Seasonal genotypic and phenotypic differentiation of a cosmopolitan freshwater diatom

Most ecosystems are characterized by seasonality, which, through biotic and abiotic changes, influences species biomass dynamics. Recent studies have shown that ecologically important traits can evolve rapidly in response to environmental changes, resulting in eco‐evolutionary dynamics with consequences for population and community dynamics. Evidence for seasonal effects on intraspecific variation is still scarce and understanding eco‐evolutionary dynamics in the presence of seasonal fluctuations remains a major challenge. Following the phytoplankton spring bloom in Lake Constance, we investigated how seasonal changes influence the intraspecific diversity of Asterionella formosa both at genotypic and phenotypic levels. We found a moderate degree of genetic and phenotypic differentiation characterizing the Asterionella population, explained by a clustering of the isolates into early and late spring according to lake thermal stratification. Yet, most traits related to environmental parameters as well as fitness in different seasonal environments did not show a clear response to seasonality (i.e., temperature and nutrients). The changes in genetic patterns observed after a peak in parasite relative abundance suggested that seasonal changes in biotic interactions (i.e., parasitic chytrids) might be an important driver of the observed seasonal shift in Asterionella genotypes. Our results highlight the importance of studying eco‐evolutionary processes for understanding variations in population and community dynamics in response to seasonal environmental fluctuations.