Marine phytoplankton in subtropical coastal waters showing lower thermal sensitivity than microzooplankton

Temperature sensitivity of plankton in terms of activation energy (E-a, eV) in the Arrhenius equation is critical for predicting how marine productivity and carbon export will respond to ocean warming. In this study, we quantified the temperature responses of phytoplankton growth rate and microzooplankton grazing rate by conducting short-term temperature modulation experiments on natural communities at two subtropical sites with contrasting nutrient conditions. Our results showed that the activation energy of phytoplankton growth rate (E-a = 0.36 eV, 95% confidence interval [CI] = 0.28-0.44 eV) at each station was less than that of microzooplankton grazing rate (E-a = 0.53 eV, 95% CI = 0.47-0.59 eV), indicating an increasing grazing pressure on phytoplankton under warming conditions. Although the difference is consistent with that reported in previous studies, it is very likely to arise from another reason, i.e., differential proximities of the optimal temperature (T-opt in nonlinear temperature responses of rates) of phytoplankton and microzooplankton to the environmental temperature, as we found that the environmental temperature is closer to the optimal temperature of phytoplankton growth than to that of microzooplankton grazing in this subtropical environment. Our results suggest that nonlinear temperature responses of plankton should be considered when evaluating and predicting the effects of ocean warming on ecosystem productivity and food web dynamics, especially in subtropical and tropical waters.