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Arctic freshwaters support biota adapted to the harsh conditions at these latitudes, but the climate is changing rapidly and so are the underlying environmental filters. Currently, we have limited understanding of broad‐scale patterns of Arctic riverine biodiversity and the correlates of α‐ and β‐diversity.Using information from a database set up within the scope of the Arctic Council's Conservation of Arctic Flora and Fauna Circumpolar Biodiversity Monitoring Plan, we analysed patterns and correlates of α‐ and β‐diversity in benthic diatom and macroinvertebrate communities across northern Norway, Sweden, and Finland. We analysed variation in total β‐diversity and its replacement and richness difference components in relation to location of the river reach and its drainage basin (Baltic Sea in the south, the Barents Sea in the east and the north, and the Norwegian Sea in the west), in addition to climate and environmental variables.In both macroinvertebrates and diatoms, the replacement and richness difference components showed wide variation. For macroinvertebrates, the richness difference component was the more important, whereas for diatoms, the replacement component was the more important in contributing to variation in β‐diversity. There was no significant difference in β‐diversity between the three main drainage basins, but species composition differed among the drainage basins.Based on the richness difference component of β‐diversity, climate variables were most strongly associated with community variation in macroinvertebrates. In diatoms, both environmental and climate variables were strongly correlated with community compositional variation. In both groups, there were also significant differences in α‐diversity among the three main drainage basins, and several taxa were significant indicators of one of these drainage basins. Alpha diversity was greater in areas with a continental climate, while the oceanic areas in the west harboured greatly reduced flora and fauna.The correlates of biodiversity were relatively similar in macroinvertebrates and diatoms. Climate variables, in particular temperature, were the most strongly associated with biodiversity patterns in the Arctic rivers of Fennoscandia. Sedimentary geology may be associated with increased productivity and, to a lesser extent, with sensitivity to acidification. There was considerable variation in community composition across Arctic Fennoscandia, indicating the necessity of protecting several stream reaches or even whole catchments within each region to conserve total riverine biodiversity. Furthermore, it is likely that the predicted changes in temperature in Arctic areas will influence riverine diversity patterns across Fennoscandia. [ABSTRACT FROM AUTHOR]

Arctic and sub‐Arctic lakes in northern Europe are increasingly threatened by climate change, which can affect their biodiversity directly by shifting thermal and hydrological regimes, and indirectly by altering landscape processes and catchment vegetation. Most previous studies of northern lake biodiversity responses to environmental changes have focused on only a single organismal group. Investigations at whole‐lake scales that integrate different habitats and trophic levels are currently rare, but highly necessary for future lake monitoring and management.We analysed spatial biodiversity patterns of 74 sub‐Arctic lakes in Norway, Sweden, Finland, and the Faroe Islands with monitoring data for at least three biological focal ecosystem components (FECs)—benthic diatoms, macrophytes, phytoplankton, littoral benthic macroinvertebrates, zooplankton, and fish—that covered both pelagic and benthic habitats and multiple trophic levels.We calculated the richnessrelative (i.e. taxon richness of a FEC in the lake divided by the total richness of that FEC in all 74 lakes) and the biodiversity metrics (i.e. taxon richness, inverse Simpson index (diversity), and taxon evenness) of individual FECs using presence–absence and abundance data, respectively. We then investigated whether the FEC richnessrelative and biodiversity metrics were correlated with lake abiotic and geospatial variables. We hypothesised that (1) individual FECs would be more diverse in a warmer and wetter climate (e.g. at lower latitudes and/or elevations), and in hydrobasins with greater forest cover that could enhance the supply of terrestrial organic matter and nutrients that stimulated lake productivity; and (2) patterns in FEC responses would be coupled among trophic levels.Results from redundancy analyses showed that the richnessrelative of phytoplankton, macrophytes, and fish decreased, but those of the intermediate trophic levels (i.e. macroinvertebrates and zooplankton) increased with decreasing latitude and/or elevation. Fish richnessrelative and diversity increased with increasing temporal variation in climate (temperature and/or precipitation), ambient nutrient concentrations (e.g. total nitrogen) in lakes, and woody vegetation (e.g. taiga forest) cover in hydrobasins, whereas taxon richness of macroinvertebrates and zooplankton decreased with increasing temporal variation in climate.The similar patterns detected for richnessrelative of fish, macrophytes, and phytoplankton could be caused by similar responses to the environmental descriptors, and/or the beneficial effects of macrophytes as habitat structure. By creating habitat, macrophytes may increase fish diversity and production, which in turn may promote higher densities and probably more diverse assemblages of phytoplankton through trophic cascades. Lakes with greater fish richnessrelative tended to have greater average richnessrelative among FECs, suggesting that fish are a potential indicator for overall lake biodiversity.Overall, the biodiversity patterns observed along the environmental gradients were trophic‐level specific, indicating that an integrated food‐web perspective may lead to a more holistic understanding of ecosystem biodiversity in future monitoring and management of high‐latitude lakes. In future, monitoring should also focus on collecting more abundance data for fish and lower trophic levels in both benthic and pelagic habitats. This may require more concentrated sampling effort on fewer lakes at smaller spatial scales, while continuing to sample lakes distributed along environmental gradients. [ABSTRACT FROM AUTHOR]

