Effect of small water retention structures on diffusive CO₂and CH₄emissions along a highly impounded river

Abstract

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.

Greenhouse gas dynamics in river networks fragmented by drying and damming

“…Increased water residence time is associated with increased interaction between organic substrates and microorganisms, allowing for decomposition of organic matter and CO 2 production (Gómez-Gener et al, 2018). Carbon dioxide fluxes from reservoirs typically had a negative relationship with DO and Chl-a concentrations.…”

“…Carbon dioxide fluxes increase with warmer water temperature; for example, tropical reservoirs typically emit GHGs at greater rates than high latitude temperate and boreal reservoirs, which is likely to the result of higher rates of biological activity stimulated by warm water temperature and higher flooded biomass in tropical regions (Wang et al, 2018). Increased water residence time is associated with increased interaction between organic substrates and microorganisms, allowing for decomposition of organic matter and CO 2 production (Gómez‐Gener et al, 2018). Carbon dioxide fluxes from reservoirs typically had a negative relationship with DO and Chl‐ a concentrations.…”

Integrated assessment of the net carbon footprint of small hydropower plants

“…multiple and consecutive HPs within a river section or the river network) are likely to be greater than the sum of the impacts from each individual plant (Lange et al 2018). Small impoundments have recently been found to also alter the biogeochemical processes that control the production and emission of C in river networks by increasing the residence time of water and organic matter (Maeck et al 2013, Gómez-Gener et al 2018, Maavara et al 2020. Consequently, the CF of SHPs is in most cases disproportionally higher than that of LHPs (Räsänen et al 2018, Almeida et al 2019b.…”