Arctic freshwater biodiversity is rapidly changing due to climate warming, resource extraction, infrastructure development, and landscape transformation. To improve understanding, predict future responses, and inform policy formulation, research needs must be clearly identified. Using a horizon scan survey, Arctic freshwater experts from government, international agencies, and Indigenous Peoples identified 77 biodiversity research questions with 17 highlighted as most important for near term assessment. These questions span nine thematic categories: biodiversity and taxonomic challenges, hydrological change, productivity and food webs, ecosystem connectivity, methods, monitoring and assessment, permafrost change, winter ecology, anthropogenic development, and Indigenous Knowledge. Climate change emerged as the major driver among all categories and research questions. A key priority identified was the urgent need for long-term, harmonized monitoring programs among Arctic countries. Multiple knowledge gaps detected suggest that circumpolar research collaborations are required to tackle these issues.

Glaciers cover ~10% of Earth’s land surface, and their meltwater rivers are home to diverse biological communities that play central roles maintaining water quality, supporting fisheries, and subsidising the diet of terrestrial birds and mammals1,2. However, current unprecedented rates of glacier shrinkage3 are expected to cause major changes to algae and invertebrate biodiversity in rivers worldwide with largely unknown consequences4,5. Whilst knowledge of co-dependencies among biological groups is critical for predicting biodiversity change and driving conservation actions6, the intricate species-interactions shaping glacial-river food webs remain poorly understood4. Here we use >3500 newly observed feeding interactions among diatoms and invertebrates from rivers in the European Alps to demonstrate how decreasing glacier influence leads to increases in food web size as habitats become warmer, more stable, and less turbid. These responses are consistent with mountain-river biomonitoring observations from a new global meta-analysis of >190 studies. With decreasing glacier influence, diatoms became more species-rich and abundant both in real terms and relative to invertebrate consumers, and mean trophic level declined implying more efficient primary production transfer to consumers. We show this reorganisation is predictable from constituent species’ body mass and abundance, allowing projections to be made for rivers where species interactions remain unstudied. By demonstrating predictable links between species composition, traits, food web structure, and shrinking glaciers, this study significantly advances our ability to forecast ecological responses in rapidly changing mountain environments.

Approximately half of all methane (CH 4 ) emissions come from freshwaters, where they are regulated by the microbial ‘CH 4 filter’ whose efficiency describes the fraction of CH 4 produced that is subsequently oxidized back to CO 2 (methanotrophy) before emission. How the CH 4 filter efficiency responds to natural warming over centuries or millennia remains unknown. Here we address this question using a natural experiment comprising high-latitude, geothermally warmed streams in five regions spanning the Northern Hemisphere. CH 4 production becomes more efficient with warming, linked to increased abundance of methanogens and underpinned by community shifts. In contrast, while CH 4 oxidation activity increases, its process-level efficiency does not, and methanotrophs shift towards less efficient taxa. Consequently, the system-level CH 4 filter efficiency remains fixed, and CH 4 emissions increase. If this fixed CH 4 filter efficiency under warming is common to freshwaters worldwide (wetlands, lakes and rivers), then an upward trajectory for CH 4 emissions through future climate change appears inevitable.

The River Invertebrate Prediction and Classification System (RIVPACS) is used widely in freshwater management to set targets for macroinvertebrate ecological health based on the expected scores of metrics such as WHPT or LIFE in the absence of anthropogenic stressors. An underutilised capability of RIVPACS-type models is the capability to predict expected macroinvertebrate community composition, which could function as a novel management metric for river health. We present a novel Monte-Carlo simulation approach that generates simulated expected communities for England’s rivers based on RIVPACS predictions. This allows for assessments of macroinvertebrate health using similarity calculations between observed and expected communities. We assess 10-year trends in similarity between 2010 and 2019 at 4172 sites in England, and contrast these trends with WHPT ASPT O/E trends in the same period. Similarity scores include both Chi-Squared and Hellinger methods, to prioritise rare and common species, respectively. We find that whilst most sites (63.3%) showed improvement in WHPT ASPT O/E in this period, most sites showed declines in similarity for Chi-Squared and Hellinger O/E (51.1% and 58.8%, respectively). We identified three case study regions showing contrasting trends and illustrate how the new RIVPACS-derived similarity calculations can track meaningful shifts in composition associated with water quality and multiple stressors including invasive species. RIVPACS-derived similarity calculations potentially provide a sensitive and practical management metric to assess ecosystem health, although further work is required to understand the composition of communities in changing environments with clear changes in stressor regimes. ### Competing Interest Statement The authors have declared no competing interest.

