• Energy Research

AbstractStreams are significant contributors of greenhouse gases (GHG) to the atmosphere, and the increasing number of stressors degrading freshwaters may exacerbate this process, posing a threat to climatic stability. However, it is unclear whether the influence of multiple stressors on GHG concentrations in streams results from increases of in‐situ metabolism (i.e., local processes) or from changes in upstream and terrestrial GHG production (i.e., distal processes). Here, we hypothesize that the mechanisms controlling multiple stressor effects vary between carbon dioxide (CO2) and methane (CH4), with the latter being more influenced by changes in local stream metabolism, and the former mainly responding to distal processes. To test this hypothesis, we measured stream metabolism and the concentrations of CO2 (pCO2) and CH4 (pCH4) in 50 stream sites that encompass gradients of nutrient enrichment, oxygen depletion, thermal stress, riparian degradation and discharge. Our results indicate that these stressors had additive effects on stream metabolism and GHG concentrations, with stressor interactions explaining limited variance. Nutrient enrichment was associated with higher stream heterotrophy and pCO2, whereas pCH4 increased with oxygen depletion and water temperature. Discharge was positively linked to primary production, respiration and heterotrophy but correlated negatively with pCO2. Our models indicate that CO2‐equivalent concentrations can more than double in streams that experience high nutrient enrichment and oxygen depletion, compared to those with oligotrophic and oxic conditions. Structural equation models revealed that the effects of nutrient enrichment and discharge on pCO2 were related to distal processes rather than local metabolism. In contrast, pCH4 responses to nutrient enrichment, discharge and temperature were related to both local metabolism and distal processes. Collectively, our study illustrates potential climatic feedbacks resulting from freshwater degradation and provides insight into the processes mediating stressor impacts on the production of GHG in streams.

Effluents from urban wastewater treatment plants (WWTP) consist of complex mixtures of substances that can affect processes in the receiving ecosystems. Some of these substances (toxic contaminants) stress biological activity at all concentrations, while others (e.g., nutrients) subsidize it at low concentrations and stress it above a threshold, causing subsidy-stress responses. Thus, the overall effects of WWTP effluents depend mostly on their composition and the dilution capacity of the receiving water bodies. We assessed the immediate and legacy effects of WWTP effluents in artificial streams, where we measured the uptake of soluble reactive phosphorus (SRP) by the biofilm, biomass accrual, benthic metabolism and organic matter decomposition (OMD). In a first phase (32 d), the channels were subjected to a gradient of effluent contribution, from pure stream water to pure effluent. WWTP effluent affected the ecosystem processes we measured, although we found no clear subsidy-stress patterns except for biofilm biomass accrual. Instead, most of the processes were subsidized, although they showed complex and process-specific patterns. Benthic metabolism and OMD were subsidized without saturation, as they peaked at medium and high levels of pollution, respectively, but they never fell below control levels. SRP uptake was the only process that decreased with increasing effluent concentration. In a second phase of the experiment (23 d), all channels were kept on pure stream water to analyse the legacy effects of the effluent. For most of the processes, there were clear legacy effects, which followed either subsidy, stress, or subsidy-stress patterns. SRP uptake capacity was stressed with increasing pollution legacy, whereas algal accrual and benthic metabolism continued being subsidized. Conversely, biofilm biomass accrual and OMD showed no legacy effects. Overall, the WWTP effluent caused complex and process-specific responses in our experiment, mainly driven by the mixed contribution of subsidizers and stressors. These results help improving our understanding of the effects of urban pollution on stream ecosystem functioning.

