Zooplankton Functional Structure in Relation to Ecosystem Stability and Resilience Along an Anthropogenic Gradient

Urban aquatic ecosystems are increasingly shaped by anthropogenic pressures that alter community structure and ecological functioning. This study investigates how the functional organization of zooplankton communities reflects ecosystem stability along an urbanization gradient in the Colentina River–Lake system (Romania). Zooplankton taxa were classified into trophic guilds and size-based functional groups, and functional diversity was quantified using the FEve, FDiv, FDis, and RaoQ indices, based on functional trait structure and distribution within communities. Ecosystem stability patterns were estimated through zooplankton community resilience (RSL) and resistance (RST), indices derived from Shannon diversity and Pielou evenness. Across the system, filter feeders dominated, and their density increased toward downstream, highly urbanized sectors. Also, small-sized organisms (SMC) were consistently prevalent, representing a zooplankton component commonly associated with stress tolerance and opportunism in disturbed aquatic environments. Functional diversity patterns showed low evenness but high divergence, suggesting that although few functional strategies dominate, communities maintain internal differentiation. While filter feeders remained dominant even in disturbed sectors, the uneven distribution of other groups, especially scrapers, may reflect greater sensitivity to anthropogenic conditions. These findings suggest that functional trait composition, in addition to diversity, plays an important role in shaping structural stability patterns. These findings indicate that functional trait composition, in addition to diversity, was associated with the observed stability patterns. The study reinforces the value of zooplankton as sensitive indicators of functional integrity in anthropogenically impacted freshwater systems and provides insights relevant for sustainable urban water management.