Scientists, managers, and citizens use the concept of the trophic state to characterize lakes by categorizing them into oligo-, meso-, or eutrophic states based on a few snapshot measurements. However, lakes have seasonal variability in phytoplankton biomass with the possibility for periodic blooms that impair water quality and ecosystem services. First, we developed a dynamic mathematical simulation model to understand seasonal cycles of nutrient uptake and phytoplankton growth. We found predictable cycles of phytoplankton biomass and available nutrients related to seasonal internal loading. Second, we assessed three years of nutrient limitation data from in situ incubations in 9 regional lakes and found that most of the lakes had interannual variability in nutrient limitation type, with colimitation being most common. Together, these results support the need for a more dynamic framework based on function, process, and temporal change that would better inform management actions and understanding of lake ecosystems.