Temperature and resources interact to shape phytoplankton size-abundance relationships at a continental scale
Communities contain more individuals of small species and fewer individuals of large species. The observed -3/4 slope relating mean size and mean abundance across communities (the cross-community scaling relationship or CCSR) is thought to arise from a 3/4-power scaling of metabolic rate with body size. Assuming that 3/4-power metabolic scaling is universal, as claimed by the metabolic theory of ecology (MTE), size-abundance relationships should also be invariant with a scaling exponent of -3/4 or nearly so, across environmental conditions. However, we find that the CCSR slope is actually -1 overall (perhaps due to isometric metabolic scaling), and varies substantially across temperature and nutrient gradients in 1048 freshwater lake phytoplankton communities in the USA. The size-abundance relationship is most shallow at low temperatures and high nutrients, and steeper and relatively insensitive to nutrients at high temperatures. Phytoplankton communities have orders of magnitude more small or large cells depending on environmental conditions than would be predicted from the MTE. Although based on observational data, these results suggest that the environment alters either metabolic scaling or other poorly understood processes shaping community size distributions.