Contrasting methane emissions from upstream and downstream rivers and their associated subtropical reservoir in eastern China

Abstract

Subtropical reservoirs are an important source of atmospheric methane (CH 4 ). This study investigated the spatiotemporal variability of bubble and diffusive CH 4 emissions from a subtropical reservoir, including its upstream and downstream rivers, in eastern China. There was no obvious seasonal variation in CH 4 emissions from the main reservoir, which increased slightly from the first half year to the next half year. In the upstream river, CH 4 emissions were low from February to June and fluctuated widely from July to January due to bubble activity. In the downstream river, CH 4 emissions were lowest in February, which was possibly influenced by the low streamflow rate from the reservoir (275 m 3 s −1 ) and a short period of mixing. There was spatial variability in CH 4 emissions, where fluxes were highest from the upstream river (3.65 ± 3.24 mg CH 4 m −2 h −1 ) and lowest from the main reservoir (0.082 ± 0.061 mg CH 4 m −2 h −1 ), and emissions from the downstream river were 0.49 ± 0.20 mg CH 4 m −2 h −1 . Inflow rivers are hot spots in bubble CH 4 emissions that should be examined using field-sampling strategies. This study will improve the accuracy of current and future estimations of CH 4 emissions from hydroelectric systems and will help guide mitigation strategies for greenhouse gas emissions.

Spatio-temporal variability of carbon dioxide and methane emissions from a Mediterranean reservoir

“…In El Gergal both CO 2 and CH 4 emissions showed marked differences between riverine and lacustrine zones, highlighting the importance of including riverine areas to achieve accurate reservoir CO 2 and CH 4 emission assessments (Beaulieu et al, 2014;Yang, 2019), especially in heterogeneous ecosystems such as reservoirs.…”

“…Ebullitive and diffusive fluxes were respectively 40 and 6 times greater in the riverine zone than those measured in the lacustrine. Shallow riverine areas have been previously identified as CH 4 emission hot spots in lakes and reservoirs (Bastviken et al, 2002;Beaulieu et al, 2014;Paraníba et al 2018;Yang, 2019), where CH 4 production is enhanced by deposition of fluvial organic matter, and a large fraction of sedimentary CH 4 is directly evading to the atmosphere through bubbling. The ebullitive CH 4 emissions measured in the shallow riverine zone of El Gergal were comparable to ebullitive fluxes found in temperate shallow ponds (Dove et al, 1999;DelSontro et al, 2016), a kind of ecosystem typically supporting higher CH 4 emission rates than temperate deep lakes and reservoirs.…”

“…Recent studies have demonstrated that CO 2 and CH 4 fluxes in lakes and reservoirs depicts a marked spatial variability (Beaulieu et al, 2014;Natchimuthu et al, 2016, Paranaíba et al, 2018Yang, 2019, McClure et al, 2020, and that greenhouse gas emissions assessments which do not account for this spatial heterogeneity could substantially (and systematically) bias annual estimations of carbon fluxes to the atmosphere. In spite of this, there are still few studies on inland waters greenhouse gas emissions taking into account spatial and temporal variability simultaneously, and including both diffusive and ebullitive fluxes (i.e.…”

Greenhouse gas dynamics in river networks fragmented by drying and damming

“…Global N 2 O fluxes from reservoirs was estimated at 31.7 Tg CO 2 eq./year (Deemer et al, 2016), which is notable, but a smaller contribution than CO 2 (328) and CH 4 (748) (Harrison et al, 2021). There was no clear trend in the spatial variation of N 2 O fluxes (Figure 3b; Table S6) with some studies reporting the highest average fluxes at the upstream river (Cheng et al, 2019), downstream river (Yang, 2019) and reservoir surface (Yang et al, 2020). Anoxic conditions in reservoir sediments can lead to conditions favourable for denitrification, generating N 2 O emissions (Wang et al, 2018).…”

“…On average, CH 4 fluxes were an order of magnitude higher from the downstream reaches than from the reservoir surface and upstream reaches (Figure 3b). However, in individual studies the results were variable (Table S6), with observed CH 4 fluxes at the reservoir surface that were greater or lower than upstream (Yang, 2019; Yang et al, 2013) and downstream waters (Guérin et al, 2006; Zhao et al, 2013). Methane production may not translate directly to CH 4 efflux, as the upwardly diffusing CH 4 can be converted to CO 2 by methanotrophic bacteria (Wang et al, 2018).…”