Heavy Rainfall Increases CO2 Emissions from Rivers in a Typical Human-Impacted Region

Rivers emit substantial amounts of carbon dioxide (CO2) to the atmosphere, yet its response to heavy rainfall remains unclear with intensive anthropogenic disturbances. To fill the knowledge gap, this study investigated the dynamic variability of CO2 partial pressure (pCO2) and CO2 emissions flux at the Chaohu Lake Basin, a watershed under intensive anthropogenic perturbations, based on field campaigns across diverse river systems during dry season, normal season, and post-rainfall periods. Results demonstrated marked differences in aquatic pCO2 across river types, with urban rivers (3949 µatm) exhibiting significantly higher levels than non-urban counterparts (1423 µatm). Rainfall events elevated riverine pCO2, but the effect size varied between river types (urban river versus non-urban river). In non-urban rivers, pCO2 following heavy rainfall (2461 μatm) was significantly higher (p < 0.05) than those observed during both dry season (1096 μatm) and normal season (712 μatm). In contrast, urban rivers demonstrated only marginal pCO2 elevation after rainfall (20–30%). Statistical analysis revealed that discharge, total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH4+-N) showed significantly positive correlations with pCO2, while dissolved oxygen (DO) and pH exhibited significantly negative correlations with pCO2. Overall, rivers in the Chaohu Lake Basin act as significant sources of atmospheric CO2, with an annual mean CO2 emission flux of 297.84 mmol·m−2·d−1, and the heavy rainfall events amplify riverine CO2 emissions (629.91 mmol·m−2·d−1), with observed enhancement effects exceeding 300% compared to baseline conditions. To accurately estimate the CO2 emissions from human-dominated rivers, future research should emphasize the impacts of extreme or heavy rainfall events.