Scale-dependent processes drive spatially discontinuous methane emissions from small cascade reservoirs

The river continuum is fundamentally disrupted by dense cascade damming, yet the resultant CH₄ emission patterns remain poorly understood. This study conducted a comprehensive annual investigation of CH₄ dynamics across 15 small cascade reservoirs in the montane Wubu River, China. We found these reservoirs constitute significant CH₄ hotspots, with a mean flux of 2.73 ± 2.70 mmol m^-2 d^-1 calculated from monthly measurements across all reservoirs. CH₄ fluxes exhibited highly spatio-temporal heterogeneity in the small cascade reservoirs system. In contrast to previous reports in large river-reservoir systems, where CH₄ fluxes often peak in upstream reservoirs and exhibit a pronounced longitudinal attenuation due to efficient organic matter interception, we observed marked spatial discontinuity in CH₄ fluxes along this small cascade system. Rather than following longitudinal position, CH₄ emission hotspots were primarily governed by reservoir scale, with relatively larger reservoir consistently exhibiting higher fluxes associated with enhanced organic matter retention and a consequent shift in sediment microbial communities toward elevated methanogenic potential (higher mcrA/pmoA gene ratio). Ebullition was the dominant emission pathway, contributing contributed an average of approximately 70.5% of total fluxes and exhibiting strong sensitivity to reservoir size, whereas diffusive fluxes showed no significant scale dependence. We calculated the annually total CH₄-C release from each reservoir based on extrapolation and further quantified its ratio to the organic carbon stock within the water body (defined as "CH₄ release efficiency"). We identified a progressive downstream enhancement in "CH₄ release efficiency" despite spatial discontinuity in fluxes, revealing a novel "cascading efficiency" effect of intensifies methanogenic carbon processing. These findings establish that small cascade reservoirs operate under distinct biogeochemical rules where physical configuration and material transport create a spatially discontinuous yet highly efficient CH₄ emission regime. Accurate assessment of their climate impact requires integration of both scale-dependent emission patterns and downstream efficiency gains in global greenhouse gas inventories.