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Si us plau utilitzeu sempre aquest identificador per citar o enllaçar aquest document: https://hdl.handle.net/2445/110882
Effects of flow discontinuities on carbon gas fluxes in a Mediterranean fluvial network = Efecte de les dicontinuitats hidrològiques sobre els fluxes gasosos de carboni en una xarxa fluvial Mediterrània
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[eng] Inland waters are active components of the global carbon (C) cycle that transform, store and outgas more than half of the C they receive from adjacent terrestrial ecosystems. However, fundamental uncertainties regarding the spatiotemporal patterns, controls and sources of C gas fluxes in fluvial networks still exist. For instance, current biogeochemical models addressing C transport and processing in fluvial networks from a continuous perspective, do not integrate the effects of local discontinuities such as river impoundment or stream flow intermittency on the dynamics of C gas fluxes. The present dissertation aims to examine how flow discontinuities (i.e., river impoundment, flow fragmentation and drying) shape the spatiotemporal patterns, the controls and the sources of C gas fluxes in a Mediterranean fluvial network. The study was performed from December 2012 to March 2015 in the Fluvià river (NE Iberian Peninsula), characterized by a high density of impounded waters associated to small water retention structures (SWRS; i.e., weirs and small to very small impoundments with surface area < 0.1 km2 and a volume < 0.2 hm3) as well as fragmented river sections dominated by isolated water pools and dry riverbeds coinciding with dry periods. Results of this dissertation show that river discontinuities associated to SWRS and flow intermittency modulate the spatiotemporal patterns, controls and sources of C gas fluxes in the studied fluvial network. However, the magnitude of these effects varied depending on the nature of the discontinuity (i.e., river impoundment or flow intermittency), the type of C gas (i.e., carbon dioxide (CO2) or methane (CH4)) and the hydrological condition (i.e., high or low flow). The presence of SWRS, despite their relatively small water capacity, attenuated the turbulent conditions occurring in free-flowing river sections. As a consequence, the diffusive CO2 emissions from impounded waters were significantly lower than from free-flowing river sections. Contrarily, no reduction in CH4 emissions from impounded river sections associated to the presence of SWRS was detected. This result suggests that the higher internal CH4 production at the impounded river sections, which remained very stable over time, compensated the attenuated physical effect on CH4 emissions. Despite potential inaccuracies in capturing the temporal and spatial heterogeneity, the ebullition was the predominant pathway of CH4 emissions in impounded river sections. Moreover, sources other than internal metabolism (i.e., external inputs, internal geochemical reactions or photochemical mineralization) sustained most of the fluvial network CO2 emissions. Specifically, the magnitude and sources of CO2 emissions depended on flow conditions in the free-flowing sections, whereas they remained relatively stable and independent of hydrological variation in the impounded river sections. The channels of temporary rivers remain as active biogeochemical habitats processing and degassing significant amounts of CO2 to the atmosphere after flow cessation. In contrast, the CH4 efflux from dry beds was undetectable in almost all cases, most likely due to the high aeration limiting the redox requirements for microbial CH4 production. Our results also suggest that the source of CO2 emitted from dry riverbeds remains unclear, although CO2 produced from biological mineralization of fresh and labile organic matter fractions could be an important source. Future hydrological scenarios considering the combined effects of climate change and human pressures on water resources in the Mediterranean regions show the rather low sensitivity of the annual CO2, CH4 and total C emissions to shifts in river discharge. In contrast, they stress the high sensitivity of annual CH4 and total C emissions to shifts in the surface area of lentic waterbodies associated to SWRS. Overall, the main findings of this dissertation point to the need for a shift away from a continuous and system-centric view to a more inclusive approach that incorporates spatiotemporal discontinuities (i.e., SWRS and flow fragmentation and drying) as a suitable framework to understand the dynamics of C gas fluxes in fluvial networks.
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GÓMEZ GENER, Lluís. Effects of flow discontinuities on carbon gas fluxes in a Mediterranean fluvial network = Efecte de les dicontinuitats hidrològiques sobre els fluxes gasosos de carboni en una xarxa fluvial Mediterrània. [consulta: 28 de novembre de 2025]. [Disponible a: https://hdl.handle.net/2445/110882]