The drawdown phase of dam decommissioning is a hot moment of gaseous carbon emissions from a temperate reservoir

Exposed sediments in a temperate-climate reservoir under dam decommissioning contain large stocks of highly bioreactive organic matter

“…Furthermore, sediment texture did not affect the content and reactivity of sediment OM in the Enobieta Reservoir, and previous studies also reported conflicting results on the role sediment texture in the reactivity of sediment OM (Mendoza-Lera et al, 2017;von Schiller et al, 2019). Amani et al (2022) hypothesized that the areal CO 2 fluxes in exposed sediments decreased with the sediment exposure time due to a decrease in the content and reactivity of sediment OM in the Enobieta Reservoir after the drawdown phase of DD. This study rejects the hypothesis that the content and reactivity of sediment OM in the Enobieta Reservoir decreased with sediment exposure time.…”

“…Subsequently, the Enobieta Reservoir was not used to provide water (Larrañaga et al, 2019) and was not maintained properly to the point that it became a safety prob-lem. Due to the early structural instability and the conservation status of the Artikutza Valley (it is part of the Natura 2000 Network and a special conservation zone), the decommissioning of the Enobieta Reservoir began in 2017 and continued throughout 2018 and 2019 (Amani et al, 2022). The phase of reservoir drawdown of the Enobieta Reservoir ended on 25 February 2019.…”

“…We determined the last inundation date for each site from the site elevation (Table S2, available at https://www. limnetica.net/en/limnetica), the reservoir bathymetry, and the daily evolution of water level in the reservoir (available in Amani et al (2022)). The range of sediment exposure time was 9-580 days (Table S1).…”

“…Dam decommissioning is a widely hailed option to restore riverine connectivity, biodiversity, and ecosystem function (Allen et al, 2016;Bednarek, 2001;Magilligan et al, 2016). However, DD can also allow the downstream export and decomposition of organic matter (OM) buried in reservoir sediments (i.e., sediment OM) into greenhouse gases (GHGs) (Amani et al, 2022;Pacca, 2007). The decomposition rate of sediment OM depends on extrinsic environmental factors and the content and reactivity of sediment OM (Keller et al, 2020;Obrador et al, 2018;Paranaíba et al, 2021).…”

“…Thus, DD can affect extrinsic factors of the decomposition of OM in sediments, and hence the content and reactivity of sediment OM by exposing the anoxic sediments to the atmosphere. For instance, exposed sediments in a large temperate reservoir under DD were reported to be a hotspot for carbon dioxide (CO 2 ) emissions (Amani et al, 2022). These CO 2 emissions decreased with sediment exposure time, hypothetically, due to a decrease in the content and reactivity of sediment OM.…”