1. Understanding and explaining the structure of communities in response to environmental gradients is a central goal in ecology. Trait-based approaches are promising but yet rarely applied to understand community dynamics in response to changing environmental conditions.

2. Here we investigate seasonal succession patterns of functional traits in phytoplankton communities and how nutrient reductions (oligotrophication) alter these patterns. We used phytoplankton data from 40 years of observation from the Rappbode Reservoir (Germany), which underwent a strong shift in trophic conditions, and translated taxonomic composition into functional traits by assigning trait values compiled from the literature.

3. All studied traits (morphological, behavioral and physiological traits) responded to changing environmental conditions and showed consistent, reoccurring seasonal developments. The seasonal succession of phytoplankton communities was shaped by a trade-off between small celled, fast growing species that are able to rapidly incorporate existing resources (r-strategists) and large celled species with more complex and efficient mechanisms to exploit scarce mineral nutrients or acquire previously unexploited nutrient pools (k-strategists). In summer, when nutrients were scarce, the k-strategy was prevailing (important traits: phosphate affinity, nitrogen fixation, motility and mixotrophy). During the rest of the year, nutrients and turbulence were high and r-strategists dominated (important traits: maximum growth rate, light affinity).

4. A comparison between eutrophic and oligotrophic years revealed that the main features of functional trait succession were largely preserved, but intra-annual fluctuations from spring to summer were stronger during eutrophic years. Nutrient reductions mainly affected functional traits and biomass in spring, while in summer the functional community composition changed little.

5. Synthesis. This study provides for the first time a quantitatively supported functional template for trait-based succession patterns in lakes under different nutrient conditions. By translating taxonomic composition into trait information, we demonstrate that the quantification of functional characteristics enables ecological interpretation of observed community dynamics and provides not only a testable template but also a powerful tool towards a more mechanistic understanding. The quantification of functional traits further improves the predictability of community shifts in response to changing environmental conditions and thus opens new perspectives for predictive limnology using lake-ecosystem models.