Warming can have profound impacts on ecological communities. However, explorations of how differences in biogeography and productivity might reshape the effect of warming have been limited to theoretical or proxy-based approaches: for instance, studies of latitudinal temperature gradients are often conflated with other drivers (e.g., species richness). Here, we overcome these limitations by using local geothermal temperature gradients across multiple high-latitude stream ecosystems. Each suite of streams (6-11 warmed by 1-15°C above ambient) is set within one of five regions (37 streams total); because the heating comes from the bedrock and is not confounded by changes in chemistry, we can isolate the effect of temperature. We found a negative overall relationship between diatom and invertebrate species richness and temperature, but the strength of the relationship varied regionally, declining more strongly in regions with low terrestrial productivity. Total invertebrate biomass increased with temperature in all regions. The latter pattern combined with the former suggests that the increased biomass of tolerant species might compensate for the loss of sensitive species. Our results show that the impact of warming can be dependent on regional conditions, demonstrating that local variation should be included in future climate projections rather than simply assuming universal relationships.

Global change threatens invertebrate biodiversity and its central role in numerous ecosystem functions and services. Functional trait analyses have been advocated to uncover global mechanisms behind biodiversity responses to environmental change, but the application of this approach for invertebrates is underdeveloped relative to other organism groups. From an evaluation of 363 records comprising >1.23 million invertebrates collected from rivers across nine biogeographic regions on three continents, consistent responses of community trait composition and diversity to replicated gradients of reduced glacier cover are demonstrated. After accounting for a systematic regional effect of latitude, the processes shaping river invertebrate functional diversity are globally consistent. Analyses nested within individual regions identified an increase in functional diversity as glacier cover decreases. Community assembly models demonstrated that dispersal limitation was the dominant process underlying these patterns, although environmental filtering was also evident in highly glacierized basins. These findings indicate that predictable mechanisms govern river invertebrate community responses to decreasing glacier cover globally.

Long-term records of benthic macroinvertebrates in high-latitude streams are essential for understanding climatic changes, including extreme events (e.g. floods). Data extending over multiple decades are typically scarce. Here, we investigated macroinvertebrate community structural change (including alpha and beta diversity and gain and loss of species) over 22 years (1994-2016) in 10 stream systems across Denali National Park (Alaska, USA) in relation to climatological and meteorological drivers (e.g. air temperature, snowpack depth, precipitation). We hypothesised that increases in air temperature and reduced snowpack depth, due to climatic change, would reduce beta and gamma diversity but increase alpha diversity. Findings showed temporal trends in alpha diversity were variable across streams, with oscillating patterns in many snowmelt- and rainfall runoff-fed streams linked to climatic variation (temperature and precipitation), but increased over time in several streams supported by a mixture of water sources, including more stable groundwater-fed streams. Beta-diversity over the time series was highly variable, yet marked transitions were observed in response to extreme snowpack accumulation (1999-2000), where species loss drove turnover. Gamma diversity did not significantly increase or decrease over time. Investigating trends in individual taxa, several taxa were lost and gained during a relative constrained time period (2000-2006), likely in response to climatic variability and significant shifts in instream environmental conditions. Findings demonstrate the importance of long-term biological studies in stream ecosystems and highlight the vulnerability of high-latitude streams to climate change.

