Detecting long-term trajectories in ecosystem functioning across European land river sea continuum: insights from four major river basins - Danube, Po, Elbe and Guadalquivir

Long-term changes in freshwater and coastal ecosystems is tightly linked to anthropogenic drivers, yet understanding these trajectories requires coherent, high-resolution time series that span entire land-river-sea continuum gradients. Using the source-to-sea paradigm and a conceptual model linking upstream protected areas to downstream coastal conditions (ecosystem services, mediating flows and ecological responses) we evaluated how upstream terrestrial and freshwater conservation areas influence ecosystems across four major European land–river–sea systems: the Danube River - Black Sea, Po River - Adriatic Sea, Elbe River - North Sea and Guadalquivir River - Atlantic Ocean. Drawing on available multi-decadal datasets on protected areas, nutrient (nitrogen and phosphorus) concentrations, ecological water quality based on Water Framework Directive reported ecological quality ratios (EQRs) and chlorophyll-a we developed an integrated conceptual and empirical framework spanning 1990–2024. We analyse how basin wide protected areas extent shape nutrient attenuation and ecosystem functioning across freshwater, transitional, and coastal domains. Strong multi-decadal declines in nitrogen—and to a lesser extent phosphorus—emerge in the Danube and Elbe rivers, particularly in freshwater and transitional waters, where increased protected-area basins coverage aligns with reduced nutrient concentrations and lower chlorophyll-a, signalling improved ecosystem functioning and reduced eutrophication risk. These effects propagate, albeit attenuated, into coastal zones, demonstrating that upstream conservation can generate measurable downstream benefits. In contrast, the Po and Guadalquivir systems exhibit weak or undetectable trends, reflecting extensive hydromorphological alteration, fragmented wetlands, or severe monitoring gaps that obscure underlying functional changes. Across basins, results highlight the critical role of floodplain connectivity, land-use intensity, and legacy pollution in shaping long-term nutrient trajectories, as well as the limits imposed by non-harmonised, discontinuous monitoring—especially in transitional waters. The analysis underscores the urgent need for integrated basin-to-coast observation systems, such as those being advanced by eLTER, to co-locate biodiversity, ecosystem-function, and driver data. By linking nutrient and chlorophyll dynamics to ecosystem service pathways, the study provides a functional lens to interpret biodiversity change in the Anthropocene and demonstrates how data limitations currently bias our capacity to detect trends and attribute them to conservation actions.