Functional Identity Drives Directional Shifts in Freshwater Plankton Biomass-Size Relationships

Body size is a master variable in ecology that can predict ecosystem-level processes and reflect environmental conditions across diverse ecosystems. In freshwater ecosystems, the relationship between body size and biomass provides a powerful tool for characterizing community structure and trophic transfer efficiency. While these relationships are well established at the whole-community level, responses within plankton functional groups with distinct ecological characteristics have yet to be explored. In this study, we analysed plankton biomass-size relationships across > 550 Canadian lakes. For phytoplankton, we found that mixotrophs exhibited positive biomass-size slopes, while autotrophs exhibited negative slopes. For zooplankton, herbivores exhibited relatively flat slopes, while non-herbivores exhibited negative slopes. Biomass-size relationships also differed among lake trophic states, but only for phytoplankton functional groups. This work should improve predictions and assessments of freshwater ecosystem functioning and food web pathways, based on our observation of directional shifts in biomass-size relationships across functional groups and trophic states.