Groundwater contributions to streamflow significantly influence the structure and function of riverine ecosystems, particularly in glacierized catchments where there are marked differences in water sources and subsurface flow paths. Here, we investigated spatial and temporal variation in relationships between water sources, flow paths, physical and chemical processes, organic matter, microbial biofilms, and macroinvertebrates across groundwater-fed streams in the glacierized Toklat River catchment of Denali National Park, Alaska. Streams fed predominantly by seepage from the valley sides were perennial, whereas streams sustained by glacial meltwater seepage were ephemeral. Differences in environmental conditions between flow regimes appeared to influence spatial and temporal patterns of organic matter, linking to macroinvertebrate community dynamics. Macroinvertebrates in perennial streams were supported by fine particulate organic matter from subsurface flow paths during summer, transitioning to a combination of fine particulate matter and leaf litter in autumn. In comparison, macroinvertebrates inhabiting ephemeral streams, which only flowed during autumn, were supported by leaf litter. Some macroinvertebrate taxa were unaffected by turnover in organic matter, indicating potential plasticity in organic matter resource use. Findings highlight the importance of considering spatial and temporal variation in groundwater-fed streams, considering that projected hydrological changes under a changing climate may have significant implications for these systems.

Hyporheic zones increase freshwater ecosystem resilience to hydrological extremes and global environmental change. However, current conceptualizations of hyporheic exchange, residence time distributions, and the associated biogeochemical cycling in streambed sediments do not always accurately explain the hydrological and biogeochemical complexity observed in streams and rivers. Specifically, existing conceptual models insufficiently represent the coupled transport and reactivity along groundwater and surface water flow paths, the role of autochthonous organic matter in streambed biogeochemical functioning, and the feedbacks between surface‐subsurface ecological processes, both within and across spatial and temporal scales. While simplified approaches to these issues are justifiable and necessary for transferability, the exclusion of important hyporheic processes from our conceptualizations can lead to erroneous conclusions and inadequate understanding and management of interconnected surface water and groundwater environments. This is particularly true at the landscape scale, where the organizational principles of spatio‐temporal dynamics of hyporheic exchange flow (HEF) and biogeochemical processes remain largely uncharacterized. This article seeks to identify the most important drivers and controls of HEF and biogeochemical cycling based on a comprehensive synthesis of findings from a wide range of river systems. We use these observations to test current paradigms and conceptual models, discussing the interactions of local‐to‐regional hydrological, geomorphological, and ecological controls of hyporheic zone functioning. This improved conceptualization of the landscape organizational principles of drivers of HEF and biogeochemical processes from reach to catchment scales will inform future river research directions and watershed management strategies.

This chapter provides an overview of current understanding of alpine lotic ecosystems. The global distribution of alpine streams is presented and links between climate and hydrology are discussed. The different stream types found in the alpine zone, associated habitat properties and the concept of dynamic water source contributions (e.g., snow, ice/permafrost, groundwater and rainfall) are all introduced. Current knowledge of alpine stream ecosystem structure and function is presented, with an emphasis on biodiversity patterns of key biotic groups (i.e., microbes, algae, invertebrates and fish), biogeochemical cycling and species interactions. A discussion of anthropogenic threats and future predictions for alpine rivers, with a particular focus on glacier-fed rivers, concludes the chapter.

1. Warming in the Arctic is predicted to change freshwater biodiversity through loss of unique taxa and northward range expansion of lower latitude taxa. Detecting such changes requires establishing circumpolar baselines for diversity, and under -standing the primary drivers of diversity. 2. We examined benthic macroinvertebrate diversity using a circumpolar dataset of > 1,500 Arctic lake and river sites. Rarefied α diversity within catchments was as -sessed along latitude and temperature gradients. Community composition was

