Changing Rates and Shifting Ranges: Assessing the Phytoplankton Global Response to Ocean Warming

Ocean warming will influence the metabolic rates and geographic ranges of phytoplankton, with implications for global primary production and biogeochemical cycling. Variation in the physiological effects of temperature across functional groups could lead to altered marine phytoplankton biogeography and community structure. To assess the relative capacity of phytoplankton functional groups to cope with thermal change, we aggregated and analyzed empirically derived thermal reaction norms from diatoms, dinoflagellates, cyanobacteria, and coccolithophores. Using Earth system model data to estimate sea surface changes between 1950-1970 and 2080-2100, we explored potential alterations to each group’s growth rates and temporal and geographical distribution. Our data suggest phytoplankton functional groups may be differentiated by their thermal envelopes and temperature coefficients. Between each functional group, thermal reaction norms exhibit varied levels of skewness, which would drive dissimilar responses to each degree of change below or above their thermal optima. Among the functional groups, low-latitude diatoms and dinoflagellates appear the most susceptible to rising temperatures, while cyanobacteria may be subject to the greatest increases in both growth and geographic range. However, for all groups, thermal tolerance is greater than expected. These results suggest that the singular effect of temperature may alter phytoplankton global community structure and biogeography, owing to the significant thermal variability between phytoplankton functional groups.