The impoundment of running waters through the construction of large dams is recognised as one of the most important factors determining the transport, transformation, and outgassing of carbon (C) in fluvial networks. However, the effects of small and very small water retention structures (SWRS) on the magnitude and spatiotemporal patterns of C emissions are still unknown, even though SWRS are the most common type of water retention structure causing river fragmentation worldwide. Here we evaluated and compared diffusive carbon dioxide (CO 2 ) and methane (CH 4 ) emissions from river sections impounded by SWRS and from their adjacent freeflowing sections along a highly impounded river. Emissions from impounded river sections (mean [SE] = 17.7 [2.8] and 0.67 [0.14] mmol m −2 d −1 , for CO 2 and CH 4 , respectively) never exceeded those from their adjacent free-flowing river sections (230.6 [49.7] and 2.14 [0.54] mmol m −2 d −1 ). We attribute this finding to the reduced turbulence in impounded river sections induced by SWRS compared to free-flowing river sections (i.e., physical driver). Likewise, the presence of SWRS favoured an increase of the concentration of CH 4 in impounded waters, but this increase was not sufficient to cause a significant influence in the CH 4 efflux from the downstream free-flowing river sections. By contrast, this influenced the larger-scale longitudinal patterns of dissolved CH 4 , which exhibited a clear shifting pattern along the study stretch, modulated by variables associated with the presence of SWRS, such as higher water residence times, higher sedimentation rates, and higher temperatures. Overall, our results show that the presence of SWRS can modify the concentrations of C gases in highly impounded rivers but exerts a minor influence on diffusive C emissions.

Global assessments evaluating greenhouse gas emissions and climate benefits of hydropower rely on life cycle assessments (LCAs). However, small hydropower plants (i.e., installations with less than 10 MW; SHPs), are largely underrepresented in such schemes, despite their widespread proliferation and well known ecological concerns. Here we quantified, partitioned, and compared the net carbon (C) footprint of four temperate SHPs with different operation designs over a 100-year time horizon. In contrast with previous hydropower LCAs studies, we followed an integrative net C footprint approach accounting for all potential sources and sinks of C within the life cycle of the studied SHPs, including both biogenic and non-biogenic sources, as well as for the pre- and post-impoundment stages involved in the flooding of a reservoir. We found that the areal and system-level C emissions were mostly driven by the residence time of the impounded water, which in turn was linked to the SHP operation type. The power installed in the SHPs did not have a relevant role on the net C fluxes. Accordingly, SHPs with smaller water storage capacity were almost neutral in terms of the C footprint. In contrast, SHPs with water storage facilities prolonged the water residence time in the reservoir and either acted as a source or sink of C. The long water residence time in these SHPs promoted either emission of biogenic gases from the surface or C storage in the sediments. Our work shows that integrative net C footprint assessments accounting for different operation designs are necessary to improve our understanding of the environmental effects of SHPs.