Succession is defined as change in community composition following a disturbance and is one of the oldest key concepts in ecology. Succession is a change in community structure or evenness at a site following a disturbance and may involve colonization/extinction but not always. The study of succession has generated insight into the various mechanisms by which communities are assembled and species co-exist, including the relative role of deterministic and stochastic processes. Succession has been considered as a cyclical temporal process (i.e., seasonal succession) or a process occurring across space (i.e., longitudinal succession). However, the focus of this chapter is site-specific, temporal succession (i.e., following a disturbance or creation of new habitat). Succession in streams to date has not contributed widely to general successional theory which has focused principally on plants. The aim of this chapter is to expand and update Fisher's classic chapter on succession ( Fisher, 1983 ) by providing a comprehensive review of this process in streams. We start by defining disturbance regimes in the context of succession and then introduce the idea of space and time considerations into successional trajectories. We then explore examples of succession for different groups of aquatic organisms from microbes to fish. We conclude with how stream succession may be influenced by legacy effects following disturbance and links to other ecosystems, such as the terrestrial riparian zone, or the underlying hyporheic and groundwater systems as refugia. Publications in the last 35 years or so have added substantially to our understanding, particularly with respect to interactions with other ecosystems, the role of anthropogenic impacts and the application of stream successional theory to restoration and management.

The Japanese macaque (Macaca fuscata) is native to the main islands of Japan, except Hokkaido, and is the most northerly living non-human primate. In the Chubu Sangaku National Park of the Japanese Alps, macaques live in one of the coldest areas of the world, with snow cover limiting the availability of preferred food sources. Winter is typically a bottleneck for food availability potentially resulting in marked energy deficits, and mortality may result from famine. However, streams with groundwater upwelling flow during the winter with a constant water temperature of about 5 °C are easily accessible for Japanese macaques to search for riverine biota. We used metabarcoding (Cytochrome c oxidase I) of fecal samples from Japanese macaques to determine their wintertime diet. Here we provide the first robust evidence that Japanese macaques feed on freshwater biota, including brown trout, riverine insects and molluscs, in Chubu Sangaku National Park. These additional food sources likely aid their winter survival.

Climate change is decreasing glacier cover and increasing the frequency and magnitude of precipitation‐driven high flows and floods in many regions of the world. Precipitation may become the dominant water source for river systems in recently deglaciated catchments, with major rainfall events driving significant changes in river channel morphology. Few studies, however, have examined river channel response to repeated precipitation‐driven high flows. In this study, we measured the geomorphological condition of four low‐order rivers in recently deglaciated catchments (70–210 years ice free) before and after a series of repeated precipitation‐driven high flows during summer 2014. High flows drove substantial initial morphological change, with up to 75% change in baseflow channel planform position and active channel form change from pre‐ to post‐high flow. Post‐high flow years were associated with increased instream wood and geomorphological complexity at all but the youngest river. Channel changes were part of an active relaxation stage at all rivers, where channels continued to migrate, and complexity varied through time. Overall, these measurements permit us to propose a conceptual model of the role of geomorphologically effective high flows in the context of paraglacial adjustment theory. Specifically, we suggest that older rivers in recently deglaciated catchments can undergo a short‐term (<10 years) increase in the rate of geomorphological development as a result of the recruitment of instream wood and channel migration during and following repeated precipitation‐driven high flows. Enhancing our knowledge of these geomorphological and paraglacial processes in response to high flows is important for the effective management of riverine water and ecosystem resources in rapidly changing environments.

A series of files with hydrological, physicochemical and metabolic activity data from a study investigating the environmental dynamics of six stream systems in the Japanese Alps.

