Assessing Urban River Health: Phytoplankton as a Proxy for Resource Use Efficiency and Human Impact

This study assesses the ecological conditions of the Colentina urban river system by investigating phytoplankton community traits, with a focus on resource use efficiency (RUE) as a functional indicator. Using phytoplankton biomass, taxonomic composition, and RUE, we assessed the ecological effects of anthropogenic pressures. Our results showed that total phosphorus values indicated chronic eutrophication conditions but supported increased phytoplankton biomass, especially in spring and summer. RUE varied independently of biomass, with maximum values recorded in autumn, suggesting a functional recovery phase, characterized by higher RUE under nutrient decline. The analysis at the phytoplankton group level highlighted distinct ecological strategies: cyanobacteria presented a high RUE in autumn, diatoms increased their efficiency during nutrient limitation periods, and green algae showed a functional flexibility throughout the study period. In contrast, spatial analyses indicated a decoupling between biomass and RUE, reflecting the influence of local environmental conditions on ecosystem functioning. RUE was significantly influenced by total phosphorus, nitrogen forms, temperature and light availability. Our results strengthen the combined approach of structural (biomass) and functional (RUE) indicators for the assessment of communities and anthropogenic impacts in urban and peri-urban aquatic ecosystems.