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

Area of Science:

• Environmental Science
• Geochemistry
• Hydrology

Background:

• River damming fundamentally alters aquatic ecosystems, yet methane (CH4) emissions from cascade reservoirs are poorly understood.
• Previous studies on large river-reservoir systems show longitudinal patterns in CH4 flux, often decreasing downstream.
• Small cascade systems present unique biogeochemical dynamics due to their physical configuration and material transport.

Purpose of the Study:

• To investigate the spatio-temporal patterns and controlling factors of CH4 dynamics in small cascade reservoirs.
• To quantify CH4 emission fluxes and identify dominant emission pathways.
• To assess the relationship between reservoir scale, organic carbon retention, and methanogenic potential.

Main Methods:

• Conducted annual investigations of CH4 dynamics across 15 small cascade reservoirs.
• Measured monthly CH4 fluxes (ebullition and diffusion) in each reservoir.
• Analyzed the ratio of methanogenic (mcrA) to aerobic (pmoA) genes in sediments.
• Calculated CH4 release efficiency relative to organic carbon stock.

Main Results:

• Small cascade reservoirs are significant CH4 hotspots with a mean flux of 2.73 ± 2.70 mmol m-2 d-1.
• CH4 fluxes showed spatial discontinuity, primarily governed by reservoir scale, not longitudinal position.
• Larger reservoirs exhibited higher CH4 fluxes due to enhanced organic matter retention and increased methanogenic potential.
• Ebullition was the dominant pathway (~70.5%), sensitive to reservoir size, while diffusion was scale-independent.
• A downstream increase in CH4 release efficiency, termed

Conclusions:

• Small cascade reservoirs create a unique, spatially discontinuous yet highly efficient CH4 emission regime.
• Reservoir scale is a key factor controlling CH4 hotspots and emission pathways.
• Accurate climate impact assessment requires accounting for scale-dependent emissions and downstream efficiency gains in global inventories.