Impact of drought periods on carbon processing across surface-hyporheic interfaces in fluvial systems = Impacte dels períodes de sequera sobre el processat de carboni a través de la interfase superficial i zona hiporreica en sistemes fluvials

[eng] Headwater streams essentially link the terrestrial and the aquatic carbon cycle because they transport terrestrial organic and inorganic carbon downstream towards the oceans. However, most of these inputs are processed during this journey. These processes include down-break of particulate organic matter, transformation and respiration of dissolved organic matter and furthermore, in-stream production of organic matter. In particular, during drought periods the aquatic processes gain importance because terrestrial inputs are diminished. Therefore, carbon cycling in the remaining surface and subsurface flow of the main channel is accelerated and driven by the connectivity of these compartments. As the surface flow ceases carbon processing rates along subsurface flow paths, namely the hyporheic zone, increase. In the light of climate change, longer drought periods, including in currently humid areas, are expected. Within this context, this thesis aims to understand carbon processing across the surface-hyporheic interface of a Mediterranean intermittent stream during a summer drought. Since dissolved organic matter represents the key energy source of aquatic metabolism that ultimately determine in-stream carbon cycling, we focused on the organic matter quantity and quality. We found increasing retention rates of dissolved organic matter along hyporheic flowpaths as water residence time in this compartment increases with the ceasing of surface flow. The evaluation of optical indices of dissolved organic matter quality revealed that the molecular weight decreased and indices related to biological activity increased. Furthermore, we showed that dissolved organic matter from primary production is rapidly respired in the remaining surface water, while humic-like compounds are processed and respired in the hyporheic zone. The dissolved organic matter processing in the hyporheic zone was paralleled with observations of disproportional high partial pressure of CO2 in the interstitial pore water. These CO2 pulses were related to the desiccation of the streambed, as well as dissolved organic matter availability. Our results suggested that the hyporheic zone acts as a humid refuge for microbial activity and that respiration activity immediately restarts when rain events reestablish subsurface flow paths. Associated with this microbial activity, during drought the processing rates of dissolved organic matter, as well as the processing rates of inorganic nutrients were enhanced. Moreover, we explored the effects of a summer drought on subalpine streams by applying different discharge levels in stream-side flumes fed by the water of a subalpine stream. In this experiment, we found high dissolved organic carbon release from in-stream processes in the flumes with the lowest discharges. This dissolved organic carbon release was at the beginning paralleled with a transient increase in gross primary production but continued to rise even when primary production collapsed. While the collapse of primary production might be a consequence of phosphor limitation, respiration and degradation of dissolved and particulate organic matter in the sediment continued throughout. In line with our findings from the Mediterranean stream, this mesocosm experiment highlighted the importance of the hyporheic zone and organic matter stored therein for carbon processing during drought periods. In both study sites, the surface water metabolism was ultimately dominated by respiration, and dissolved organic matter quality of the surface water played an important role on processes in the hyporheic zone. Although the investigated study sites were different in many aspects we found surprising similarities in carbon processing with flow reduction. This suggests that findings from Mediterranean streams might be transferable to other climatic regions under global change scenarios.[cat] Els rius de capçalera connecten essencialment el cicle del carboni terrestre i aquàtic, donat que transporten el carboni orgànic i inorgànic terrestre aigües avall. Durant els períodes de sequera, els processos aquàtics guanyen importància perquè les aportacions terrestres es redueixen. A mesura que l’aigua de la superfície desapareix, les taxes de processament de carboni al llarg de la zona hiporreica augmenta. Atès que, la matèria orgànica dissolta és la principal font d'energia del metabolisme aquàtic que, en aquesta tesis se centra en la quantitat i la qualitat de la matèria orgànica. Hem trobat un augment de la retenció de la matèria orgànica dissolta en medi hiporreic, ja que el temps de residència de l'aigua en aquest compartiment augmenta amb el cessament de aigües superficials. L'avaluació dels índexs òptics de la qualitat de la matèria orgànica dissolta va revelar que el pes molecular es va reduir, i es van incrementar els índexs relacionats amb l'activitat biològica. Vam demostrar que la matèria orgànica dissolta de la producció primària es respira ràpidament en l'aigua superficial restant, mentre que els compostos més húmics es processen i es respiren a la zona hiporreica. Els nostres resultats suggereixen que la zona hiporreica actua com un refugi humit per a l'activitat microbiana i que l'activitat de la respiració es reinicia immediatament quan els esdeveniments de pluja restableixen el flux hyporheic. A més a més, es van explorar els efectes d'una sequera en rius de capçalera subalpins aplicant diferents nivells de caudal en mesocosms. En aquest experiment, vam trobar una elevada alliberació de carboni orgànic dissolt fruit dels processos aquàtics en els mesocosms de cabals més baixos. Al principi, aquest augment de carboni orgànic dissolt va anar acompanyat per un increment transitori de la producció primària bruta, però va continuar augmentant fins i tot quan la producció primària es va col·lapsar. En els dos llocs d'estudi, el metabolisme de l'aigua superficial va estar dominat per la respiració; i la qualitat de la matèria orgànica dissolta de l'aigua superficial va jugar un paper important en els processos de la zona hiporreica

