Biomass-weighted mean traits of a community's constituent species are a useful tool to assess environmental filtering in community function in response to environmental change. We show that annually averaged phytoplankton community function, expressed by the community mean traits phosphate and light affinity, responded strongly and reversibly to long-term changes in nutrient supply over a 42-year period of eutrophication and re-oligotrophication of Lake Constance. Within the lake's species pool, phosphate and light affinities were weakly negatively correlated, suggesting a weak physiological trade-off. Yet, a strong trade-off between these traits emerged when species were weighted by their biomass, suggesting species sorting along the trade-off line across years of shifting nutrient status. Emergent trade-offs, that is, trade-offs that become apparent first when trait combinations are weighted by the contributions of the trait-bearing organisms to community biomass, may be a useful, novel concept in trait-based ecology of potentially similar importance as commonly considered physiological trade-offs.