Climate change, biological invasions, and anthropogenic disturbance pose a threat to the biodiversity and function of Arctic freshwater ecosystems. Understanding potential changes in fish species distribution and richness is necessary, given the great importance of fish to the function of freshwater ecosystems and as a resource to humans. However, information gaps limit large‐scale studies and our ability to determine patterns and trends in space and time. This study takes the first step in determining circumpolar patterns of fish species richness and composition, which provides a baseline to improve both monitoring and conservation of Arctic freshwater biodiversity.Information on species presence/absence was gathered from the Circumpolar Biodiversity Monitoring Program's Freshwater Database and used to examine patterns of freshwater fish γ‐, α‐, and β‐diversity across 234° of longitude in the Arctic. The metrics of diversity provided information on species richness and composition across hydrobasins, ecoregions, and Arctic zones.Circumpolar patterns of fish species biodiversity varied with latitude, isolation, and coarse ecoregion characteristics; patterns were consistent with historic and contemporary barriers to colonisation and environmental characteristics. Gamma‐diversity was lower in the high Arctic compared to lower latitude zones, but α‐diversity did not decrease with increasing latitude below 71°N, reflecting glacial history. Alpha‐diversity was reduced to a single species, Arctic charr Salvelinus alpinus, in ecoregions above 71°N, where γ‐diversity was the lowest. Beta‐diversity indicated little variation in the composition and richness of species across the High Arctic; at lower latitudes, ecoregions contained more species, although species composition turned over across large spatial extents.In an analysis of five ecoregions in the circumpolar Arctic, physical isolation, and ecoregion area and topography were identified as strong drivers of γ‐, α‐, and β‐diversity. Physical isolation reduced the γ‐ and α‐diversity, and changes in β‐diversity between adjacent locations were due mainly to losses in species richness, rather than due to differences in species composition. Heterogeneity of habitats, environmental gradients, and geographic distance probably contributed to patterns of fish dissimilarity within and across ecoregions.This study presents the first analysis of large‐scale patterns of freshwater fish biodiversity in the circumpolar Arctic. However, information gaps in space, time, and among taxonomic groups remain. Future inclusion of extensive archive and new data will allow future studies to test for changes and drivers of the observed patterns of biodiversity. This is important given the potential impacts of ongoing and accelerating climate change, land use, and biotic exchange on Arctic fish biodiversity. [ABSTRACT FROM AUTHOR]