Impact of drought periods on carbon processing across surface-hyporheic interfaces in fluvial systems = Impacte dels períodes de sequera sobre el processat de carboni a través de la interfase superficial i zona hiporreica en sistemes fluvials

Headwater streams essentially link the terrestrial and the aquatic carbon cycle because they transport terrestrial organic and inorganic carbon downstream towards the oceans. However, most of these inputs are processed during this journey. These processes include down-break of particulate organic matter, transformation and respiration of dissolved organic matter and furthermore, in-stream production of organic matter. In particular, during drought periods the aquatic processes gain importance because terrestrial inputs are diminished. Therefore, carbon cycling in the remaining surface and subsurface flow of the main channel is accelerated and driven by the connectivity of these compartments. As the surface flow ceases carbon processing rates along subsurface flow paths, namely the hyporheic zone, increase. In the light of climate change, longer drought periods, including in currently humid areas, are expected. Within this context, this thesis aims to understand carbon processing across the surface-hyporheic interface of a Mediterranean intermittent stream during a summer drought. Since dissolved organic matter represents the key energy source of aquatic metabolism that ultimately determine in-stream carbon cycling, we focused on the organic matter quantity and quality. We found increasing retention rates of dissolved organic matter along hyporheic flowpaths as water residence time in this compartment increases with the ceasing of surface flow. The evaluation of optical indices of dissolved organic matter quality revealed that the molecular weight decreased and indices related to biological activity increased. Furthermore, we showed that dissolved organic matter from primary production is rapidly respired in the remaining surface water, while humic-like compounds are processed and respired in the hyporheic zone. The dissolved organic matter processing in the hyporheic zone was paralleled with observations of disproportional high partial pressure of CO2 in the interstitial pore water. These CO2 pulses were related to the desiccation of the streambed, as well as dissolved organic matter availability. Our results suggested that the hyporheic zone acts as a humid refuge for microbial activity and that respiration activity immediately restarts when rain events reestablish subsurface flow paths. Associated with this microbial activity, during drought the processing rates of dissolved organic matter, as well as the processing rates of inorganic nutrients were enhanced. Moreover, we explored the effects of a summer drought on subalpine streams by applying different discharge levels in stream-side flumes fed by the water of a subalpine stream. In this experiment, we found high dissolved organic carbon release from in-stream processes in the flumes with the lowest discharges. This dissolved organic carbon release was at the beginning paralleled with a transient increase in gross primary production but continued to rise even when primary production collapsed. While the collapse of primary production might be a consequence of phosphor limitation, respiration and degradation of dissolved and particulate organic matter in the sediment continued throughout. In line with our findings from the Mediterranean stream, this mesocosm experiment highlighted the importance of the hyporheic zone and organic matter stored therein for carbon processing during drought periods. In both study sites, the surface water metabolism was ultimately dominated by respiration, and dissolved organic matter quality of the surface water played an important role on processes in the hyporheic zone. Although the investigated study sites were different in many aspects we found surprising similarities in carbon processing with flow reduction. This suggests that findings from Mediterranean streams might be transferable to other climatic regions under global change scenarios.Els rius de capçalera connecten essencialment el cicle del carboni terrestre i aquàtic, donat que transporten el carboni orgànic i inorgànic terrestre aigües avall. Durant els períodes de sequera, els processos aquàtics guanyen importància perquè les aportacions terrestres es redueixen. A mesura que l’aigua de la superfície desapareix, les taxes de processament de carboni al llarg de la zona hiporreica augmenta. Atès que, la matèria orgànica dissolta és la principal font d'energia del metabolisme aquàtic que, en aquesta tesis se centra en la quantitat i la qualitat de la matèria orgànica. Hem trobat un augment de la retenció de la matèria orgànica dissolta en medi hiporreic, ja que el temps de residència de l'aigua en aquest compartiment augmenta amb el cessament de aigües superficials. L'avaluació dels índexs òptics de la qualitat de la matèria orgànica dissolta va revelar que el pes molecular es va reduir, i es van incrementar els índexs relacionats amb l'activitat biològica. Vam demostrar que la matèria orgànica dissolta de la producció primària es respira ràpidament en l'aigua superficial restant, mentre que els compostos més húmics es processen i es respiren a la zona hiporreica. Els nostres resultats suggereixen que la zona hiporreica actua com un refugi humit per a l'activitat microbiana i que l'activitat de la respiració es reinicia immediatament quan els esdeveniments de pluja restableixen el flux hyporheic. A més a més, es van explorar els efectes d'una sequera en rius de capçalera subalpins aplicant diferents nivells de caudal en mesocosms. En aquest experiment, vam trobar una elevada alliberació de carboni orgànic dissolt fruit dels processos aquàtics en els mesocosms de cabals més baixos. Al principi, aquest augment de carboni orgànic dissolt va anar acompanyat per un increment transitori de la producció primària bruta, però va continuar augmentant fins i tot quan la producció primària es va col·lapsar. En els dos llocs d'estudi, el metabolisme de l'aigua superficial va estar dominat per la respiració; i la qualitat de la matèria orgànica dissolta de l'aigua superficial va jugar un paper important en els processos de la zona hiporreica

Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

Subalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream‐side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream

Responses of microbial activity in hyporheic pore water to biogeochemical changes in a drying headwater stream

Microbial heterotrophic activity is a major driver of nutrient and organic matter processing in the hyporheic zone of headwater streams. Additionally, the hyporheic zone might provide refuge for microbes when surface flow ceases during drought events. We investigated chemical (organic and inorganic nutrients) and microbiological parameters (bacterial cell concentration, live-dead ratios, and extracellular enzyme activities) of surface and interstitial pore water in a period of progressive surface‐hyporheic disconnection due to summer drying. The special situation of the chosen study reach, where groundwater mixing is impeded by the bedrock forming a natural channel filled with sediment, allowed as to study the transformation of these parameters along hyporheic flow paths. The chemical composition of the hyporheic pore water reflected the connectivity with the surface water, as expressed in the availability of nitrate and oxygen. Conversely, microbiological parameters in all hyporheic locations were different from the surface waters, suggesting that the microbial activity in the water changes rapidly once the water enters the hyporheic zone. This feature was principally manifested in higher live-dead ratios and lower leucine aminopeptidase (an activity related to nitrogen acquisition) in the hyporheic pore waters. Overall, bacterial cell concentration and extracellular enzyme activities increased along hyporheic flow paths, with a congruent decrease in inorganic nutrients and dissolved organic matter quantity and apparent molecular size. Our findings show two important functions of the hyporheic zone during drought: (1) deeper (−50 cm) water‐saturated layers can act as a refuge for microbial activity; and (2) the hyporheic zone shows high rates of carbon and nitrogen turnover when water residence times are longer during drought. These rates might be even enhanced by an increase in living microbes in the remaining moist locations of the hyporheic zone

Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

Subalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream-side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream

Continuous monitoring of dissolved gases with membrane inlet massspectrometry to fingerprint river biochemical activity

