relationship between growth rate and toxin production, predominantly in the exponential phase. Grazer-induced toxin production was evident throughout the bloom, but highest during the exponential phase. Induction was greater when cells were directly exposed to grazers rather than their cues alone. In the presence of grazers toxin production and cell growth rate were negatively related, indicating a defense-growth trade-off. Further, a fitness reduction associated with toxin production was more evident in the presence than the absence of grazers. Consequently, the relationship between toxin production and cell growth is fundamentally different between constitutive and inducible defense. This suggests that understanding and predicting bloom dynamics requires considering both constitutive and grazer-induced toxin production.