Predicting temperature impacts on aquatic productivity: Questioning the metabolic theory of ecology's “canonical” activation energies

Microalgae contribute ~ 50% to global primary production, most of which is consumed by protozoa. Determining the thermal‐sensitivity of this trophic interaction is, therefore, fundamental to predicting impacts of climate change. Here, we question the application of current predictive approaches. Thermal responses are commonly described by the Arrhenius function: urn:x-wiley:00243590:media:lno11105:lno11105-math-0002, where r is a rate (e.g., growth), A is a scaling factor, Ea is the activation energy, k is the Boltzmann‐constant, and T is absolute temperature. The influential metabolic theory of ecology (MTE) proposes that estimates of Ea for heterotrophs and autotrophs are 0.65 eV and 0.32 eV, respectively; when applied to specific growth rate of algae and protozoa, this difference has significant predictive consequences. Through literature review and statistical evaluation, we show that the MTE predictions do not apply to taxon‐specific responses of protozoa (n = 103) or algae (n = 183), with mean Ea of 0.71 eV (95% confidence interval [CI]: 0.69–0.74) and 0.61 eV (95% CI: 0.58–0.63), respectively. To obtain these, we fitted a series of models where Ea was constant within a defined group (e.g., protozoa), and the amplitude A depended on the individual responses within the group. Then, by applying the MTE and our predictions to a generic protozoan‐algal, predator‐prey model we show that: (1) the “canonical” MTE values lead to misrepresenting productivity by several fold; (2) a general response encompassing both groups (0.69 eV) should suffice for such models; and (3) applying our new responses has substantial effects on algal‐protozoan population dynamics over temperature shifts of ~ 5°C.