Small Water Impoundments Have Local and Longitudinal Impacts on Carbon Cycling Along a River Network

ABSTRACT Damming may be one of the most significant anthropogenic modifications fragmenting river networks. Damming alters the transport and processing of coarse particulate organic matter (OM), hence influencing carbon (C) cycling, including the production and emission of carbon dioxide (CO 2 ) and methane (CH 4 ). Most studies assessing the impact of dams tend to focus on large impoundments, although small impoundments (< 15 m high) are likely to have notable local‐ and river continuum‐scale effects by fragmenting the river network. We aimed to determine if and how small impoundments affect OM stocks, leaf litter decomposition, and gaseous C emissions to the atmosphere. We monitored 21 sites across an impounded river network in central France over three seasons. We measured in situ diffusive CO 2 and CH 4 fluxes, as well as leaf litter decomposition rates, OM stocks, and OM transport. Two small impoundments and the river reaches immediately upstream and downstream had varying patterns of C emissions, with CO 2 and CH 4 fluxes highest immediately downstream of one impoundment and high seasonal variability in fluxes at the other impoundment. Small impoundments impacted the expected river continuum pattern of OM stocks, but not leaf litter decomposition nor CO 2 emissions. In sum, small impoundments can have contrasting local impacts and combined river continuum‐scale effects on C cycling. Our study highlights the importance of considering both the local‐ and river continuum‐scales when examining river C dynamics and how they are influenced by anthropogenic impacts.