Understanding how marine phytoplankton and their primary consumer (i.e., microzooplankton) will respond to projected climate warming is critical for enhancing our ability to predict the response of marine ecosystems to climate changes over the next century. There is still poor understanding of thermal responses of phytoplankton and microzooplankton at community levels and how adaptation and resource availability affect their thermal sensitivities. To gain more in-depth insights into the response of marine plankton to warming, we conducted a series of experiments and evaluated the temperature sensitivity of plankton through activation energy (E_a) based on the framework of Metabolic Theory of Ecology (MTE). We first estimated the E_a of phytoplankton growth rate and microzooplankt...[ Read more ]

Understanding how marine phytoplankton and their primary consumer (i.e., microzooplankton) will respond to projected climate warming is critical for enhancing our ability to predict the response of marine ecosystems to climate changes over the next century. There is still poor understanding of thermal responses of phytoplankton and microzooplankton at community levels and how adaptation and resource availability affect their thermal sensitivities. To gain more in-depth insights into the response of marine plankton to warming, we conducted a series of experiments and evaluated the temperature sensitivity of plankton through activation energy (E_a) based on the framework of Metabolic Theory of Ecology (MTE). We first estimated the E_a of phytoplankton growth rate and microzooplankton grazing rate in subtropical coastal waters. We found that phytoplankton growth has a lower E_a (0.36 eV, 95% CI = 0.28-0.44 eV) than microzooplankton grazing (0.53 eV, 95% CI= 0.47-0.59 eV). This result is consistent with previous studies which states that the temperature sensitivity of autotrophic rates is lower than heterotrophic rates. However, we attribute this difference to the differential proximities of optimal temperature (T_opt in nonlinear thermal reaction curve), as we also found T_opt of phytoplankton growth rate is lower than that of microzooplankton and closer to environmental temperature. Subsequently, with the finding of T_opt of phytoplankton community, we further examined the thermal response of phytoplankton at a relatively long-term scale (one year). We found the maximal growth rate of the phytoplankton community increased with T_opt In contrast, E_a of maximal growth rate (0.47 eV, 95% CI: 0.25 - 0.69) was significantly lower than that at a short-term scale (1.65 eV, 95% CI: 1.29 - 2.02), which indicates that seasonal adaptation could compensate the acute short-term thermal response of the phytoplankton community. Next, we investigated how resource availability affects the thermal response of phytoplankton by conducting temperature and nutrient modulated experiments in the oligotrophic subtropical northwest Pacific. We found that nutrient limitation reduces the temperature sensitivity of Synechococcus growth rate, which is likely due to the temperature-dependent nature of the half-saturation constants of Synechococcus growth. In contrast, Prochlorococcus growth rate is never limited by in situ nutrient concentrations even under warming conditions. Finally, we attempted to quantify the effect of prey concentration and temperature on microzooplankton based on the framework of MTE. E_a for microzooplankton grazing rate was 0.51 eV, which supports the predicted value of MTE and demonstrates the effect of temperature on microzooplankton. We conclude that our findings in this thesis provide quantitative insights into the thermal response of marine plankton and are conducive to better inclusion of plankton in biogeochemical or ecosystem models, which, therefore, aids predictions of the impact of warming on marine ecosystems.