Stream metabolism is a key biogeochemical process in river networks, synthesising the balance between gross primary production (GPP) and ecosystem respiration (ER). Globally, more rivers and streams are drying due to climate change and water abstraction for human uses and this can alter the organic carbon residence time, leading to decoupled ER and terrestrial organic matter supply. While the consequences of drying on CO2 emissions have been recently quantified, its effects on stream metabolism are still poorly studied. We addressed the short- and long-term effects of drying on stream metabolism by monitoring oxygen dynamics at 20 reaches across a drying river network, including perennial (PR) and non-perennial reaches (NPR) for one year. We also calculated several climatic, land use variables and characterized and local abiotic conditions, and biofilm and sediment communities at five sampling dates. ER was significantly higher in NPR than in PR reaches demonstrating in-situ the effects of drying on stream metabolism. When analyzing the long-term drivers of ER and GPP, we found a direct positive effect of drying on ER and a negative effect on GPP. Drying also altered microbial community composition, with algal communities from NPRs being different from those in PRs. In the short-term, the amount of C emitted during rewetting events was positively related to the duration of precedent non-flow period. Our results show that drying had an important effect on stream metabolism both in the short and long term, and supports the need of including NPRs in global estimates of stream metabolism.

Rivers are important contributors to global greenhouse gas (GHG) exchange with the atmosphere. However, much less is known about biogeochemical dynamics in rivers when they dry, particularly in isolated pools created by drying. Our objective was to examine the effects of water temperature and allochthonous organic matter (OM) quantity on carbon dioxide (CO2) and methane (CH4) fluxes in isolated pools. We used an automated analyzer to measure CO2 and CH4 from 36 mesocosms filled with sediments and water from a non-perennial river, with temperature (20, 25, 30 °C) and Alnus glutinosa leaf litter (2g, 5g, 10 g) manipulations in triplicate. We found positive individual effects of water temperature and OM quantity on CO2 fluxes, and a synergistic effect of water temperature and OM on CH4 fluxes during the late stages of the incubation. Given the increase in water temperature and OM inputs in rivers associated with climate change, our results indicate an associated increase in CO2, and a disproportionate increase in CH4 fluxes to the atmosphere, potentially contributing towards a positive climate feedback loop.

Non-perennial streams play a crucial role in ecological communities. However, the key parameters and processes involved in stream intermittence remain poorly understood. While climate conditions, geology and land use are well identified, assessing and modeling the groundwater controls on streamflow intermittence remains a challenge. In this study, we explore new opportunities to calibrate process-based 3D groundwater flow models designed to simulate stream intermittence in groundwater-fed headwaters. Streamflow measurements and stream network maps are jointly considered to constrain aquifer’s effective hydraulic properties in hydrogeological models. The simulations were then validated using visual observations presence/absence of water, provided by a national monitoring network in France (ONDE). We tested the methodology on two pilot catchments with unconfined shallow crystalline aquifer, the Canut and Nançon (Brittany, France). We found that streamflow and expansion/contraction dynamics of the stream network are both necessary to calibrate simultaneously hydraulic conductivity K and porosity θ with low uncertainties. Conversely, calibration resulted in accurate prediction of stream intermittence - in terms of flow and spatial extent. For the two catchments studied, the Canut and Nançon, hydraulic conductivity is close reaching 1.5 x 10 -5 m/s and 4.5 x 10 -5 m/s respectively. However, they differ more by their storage capacity, with porosity estimated at 0.1 % and 2.2 % respectively. Lower storage capacities lead to higher fluctuations in the water table, increasing the proportion of intermittent streams and reducing perennial flow. This new modeling framework allowing to predict streamflow intermittence in headwaters can be deployed to improve our understanding of groundwater controls in different geomorphological, geological, and climatic contexts. It will benefit from advances in remote sensing and crowdsourcing approaches that generate new observed data products with high spatial and temporal resolution.

1. Inland waters are among the most threatened biodiversity hotspots. Ponds located in alpine areas are experiencing more rapid and dramatic water temperature increases than any other biome. Despite their prevalence, alpine ponds and their biodiversity responses to climate change have been poorly explored, reflecting their small size and difficult access. 2. To understand the effects of climate change on alpine pond biodiversity, we performed a comprehensive literature review for papers published since 1955. 3. Through analysis of their geographic distribution, environmental features, and biodiversity values, we identified which environmental factors related to climate change would have direct or indirect effects on alpine pond biodiversity. We then synthesized this information to produce a conceptual model of the effects of climate change on alpine pond biodiversity. 4. Increased water temperature, reduced hydroperiod, and loss of connectivity between alpine ponds were the main drivers of biodiversity geographic distribution, leading to predictable changes in spatial patterns of biodiversity. 5. We identified three major research gaps that, if addressed, can guide conservation and restoration strategies for alpine ponds biodiversity in an uncertain future.

Disturbance and connectivity control biodiversity, ecosystem functioning and their interactions across connected aquatic and terrestrial ecosystems, that form a meta-ecosystem. In rivers, detrital organic matter (OM) is transported across terrestrial-aquatic boundaries and along the river network and decomposed on the way by diverse communities of organisms, including microorganisms and invertebrates. Drying naturally fragments most river networks and thereby modify organism dispersal and OM transfers across ecosystems. This may prevent organisms from reaching and consuming OM, generating mismatches between community composition and decomposition. However, little evidence of the effects of drying on river network-scale OM cycling exists. Here, we aim to examine the effects of fragmentation by drying on the structure of consumer communities and ecosystem functioning within interacting aquatic-terrestrial river ecosystems. We monitored leaf resource stocks, invertebrate communities and decomposition rates in the instream and riparian habitats of 20 sites in a river network naturally fragmented by drying. Although instream resource quantity and quality increased with drying severity, decomposition decreased due to changes in invertebrate communities and particularly leaf-decomposer abundance. Invertebrate-driven decomposition peaked at intermediate levels of upstream connectivity, suggesting that intermediate levels of fragmentation can promote the functioning of downstream ecosystems. We found that the variability in community composition was unrelated to variability in decomposition at sites with low connectivity and high drying severity, suggesting that such conditions can promote mismatches between community composition and decomposition. Decomposition instream was correlated to decomposition in the riparian area, revealing one of the first network-scale evidence of the links between ecosystem functions across terrestrial-aquatic boundaries. Our river network-scale study thus demonstrates the paramount effect of drying on the dynamics of resources, communities and ecosystem functioning in river networks, with crucial implications for the adaptive management of river networks and preservation of their functional integrity.

Freshwater biodiversity is declining dramatically, and the current biodiversity crisis requires defining bold goals and mobilizing substantial resources to meet the challenges. While the reasons are varied, both research and conservation of freshwater biodiversity lag far behind efforts in the terrestrial and marine realms. We identify fifteen pressing global needs to support informed global freshwater biodiversity stewardship. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated action towards its sustainable management and conservation.