Understanding the mechanisms driving community structure and dynamics is crucial in the face of escalating climate change, including increasing incidences of extreme weather. Cell size is a master trait of small organisms that is subject to a trade-off between resistance to grazing and competition for resources, and thus holds potential to explain and predict community dynamics in response to disturbances. Here, we aimed at determining whether cell size can explain shifts in phytoplankton communities following changes in nutrient and light conditions resulting from storm-induced inputs of nutrients and colored dissolved organic matter (cDOM) to deep clearwater lakes. To ensure realistic environmental conditions, we used a crossed gradient design to conduct a large-scale enclosure experiment over 6 weeks. Cell size explained phytoplankton community structure when light availability declined as a result of cDOM supply. Initially unimodal, with small-celled species accounting for up to 60% of the total community biovolume, the cell-size distribution gradually shifted toward large-celled species as light levels declined following cDOM addition. Neither nutrients nor mesozooplankton affected the shift in cell-size distribution. These results suggest a distinct competitive advantage of larger over smaller species at reduced light levels following cDOM inputs during storm events. Importantly, the clustering of species in two distinct size classes implies that interspecific size differences matter as much as cell size per se to understand community dynamics. Given that shifts in cell-size distribution have strong implications for food-web structure and biogeochemical cycles, our results point to the importance of analyzing cell-size distributions of small organisms as an essential element to forecast community and ecosystem dynamics in response to environmental change.