The impoundment of running waters through the construction of large dams is recognised as one of the most important factors determining the transport, transformation, and outgassing of carbon (C) in fluvial networks. However, the effects of small and very small water retention structures (SWRS) on the magnitude and spatiotemporal patterns of C emissions are still unknown, even though SWRS are the most common type of water retention structure causing river fragmentation worldwide. Here we evaluated and compared diffusive carbon dioxide (CO2) and methane (CH4) emissions from river sections impounded by SWRS and from their adjacent free-flowing sections along a highly impounded river. Emissions from impounded river sections (mean [SE] = 17.7 [2.8] and 0.67 [0.14] mmol m−2 d−1, for CO2 and CH4, respectively) never exceeded those from their adjacent free-flowing river sections (230.6 [49.7] and 2.14 [0.54] mmol m−2 d−1). We attribute this finding to the reduced turbulence in impounded river sections induced by SWRS compared to free-flowing river sections (i.e., physical driver). Likewise, the presence of SWRS favoured an increase of the concentration of CH4 in impounded waters, but this increase was not sufficient to cause a significant influence in the CH4 efflux from the downstream free-flowing river sections. By contrast, this influenced the larger-scale longitudinal patterns of dissolved CH4, which exhibited a clear shifting pattern along the study stretch, modulated by variables associated with the presence of SWRS, such as higher water residence times, higher sedimentation rates, and higher temperatures. Overall, our results show that the presence of SWRS can modify the concentrations of C gases in highly impounded rivers but exerts a minor influence on diffusive C emissions.

Intermittent rivers and ephemeral streams (IRES) encompass fluvial ecosystems that eventually stop flowing and run dry at some point in space and time. During the dry phase, channels of IRES consist mainly of dry riverbeds (DRBs), prevalent yet widely unexplored ecotones between dry and wet phases that can strongly influence the biogeochemistry of fluvial networks. DRBs are often overlooked because they do not strictly belong to either domain of soil or freshwater science. Due to this dual character of DRBs, we suggest that concepts and knowledge from soil science can be used to expand the understanding of IRES biogeochemistry. Based on this idea, we propose that DRBs can be conceptually understood as early stage soils exhibiting many similarities with soils through two main forces: i) time since last sediment transport event, and ii) the development status of stabilizing structures (e.g. soil crusts and/or vascular plants). Our analysis suggests that while DRBs and soils may differ in master physical attributes (e.g. soil horizons vs fluvial sedimentary facies), they become rapidly comparable in terms of microbial communities and biogeochemical processes. We further propose that drivers of DRBs biogeochemistry are similar to those of soils and, hence, concepts and methods used in soil science are transferable to DRBs research. Finally, our paper presents future research directions to advance the knowledge of DRBs and to understand their role in the biogeochemistry of intermittent fluvial networks. Earth-Science Reviews, 188 ISSN:0012-8252 ISSN:1872-6828

Most fluvial networks worldwide include watercourses that recurrently cease to flow and run dry. The spatial and temporal extent of the dry phase of these temporary watercourses is increasing as a result of global change. Yet, current estimates of carbon emissions from fluvial networks do not consider temporary watercourses when they are dry. We characterized the magnitude and variability of carbon emissions from dry watercourses by measuring the carbon dioxide (CO2) flux from 10 dry streambeds of a fluvial network during the dry period and comparing it to the CO2 flux from the same streambeds during the flowing period and to the CO2 flux from their adjacent upland soils. We also looked for potential drivers regulating the CO2 emissions by examining the main physical and chemical properties of dry streambed sediments and adjacent upland soils. The CO2 efflux from dry streambeds (mean ± SD = 781.4 ± 390.2 mmol m−2 day−1) doubled the CO2 efflux from flowing streambeds (305.6 ± 206.1 mmol m−2 day−1) and was comparable to the CO2 efflux from upland soils (896.1 ± 263.2 mmol m−2 day−1). However, dry streambed sediments and upland soils were physicochemically distinct and differed in the variables regulating their CO2 efflux. Overall, our results indicate that dry streambeds constitute a unique and biogeochemically active habitat that can emit significant amounts of CO2 to the atmosphere. Thus, omitting CO2 emissions from temporary streams when they are dry may overlook the role of a key component of the carbon balance of fluvial networks This research was funded by the Spanish Ministry of Economy and Competitiveness through the Projects CGL2011-30474-C02-01 and CGL2014-58760-C3-1-R.