Global patterns in phytoplankton community size structure—evidence for a direct temperature effect

In this study, a global data set on size-fractionated chlorophyll distributions collected in the open ocean (depth >400 m) is used to investigate phytoplankton community size structure in relation to temperature and inorganic nutrient availability in an attempt to identify the individual and shared effects of these 2 factors. The macroecological patterns show an increase in the fraction of large phytoplankton with increasing nutrient availability and a decrease with increasing temperature. We empirically demonstrate that temperature has both a nutrient-independent effect and a nutrient-shared effect on phytoplankton community size structure. We argue that the nutrient-independent effect is likely a direct effect of temperature, whereas the nutrient-shared effect may be an indirect effect of temperature (where thermal stratification influences the introduction of nutrients to surface waters). When regional differences in the average contribution of large cells were accounted for, the nutrient-independent effect of temperature explained 8% of the variation in phytoplankton community size structure compared with the 23% explained by the nutrient-shared effect. The results suggest that the relationship between phytoplankton community size structure and temperature change is the same in all ocean regions and leads to a decrease in the relative contribution of large cells in the community as temperature increases regardless of ambient nutrient availability. As phytoplankton size is an important factor influencing carbon transport to the deep ocean, it is important to incorporate any possible direct temperature effect on phytoplankton community size composition in models addressing carbon flow and metabolism in a warming ocean.