River oligotrophication under global change: A systematic review of long-term nutrient trends and biological responses

Anthropogenic nutrient enrichment has profoundly altered riverine biogeochemical cycling and ecosystem functioning worldwide. To promote a clearer definition of “oligotrophication” in ecological studies, we define the term specifically as the physicochemical decline in nutrient concentrations, distinguishing it from “biological recovery”, which encompasses algal biomass reduction, community restructuring, and functional adaptation. Here, we synthesise global evidence for long-term (≥10 years) trophic change in rivers through a systematic review of 118 studies covering 413 river basins across six continents. Trends show marked regional contrasts: oligotrophication is widespread in Europe, whereas eutrophication persists across East Asia and Africa. North American rivers frequently exhibit stalled recovery or decoupled physicochemical and biological trajectories. Across all regions, warming, hydrological alteration, and legacy nutrients repeatedly emerge as crucial recovery constraints. Crucially, where long-term biological data are available, nutrient reductions rarely guarantee a return to historical baselines; instead, ecosystems frequently shift towards novel ecological trajectories. However, because equivalent long-term data on community composition, biodiversity, and ecosystem functioning remain exceptionally scarce compared to physicochemical records, the broader global extent of these novel biological responses remains only partially resolved. Consequently, current monitoring programmes capture chemical changes far more effectively than biological recovery. Future monitoring should emphasise long-term biological responses to accurately evaluate whether nutrient reductions translate into functional ecosystem recovery.