We evaluated a hypothesis derived from the metabolic theory of ecology (MTE) that the ratio of microzooplankton herbivory (m) to phytoplankton growth (μ) will arise in a warming ocean because of the different temperature dependencies of autotrophic and heterotrophic organisms. Using community‐level growth and grazing data from dilution experiments, generalized additive models (GAMs) were constructed to describe the effects of temperature and chlorophyll on m : μ. At low chlorophyll levels, m : μ decreases with increasing temperature, whereas at high chlorophyll levels, m : μ increases initially with temperature before reaching a peak and then declines. These complex responses of m : μ result from mixed effects of temperature and chlorophyll on microzooplankton biomass (B_z), biomass‐specific microzooplankton grazing rate (m : B_z), and phytoplankton growth rate (μ). B_z decreases with rising temperature and increases with rising chlorophyll. m : B_z increases with temperature and decreases with chlorophyll. Nutrient‐enriched growth rate of phytoplankton (μ_n) and μ increase with increasing temperature and chlorophyll. Holding chlorophyll constant, the calculated activation energies of m : B_z and μ_n are 0.67 ± 0.05 and 0.36 ± 0.05 eV, respectively, both consistent with previous MTE estimates for heterotrophs and autotrophs. Our study indicates that warming may enhance phytoplankton losses to microzooplankton herbivory in eutrophic but not in oligotrophic waters. The GAM analysis also provides important insights into underlying system relationships and reasons why community‐level responses in natural systems may depart from theory based on laboratory data and individual species.

Primary production by marine phytoplankton is one of the largest fluxes of carbon on our planet. In the past few decades, considerable progress has been made in estimating global primary production at high spatial and temporal scales by combining in situ measurements of primary production with remote-sensing observations of phytoplankton biomass. One of the major challenges in this approach lies in the assignment of the appropriate model parameters that define the photosynthetic response of phytoplankton to the light field. In the present study, a global database of in situ measurements of photosynthesis versus irradiance (P-I) parameters and a 20-year record of climate quality satellite observations were used to assess global primary production and its variability with seasons and locations as well as between years. In addition, the sensitivity of the computed primary production to potential changes in the photosynthetic response of phytoplankton cells under changing environmental conditions was investigated. Global annual primary production varied from 38.8 to 42.1 Gt C yr ${}^{-1}$ over the period of 1998–2018. Inter-annual changes in global primary production did not follow a linear trend, and regional differences in the magnitude and direction of change in primary production were observed. Trends in primary production followed directly from changes in chlorophyll-a and were related to changes in the physico-chemical conditions of the water column due to inter-annual and multidecadal climate oscillations. Moreover, the sensitivity analysis in which P-I parameters were adjusted by ±1 standard deviation showed the importance of accurately assigning photosynthetic parameters in global and regional calculations of primary production. The assimilation number of the P-I curve showed strong relationships with environmental variables such as temperature and had a practically one-to-one relationship with the magnitude of change in primary production. In the future, such empirical relationships could potentially be used for a more dynamic assignment of photosynthetic rates in the estimation of global primary production. Relationships between the initial slope of the P-I curve and environmental variables were more elusive.