International audienceWater quality in rivers results from biogeochemical processes in contributing hydrological compartments (soils,aquifers, hyporheic and riparian zones) and biochemical activity in the river network itself. Consequently,chemical fluxes fluctuate on multiple spatial and temporal scales, leading eventually to complex concentrationsignals in rivers. We characterized these fluctuations with innovative continuous monitoring of dissolved gases, toquantify transport and reaction processes occurring in different hydrological compartments.We performed stream-scale experiments in two headwater streams in Brittany, France. Factorial injections ofinorganic nitrogen (NH4NO3), inorganic phosphate (P2O5) and multiple sources of labile carbon (acetate,tryptophan) were implemented in the two streams. We used a new field application of membrane inlet massspectrometry to continuously monitor dissolved gases for multiple day-night periods (Chatton et al., 2016).Quantified gases included He, O2, N2, CO2, CH4, N2O, and 15N of dissolved N2 and N2O. We calibrated andassessed the methodology with well-established complementary techniques including gas chromatography andhigh-frequency water quality sensors. Wet chemistry and radon analysis complemented the study.The analyses provided several methodological and ecological insights and demonstrated that high frequencyvariations linked to background noise can be efficiently determined and filtered to derive effective fluxes. Froma more fundamental point of view, the tested stream segments were fully characterized with extensive samplingof riverbeds and laboratory experiments, allowing scaling of point-level microbial and invertebrate diversity andactivity on in-stream processing. This innovative technology allows fully-controlled in-situ experiments providingrich information with a high signal to noise ratio. We present the integrated nutrient demand and uptake anddiscuss limiting processes and elements at the reach and catchment scales.Eliot Chatton, Thierry Labasque, Jérôme de La Bernardie, Nicolas Guihéneuf, Olivier Bour, Luc Aquilina.2016. Field Continuous Measurement of Dissolved Gases with a CF-MIMS: Applications to the Physics andBiogeochemistry of Groundwater Flow. Environ. Sci. Technol

Using multi-tracer inference to move beyond single-catchment ecohydrology

Protecting or restoring aquatic ecosystems in the face of growing anthropogenic pressures requires an understanding of hydrological and biogeochemical functioning across multiple spatial and temporal scales. Recent technological and methodological advances have vastly increased the number and diversity of hydrological, biogeochemical, and ecological tracers available, providing potentially powerful tools to improve understanding of fundamental problems in ecohydrology, notably: 1. Identifying spatially explicit flowpaths, 2. Quantifying water residence time, and 3. Quantifying and localizing biogeochemical transformation. In this review, we synthesize the history of hydrological and biogeochemical theory, summarize modem tracer methods, and discuss how improved understanding of flowpath, residence time, and biogeochemical transformation can help ecohydrology move beyond description of site-specific heterogeneity. We focus on using multiple tracers with contrasting characteristics (crossing proxies) to infer ecosystem functioning across multiple scales. Specifically, we present how crossed proxies could test recent ecohydrological theory, combining the concepts of hotspots and hot moments with the Damkohler number in what we call the HotDam framework

Responses of microbial activity in hyporheic pore water to biogeochemical changes in a drying headwater stream

1. Microbial heterotrophic activity is a major driver of nutrient and organic matter processing in the hyporheic zone of headwater streams. Additionally, the hyporheic zone might provide refuge for microbes when surface flow ceases during drought events. 2. We investigated chemical (organic and inorganic nutrients) and microbiological parameters (bacterial cell concentration, live–dead ratios, and extracellular enzyme activities) of surface and interstitial pore water in a period of progressive surface‐hyporheic disconnection due to summer drying. The special situation of the chosen study reach, where groundwater mixing is impeded by the bedrock forming a natural channel filled with sediment, allowed as to study the transformation of these parameters along hyporheic flow paths. 3. The chemical composition of the hyporheic pore water reflected the connectivity with the surface water, as expressed in the availability of nitrate and oxygen. Conversely, microbiological parameters in all hyporheic locations were different from the surface waters, suggesting that the microbial activity in the water changes rapidly once the water enters the hyporheic zone. This feature was principally manifested in higher live–dead ratios and lower leucine aminopeptidase (an activity related to nitrogen acquisition) in the hyporheic pore waters. 4. Overall, bacterial cell concentration and extracellular enzyme activities increased along hyporheic flow paths, with a congruent decrease in inorganic nutrients and dissolved organic matter quantity and apparent molecular size. 5. Our findings show two important functions of the hyporheic zone during drought: (1) deeper (−50 cm) water‐saturated layers can act as a refuge for microbial activity; and (2) the hyporheic zone shows high rates of carbon and nitrogen turnover when water residence times are longer during drought. These rates might be even enhanced by an increase in living microbes in the remaining moist locations of the hyporheic zone.© 2019 The Author