1. Rivers are spatially organised into hierarchic dendritic networks. This unique physical structure and the associated directionality of physical flows set them apart from most other environments by regulating the dispersal of resident biota and therefore the distribution of biodiversity. 2. The aim of this special issue is to highlight the importance of the river network on structuring biodiversity, particularly through metacommunity dynamics and associated dispersal processes. 3. The issue covers a wide range of topics, including disease spread, nutrient uptake, trophic dynamics, effects of anthropogenic stressors and the joint roles of dispersal and environmental filtering. Contributions employ a broad range of approaches, including field and laboratory experiments, modelling, population genetics and conceptual synthesis. 4. Although these studies represent just a sample of the research that is being performed on biodiversity and metacommunity dynamics in river networks, several important findings have emerged; a common theme being that the structure of the network and spatial dynamics clearly influence the dynamics of populations and communities, and their functions. By taking a broad taxonomic focus (from diatoms and protists to fish), and spanning a large geographic gradient (from the tropics to the subarctic), this special issue provides a broad look at the dynamics that occur in river networks relating to their unique makeup. We hope that this selection of studies spurs additional research on these interesting, globally important, yet severely threatened ecological systems. [ABSTRACT FROM AUTHOR]

1. River networks are hierarchical dendritic habitats embedded within the terrestrial landscape, with varying connectivity between sites depending on their positions along the network. This physical organisation influences the dispersal of organisms, which ultimately affects metacommunity dynamics and biodiversity patterns. 2. We provide a conceptual synthesis of the role of river networks in structuring metacommunities in relation to dispersal processes in riverine ecosystems. We explore where the river network best explains observed metacommunity structure compared to other measurements of physical connectivity. We mostly focus on invertebrates, but also consider other taxonomic groups, including microbes, fishes, plants, and amphibians. 3. Synthesising studies that compared multiple spatial distance metrics, we found that the importance of the river network itself in explaining metacommunity patterns depended on a variety of factors, including dispersal mode (aquatic versus aerial versus terrestrial) and landscape type (arid versus mesic), as well as location- specific factors, such as network connectivity, land use, topographic heterogeneity, and biotic interactions. The river network appears to be less important for strong aerial dispersers and insects in arid systems than for other groups and biomes, but there is considerable variability. Borrowing from other literature, particularly landscape genetics, we developed a conceptual model that predicts that the explanatory power of the river network peaks in mesic systems for obligate aquatic dispersers. 4. We propose directions of future avenues of research, including the use of manipulative field and laboratory experiments that test metacommunity theory in river networks. While field and laboratory experiments have their own benefits and drawbacks (e.g. reality, control, cost), both are powerful approaches for understanding the mechanisms structuring metacommunities, by teasing apart dispersal and niche-related factors. 5. Finally, improving our knowledge of dispersal in river networks will benefit from expanding the breadth of cost-distance modelling to better infer dispersal from observational data; an improved understanding of life-history strategies rather than relying on independent traits; exploring individual-level variation in dispersal through detailed genetic studies; detailed studies on fine-scale environmental (e.g. daily hydrology) and organismal spatiotemporal variability; and synthesising comparative, experimental, and theoretical work. Expanding in these areas will help to push the current state of the science from a largely pattern-detection mode into a new phase of more mechanistically driven research. [ABSTRACT FROM AUTHOR]

1. Metacommunity theory focuses on assembly patterns in ecological communities, originally exemplified through four different, yet non-exclusive, perspectives: patch dynamics, species sorting, source-sink dynamics, and neutral theory. More recently, three exclusive components have been proposed to describe a different metacommunity framework: habitat heterogeneity, species equivalence, and dispersal. Here, we aim at evaluating the insect metacommunity of a subarctic stream network under these two different frameworks. 2. We first modelled the presence/absence of 47 stream insects in northernmost Finland, using binomial generalised linear models (GLMs). The deviance explained by pure local environmental (E), spatial (S), and climatic variables (C) was then analysed across species using beta regression. In this comparative analysis, site occupancy, as well as taxonomic and biological trait vectors obtained from principal coordinate analysis, were used as predictor variables. 3. Single-species distributions were better explained by in-stream environmental and spatial factors than by climatic forcing, but in a highly variable fashion. This variability was difficult to relate to the taxonomic relatedness among species or their biological trait similarity. Site occupancy, however, was related to model performance of the binomial GLMs based on spatial effects: as populations are likely to be better connected for common species due to their near ubiquity, spatial factors may also explain better their distributions. 4. According to the classical four-perspective framework, the observation of both environmental and spatial effects suggests a role for either mass effects or species sorting constrained by dispersal limitation, or both. Taxonomic and biological traits, including the different dispersal capability of species, were scarcely important, which undermines the patch dynamics perspective, based on differences in dispersal ability between species. The highly variable performance of models makes the reliance on an entirely neutral framework unrealistic as well. According to the three-component framework, our results suggest that the stream insect metacommunity is shaped by the effect of habitat heterogeneity (supporting both species-sorting and mass effects), rather than species equivalence or dispersal limitation. 5. While the relative importance of the source-sink dynamics perspective or the species-sorting paradigm cannot be deciphered with the data at our disposal, we can conclude that habitat heterogeneity is an important driver shaping species distributions and insect assemblages in subarctic stream metacommunities. These results exemplify that the use of the three-component metacommunity framework may be more useful than the classical four perspective paradigm in analysing metacommunities. Our findings also provide support for conservation strategies based on the preservation of heterogeneous habitats in a metacommunity context. [ABSTRACT FROM AUTHOR]

