Biodiversity affects ecosystem properties through changes in the trait composition and variation of natural communities. Using phytoplankton data obtained across 28 lakes we found that trait evenness - the regularity in distribution of morpho-physiological traits of individual organisms - was the strongest predictor of community resource use and biomass yield. Our results suggest that elucidating the mechanisms linking individual-level trait variation to community dynamics could improve our ability to forecast changes in ecosystem properties across environmental gradients.