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

Resumen

Inland waters are active components of the global carbon (C) cycle that (Maximo so linias) transform, store and outgas more than half of the C they receive from adjacent terrestrial ecosystems. However, fundamental uncertainties regarding the spatlotemporal patterns, controls and sources of C gas fluxes in fluvlal networks still exist. For Instance, current biogeochemlcal models addressing C transport and processing In fluvlal networks from a continuous perspective, do not Integrate the effects of local discontinuities such as river impoundment or stream flow intermlttency on the dynamics of C gas fluxes. ,, The present dissertation aims to examine how flow discontinuities {I.e., river lmpoundment, flow fragmentation and drying) shape the spatiotemporal patterns, the controls and the sources of C gas fluxes In a Mediterranean fluvlal network. The study was performed from December 2012 to March 2015 In the Fluvla 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 lmpouhdments 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 intermlttency modulate the spatlotemporal patterns, controls and ,, sources of C gas fluxes In the studied fluvial network. However, the magnitude of ,, these effects varl.ed dependihg on the nature of the discontinuity (I.e., river lmpoundment or flow lntermlttency), the type of C gas (I.e., carbon dioxide i> (CO2) or methane (CH4)) and the hydrological condition (I.e., high or low flow). The presence of SWRS, despite their relatively srnall 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 ebullftlon 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 photochemfcai 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 LLE impounded river sections. I The channels of temporary rivers remain as active biogeochemical habitats processing and degassing significant amounts of CO2 to the atmosphere after flow cessation. ln 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 remcilns unclear, although CO2 produced from biological mineralization of fresh and labile organic matter fractions could be an Important source. 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 incluslve approach that incorporates spatlotemporal discontinuities (I.e., SWRS and flow fragmentation and drying) as a suitable framework to understand the dynamics of C gas fluxes in fluvial networks.