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

Abstract

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 individ- ual 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 increas- ing 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 intro- duction 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 commu- nity 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 influenc- ing carbon transport to the deep ocean, it is important to incorporate any possible direct tempera- ture effect on phytoplankton community size composition in models addressing carbon flow and metabolism in a warming ocean.