Dynamic biogeochemical controls on river pCO 2 and recent changes under aggravating river impoundment: An example of the subtropical Yangtze River

This paper highlights two aspects of the dynamic biogeochemical controls of riverine pCO 2 in an increasingly impounded large subtropical river (the Yangtze): the terrestrial dominance through internal respiration of land‐derived organic carbon and the influence of increased autotrophic activity in impounded areas on river pCO 2 . River pCO 2 and total organic carbon (TOC) increase downstream on the main stem (pCO 2 : 528–1703 µatm; TOC: 137–263 µmol/L) and vary significantly among tributaries (464–3300 µatm; TOC: 109–340 µmol/L). pCO 2 displays larger spatial variability than temporal variability and is spatially correlated with river organic carbon across the river ( p  < 0.05–0.0001, seasonally independent). pCO 2 is also negatively correlated with dissolved oxygen ( r 2  = 0.46, p  < 0.0001). Respiration of allochthonous organic carbon in water column is concluded as an essential source of CO 2 supersaturation and river heterotrophy. However, significant benthic respiration and/or direct soil CO 2 transport (e.g., via groundwater, ~80%) exist at the same time. The temporal and spatial distribution of POC compositional characteristics and chlorophyll a indicate the dominant control of terrestrial processes (e.g., organic matter transport and soil erosion) on the river pCO 2 biogeochemistry, especially in warm seasons. Increased autotrophy and significant pCO 2 decrease (>60%) do occur in impounded areas (especially in nutrient‐rich rivers), but the decrease is mostly temporal and regional (~8% of the data points are significantly influenced, all from the upper reach and/or major tributaries). The paper concludes that terrestrial influence still dominates the pCO 2 biogeochemistry in this increasingly intercepted and regulated river system.