Effects of anthropogenic disturbance on stream metabolism: A global meta-analysis

Streams are experiencing substantial structural and functional alterations under increasing anthropogenic pressures. Understanding how stream metabolism responds to anthropogenic disturbances is critical for watershed management, as it is a key ecosystem function that regulates organic carbon cycling. However, global-scale assessments remain limited. In the present study, we collected data on stream metabolism, in-stream environmental variables, and watershed land use from 91 natural and 167 anthropogenically disturbed streams across the globe. We compared metabolism in natural and disturbed streams and developed a scenario analysis framework using Bayesian networks (BNs) to determine the distinct effects of agricultural and urban land use on stream metabolism. Intergroup comparisons revealed changes in stream environments and metabolism, including nutrient enrichment and hydrological and riparian alterations, with a 2.3-fold increase in median gross primary production (GPP) and greater variability in disturbed streams. We also found that the increases in GPP along the latitudinal and watershed area gradients observed in natural streams were altered or lost under disturbed conditions. In addition, the relationships between environmental variables and metabolism shifted, with discharge and photosynthetically active radiation (PAR) showing group-specific effects. Finally, scenario analysis showed that GPP responded more sensitively than ecosystem respiration (ER) to anthropogenic disturbance. Both agricultural and urban land use drove the increase in GPP by promoting nutrient enrichment, while agriculture also affected GPP through changes in discharge and water temperature. Our findings provide global evidence of changes in stream metabolism under anthropogenic pressures and highlight the need for targeted management strategies to sustain ecosystem functioning.