River networks are areas of disproportionate biogeochemical exchange compared to their occurrence in the landscape (Allen & Pavelsky, 2018). Rivers receive large amounts of terrestrially derived carbon that is subsequently stored, transformed, or ultimately transported to the ocean (Battin et al., 2008; Cole et al., 2007). Fragmentation by drying and damming, two of the most widespread forms of river fragmentation (Datry et al., 2014; Grill et al., 2019; Messager et al., 2021), may impact the production and evasion of greenhouse gases (GHGs), both locally, and at the network scale. At the local scale, CO2 and methane (CH4) fluxes from dry inland waters represent a significant component across freshwater ecosystems (Keller et al., 2020; Paranaíba et al., 2021). Organic matter (OM) and sediments accumulate in reservoirs, coupled with high water retention time and warm water temperatures, which can promote anaerobic conditions associated with denitrification and methanogenesis, and the generation of nitrous oxide (N2O) and CH4 (Wang et al., 2018). At the river network scale, rivers can be considered as meta-ecosystems made up of an aquatic continuum within a terrestrial matrix (Little et al., 2020). Fragmentation alters the lateral, vertical, and longitudinal flows of organisms, energy and resources across ecosystems (Cid et al., 2021; Datry et al., 2016). However, to date, limited research has been conducted on the effects of fragmentation on GHG dynamics of rivers at both the local and the network scales. The main objective of this thesis was to examine the effects of fragmentation by drying and damming on GHG dynamics in river networks. We tested the principle hypothesis that the spatio-temporal dynamics of drying and damming at the river network scale would be an important driver of GHG dynamics using a literature review, field studies, and a laboratory experiment (Figure 1).

Figure 1: The width of the arrows indicates the relative magnitude of process rates. Blue indicates lotic sections, yellow indicates dry sections, solid lines represent perennial sections, and dashed lines represent intermittent sections. We expect higher GHG fluxes in the upstream dry sediments, where organic matter inputs per unit area are typically higher (a). We anticipate that reservoirs in the main channel will have higher GHG fluxes per unit area due to warmer water temperatures compared to shaded upstream sections (b). The interactive additive effect of desiccation and dams may result in greater organic matter processing in the dry sections and reservoirs, leading to higher GHG emissions into the atmosphere (c). Source: Silverthorn et al., 2023 Freshwater Biology.

Our review of the literature revealed several key gaps, namely that studies about drying were lacking at the river network scale and in non-arid regions while studies about damming were lacking at small water retention structures. In the second chapter, I found that drying not only had a legacy effect on non-perennial reaches when they were flowing, but also a network-scale spatial effect. In the third chapter, I found that water temperature and allochthonous OM quantity influences CO2 and CH4 fluxes from mesocosms simulating isolated pools of non-perennial rivers. In the fourth chapter, I found that discontinuity, as a result of small impoundments, had seasonally-dependant network scale impacts on OM stocks, microbial-led decomposition, and CH4 fluxes, but not on CO2 fluxes nor on invertebrate-led decomposition. In sum, this thesis provides a compelling body of evidence attesting to the effects of fragmentation by drying and damming on GHG fluxes. Given the increasing occurrences of drying and damming in the face of global change (Döll & Schmied, 2012; Zarfl et al., 2015), the impacts of their fragmentation on river network biogeochemical dynamics at the river network scale, from isolated pools, and at small impoundments must be accounted for in global GHG budgets.

Cite the thesis

Teresa Silverthorn. Greenhouse gas flux dynamics in fragmented river networks. Hydrology. Université Claude Bernard - Lyon I, 2024. English. ⟨NNT : 2024LYO10040⟩. ⟨tel-05128075⟩

Access manuscript on HAL thèses

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