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

Abstract

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.

Les aigües continentals són uns components molt actius en cicle del carboni (C). Aquests, transformen, emmagatzemen i emeten la meitat de C que reben dels ecosistemes terrestres adjacents. No obstant, encara existeix una elevada incertesa pel que fa als patrons espaciotemporals, factors de control i principals fonts dels fluxos gasosos de C en xarxes fluvials.

Els resultats d’aquesta tesi mostren que les estructures de retenció d’aigua de mida petita (ERMP), les quals són molt comuns en rius Mediterranis, van atenuar les condicions turbulents que caracteritzen les seccions del riu amb aigües corrents. Com a conseqüència, les emissions difusives de CO2 des d’aigües represades van ser inferiors a aquells des d’aigües corrents. Contràriament, la presència de ERMP no va suposar un efecte negatiu sobre les emissions de CH4. Tanmateix, fonts diferents al metabolisme intern (això és, entrades externes i reaccions geoquímiques o fotoquímiques internes) van sostenir les emissions de CO2 de la xarxa fluvial. La magnitud i fonts d’aquestes van dependre de les condicions hidrològiques en el cas dels trams d’aigües corrents, mentre que es van mantenir relativament estables i independents de la hidrologia en aquelles seccions de riu reprssades.

Les lleres dels rius intermitents romanen actives pel que fa al processat i emissió de CO2 a l’atmosfera una vegada el flux superficial d’aigua cessa. Per contra, el flux d’emissió de CH4 des de les lleres seques va ser indetectable en gairebé tots els casos, probablement degut a les condicions d’alta aeració que limiten els requisits redox per a la producció microbiana de CH4. El flux d’emissió de CO2 des de les lleres seques va doblar a l’emès des de les lleres amb aigües corrents i va ser comparable a l’emès des dels sòls terrestres adjacents. No obstant, les lleres seques i els sòls terrestres adjacents van resultar ser mol diferents des d’un punt de vista fisicoquímic, mostrant així diferències en els principals factors i fonts que en regulen les emissions de CO2.

En resum, les principals troballes fetes en aquesta tesi apunten cap a una necessitat clara de substituir els models continus i limitats espacialment per aquells models que incorporen discontinuïtats espaciotemporals (això és, represament del riu o intermitència del règim hidrològic) per tal d’entendre en millor mesura les dinàmiques dels fluxos gasos de C en xarxes fluvials.