Adaptive plasticity evolution across urban aquatic landscapes in water fleas

Understanding how aquatic populations respond to urbanization is a central challenge in contemporary ecology and evolution. Rapid urban expansion alters hydrology and nutrient fluxes, accelerating eutrophication and promoting recurrent cyanobacterial blooms in urban waters. Yet, how these environmental changes influence the phenotypic and evolutionary trajectories of aquatic organisms remains poorly understood. Using an integrated multivariate framework combining life-history and population-genetic analyses, we examined whether and how zooplankton grazers inhabiting contrasting urban and rural aquatic landscapes have adapted to cyanobacterial stress. In a common-garden experiment with Ceriodaphnia cornuta, urban populations exhibited clear genetic differentiation and higher tolerance to toxic cyanobacteria than rural populations. This tolerance was primarily associated with reduced cyanobacteria-induced phenotypic plasticity, indicating evolutionary canalization of plastic responses under persistent eutrophic stress. Partitioning trait changes revealed that the evolution of plasticity accounted for most of the observed divergence across multiple life-history traits. Our results provide evidence for contemporary evolution of phenotypic plasticity within urban freshwater ecosystems, demonstrating that strong, predictable stressors such as cyanobacterial blooms can rapidly reshape reaction norms in zooplankton. These findings highlight the importance of incorporating evolutionary processes into urban water-quality management to sustain ecosystem function and resilience in rapidly urbanizing regions.