Top-down effects of the freshwater jellyfish Craspedacusta sowerbii : an experimental approach

Abstract The freshwater jellyfish Craspedacusta sowerbii, an invasive species with a complex life cycle and high dispersal potential, is increasingly reported across global freshwater systems. Its medusa stage acts as a gelatinous, planktonic predator—an absent functional group in most freshwater pelagic systems—capable of altering food web dynamics. Through controlled mesocosm experiments using three different lake communities, we investigated whether C. sowerbii can trigger trophic cascades similar to marine jellyfish. Our results demonstrate that even at low densities (14–114 ind. m−3), C. sowerbii significantly reduced crustacean mesozooplankton biomass, releasing phytoplankton from grazing pressure and increasing chlorophyll a concentrations up to 4-fold. Predation was size-selective, primarily targeting larger zooplankton taxa (e.g. Daphnia sp.), leading to shifts in community size structure and diversity. Phytoplankton responses were taxon-specific, with green algae and brown pigmented algae exhibiting the strongest growth. Jellyfish presence enhanced phosphorus-use efficiency and altered nutrient dynamics, indicating strong top-down control. These findings reveal that C. sowerbii can function as an ecologically significant top predator, restructuring plankton communities and energy pathways even outside bloom events. They highlight the importance of integrating gelatinous predators into lake monitoring frameworks and ecological models, especially under future climate scenarios that may favour jellyfish proliferation.