Climate change is expected to intensify the effect of environmental stressors on riverine ecosystems. Extreme events, such as low flow and heatwaves, could have profound consequences for stream ecosystem functioning, but research on the impact of these stressors and their interaction across multiple processes, remains scarce. Here, we report the results of a two-month stream mesocosm experiment testing the effect of low flow (66% water level reduction, without gravel exposure) and heatwaves (three 8-d episodes of +5 degrees C above ambient with 10-15 days recovery between each episode) on a suite of ecosystem processes (i.e. detrital decomposition, biofilm accrual, ecosystem metabolism and DOC quantity and quality). Low flow reduced whole system metab-olism, suppressing the rates of gross primary production (GPP) and ecosystem respiration (ER), but elevated DOC concentration. Overall, habitat contraction was the main driver of reduced ecosystem functioning in the low flow treatment. By contrast, heatwaves increased decomposition, algal accrual, and humic-like DOC, but reduced leaf decomposition efficiency. Net ecosystem production (NEP) generally decreased across the experiment but was most pronounced for low flow and heatwaves when occurring independently. Assessment of NEP responses to the three successive heatwave events revealed that responses later in the sequence were more reduced (i.e. more similar to controls), suggesting biofilm communities may acclimate to autumn heatwaves. However, when heatwaves co-occurred with low flow, a strong reduction in both ER and GPP was observed, suggesting in-creased microbial mortality and reduced acclimation. Our study reveals autumn heatwaves potentially elongate the growth season for primary producers and stimulate decomposers. With climate change, river ecosystems may become more heterotrophic, with faster processing of recalcitrant carbon. Further research is required to identify the impacts on higher trophic levels, meta-community dynamics and the potential for legacy effects gen-erated by successive low flows and heatwaves. (c) 2021 Elsevier B.V. All rights reserved.

The unique hydrology and physicochemistry of alpine streams provide an important influence on the structure and function of inhabiting biological communities. A substantial body of research exists on alpine streams across many regions of the globe (e.g. Europe, North and South America and Greenland). To date, however, there have been few studies investigating the environmental conditions present in alpine streams across the Japanese archipelago. The lack of information on alpine streams in Japan is problematic as unique regional climates, e.g. some of the highest levels of snowfall globally, are likely to have repercussions for morphological, hydrological, physicochemical and metabolic signatures, causing them to differ from those observed in other regions. In this study we compare the morphology, hydrology, physicochemistry and metabolic activity of snowmelt and groundwater fed streams in the Kamikochi region of the Japanese Alps. Stream discharge, water chemistry (major ions, silica, dissolved oxygen), water temperature and channel stability were measured over a period of 16 months in 2017-2018. Metabolic activity was determined using Resazurin-Resorufin (Raz-Rru) Smart Tracer and variation in the Raz transformation rate was assessed to understand the effects of hydrology and physicochemistry on ecosystem functioning. Snowmelt streams were characterised by higher variability of water temperature, water chemistry and stream discharge, both, within and between sites. Indeed, two of the snowmelt streams experienced no flow conditions for several periods and also floods. In comparison, water chemistry, water temperature and stream discharge in groundwater fed streams were more temporally stable. Metabolic activity was higher in one groundwater fed stream, attributed to significant growth of macrophytes. These findings indicate that the patterns of morphology, hydrology, physicochemistry and metabolic activity across streams in the Japanese Alps largely resemble those identified elsewhere, although there were higher levels of inter-stream variation. The diversity and inter-site variation of hydrological and physicochemical conditions are likely responsible for the unique flora and fauna in the streams. This study therefore indicates the potential importance of habitat templates for the aquatic biodiversity hotspot in this region.

Multidimensional analysis of community stability has recently emerged as an overarching approach to evaluating ecosystem response to disturbance. However, the approach has previously been applied only in experimental and modelling studies. We applied this concept to an 18-year time series (2000-2017) of macroinvertebrate community dynamics from a southeast Alaskan river to further develop and test the approach in relation to the effects of two extreme flood events occurring in 2005 (event 1) and 2014 (event 2). Five components of stability were calculated for pairs of pre- or post-event years. Individual components were tested for differences between pre- and post-event time periods. Stability components' pairwise correlations were assessed and ellipsoids of stability were developed for each time period and compared to a null model derived from the permuted dataset. Only one stability component demonstrated a significant difference between time periods. In contrast, 80% of moderate and significant correlations between stability components were degraded post-disturbance and significant changes to the form of stability ellipsoids were observed. Ellipsoids of stability for all periods after the initial disturbance (2005) were not different to the null model. Our results illustrate that the dimensionality of stability approach can be applied to natural ecosystem time-series data. The major increase in dimensionality of stability observed following disturbance potentially indicates significant shifts in the processes which drive stability following disturbance. This evidence improves our understanding of community response beyond what is possible through analysis of individual stability components.