Global patterns and hierarchical controls of metabolism from headwaters to large rivers

• GPP increases with river size. • Latitude acts as a key cross-scale driver in large rivers. • Light, temperature, and nutrients dominate metabolism in small rivers. • Global river exhibited asynchronous declines in GPP and ER. Metabolic function is a fundamental property of river ecosystems. Identifying the key controlling factors of metabolic processes remains a central challenge in predicting the role that rivers play in global carbon cycle. Here, using 3334 observations [1732 for gross primary productivity (GPP) and 1602 for ecosystem respiration (ER)] collected from 224 published studies, we quantitatively evaluated the patterns and drivers of metabolism in rivers. Results showed that (1) global rivers were predominately heterotrophic across most river scales and biome regions, with GPP ranging from −0.301 to 29.1 g O 2 m −2 d −1 and ER ranging from −39.0 to 0 g O 2 m −2 d −1 ; (2) GPP increased significantly with river size, whereas ER showed a weak relationship with river size due to the scaling effects of biotic communities; (3) factors governing metabolic processes varied significantly among different river scales, but latitude was a cross-scale regulator in large rivers (Strahler stream order ≥ 7th); (4) headwater stream (1st-3rd orders) metabolism was dominated controlled by light availability and temperature, while that in intermediate rivers (4th-6th orders) demonstrated high sensitivity to river morphology, nutrient availability, and anthropogenic pressures; and (5) over the past 30 years (1991–2021), river metabolism exhibited pronounced seasonal variation—peaking in summer—and asynchronous declines in GPP and ER. Our hierarchical regulation framework would optimize the ecological management of water resources and provide critical information for better prediction of global river ecosystem carbon fluxes.