Energy-based top-down and bottom-up relationships between fish community energy demand or production and phytoplankton across lakes at a continental scale

Fish community feeding and production rates may differ between lakes despite similar fish biomass levels because of differences in size structure and local temperature. Therefore, across-lake comparisons of the strength and direction of top-down and bottom-up fish-phytoplankton relationships should consider these factors. We used the metabolic theory of ecology to calculate size- and temperature-corrected community energy demand (CED) and community production (CP) of omnivorous fishes in 227 European lakes from major habitat types (MHTs) of polar freshwaters, temperate floodplain rivers and wetlands, and temperate coastal rivers. We related CED with total phosphorus (TP)-corrected chlorophyll a (Chl a) concentrations to evaluate a potential top-down directed trophic cascade from fish to phytoplankton. Furthermore, we related Chl a with CP to demonstrate potential bottom-up effects of phytoplankton on fish. For both analyses, we added the CED of piscivorous fishes (CED) as a predictor to account for potential predation effects on the omnivorous fish community. CED was weakly positively related with TP-corrected Chl a, but the strength of the relationship differed between MHTs. In contrast, CP was consistently positively related with Chl a in the entire dataset. CED did not contribute to top-down or bottom-up relationships. The application of metabolic variables characterizing fish community feeding and production rates makes these results robust because the approach accounted for the usually neglected effects of fish size and temperature in across-lake comparisons. Our results suggest that bottom-up effects from phytoplankton on fish secondary production in lakes are substantially stronger than top-down effects from fish on phytoplankton biomass.