1. In stream ecology, it has traditionally been proposed that local habitat conditions pose a strong environmental filter resulting in predictable community types. However, recent studies have demonstrated continuous rather than discrete community variation along environmental gradients across streams. 2. Our aim was to examine the continuity of stream communities using functional traits of benthic macroinvertebrates. Using previous studies and the environmental filtering paradigm as a background, we hypothesised that the taxonomic composition of communities should vary continuously over environmental gradients, whereas the functional composition of communities should form discrete community types according to specific habitat conditions. As stream environmental characteristics vary between seasons, we also asked if this had an effect on the characteristics of community variation in time. 3. Our data were collected from 50 streams draining into the River Tenojoki (70N, 27E) in Finland and Norway. The first data set was collected during late spring 2013 and the second data set during early autumn of the same year. Using multivariate regression trees and constrained ordination methods, we analysed the relationships of taxonomic and functional composition of macroinvertebrate communities to various in-stream environmental factors. 4. Our results suggest that the variation in the importance of environmental factors between seasons combined with the high heterogeneity of habitats precludes the formation of clear taxonomic and functional community types in northern streams. 5. Conservation efforts should hence focus on including variability in environmental conditions so as to encompass all components of biological communities. However, as there were some noticeable responses of individual traits to different environmental factors, discrete functional communities may be found if key traits of the organisms can be identified. [ABSTRACT FROM AUTHOR]

Human-induced loss of environmental heterogeneity may lead to homogenisation of biological communities. However, environmental heterogeneity is important for sustaining current levels of biodiversity. Hence, more ecological research should be directed on understanding the relationships between beta diversity and environmental heterogeneity., Benthic diatoms are among the most extensively studied taxa of primary producers in streams, yet their beta diversity patterns are not well documented at intermediate spatial scales, that is within and among streams, in a catchment., We tested how well one sampling site represents the total diatom diversity of a stream using 10 boreal streams, each comprising 10 sampled riffles. We also examined variation in diatom community structure at two hierarchical scales, within and among streams. Relationships between diatom beta diversity and environmental heterogeneity were studied using a combination of species accumulation curves, canonical analysis of principal coordinates, tests of homogeneity of dispersions and distance-based redundancy analysis., We found large variation in community structure within each stream, which indicates thatone sample is not adequate to represent the diatom diversity of an entire stream. The variation in among-stream diatom community structure was larger than among-stream habitat heterogeneity. Although no positive relationship between within-stream diatom beta diversity and environmental heterogeneity was detected, we found that diatom community variation was significantly related to physical habitat structure and, surprisingly, also to grazer abundance., We recommend that more than one sampling site is included in analyses aiming to cover the diatom diversity of a whole stream. This is especially true for pristine sites with high diatom diversity. In addition, we suggest that researchers incorporate physical and biological factors as explanatory variables when monitoring diatoms both at the within- and among-stream scales. [ABSTRACT FROM AUTHOR]

Metacommunity ecology addresses the situation where sets of local communities are connected by the dispersal of a number of potentially interacting species. Aquatic systems (e.g. lentic versus lotic versus marine) differ from each other in connectivity and environmental heterogeneity, suggesting that metacommunity organisation also differs between major aquatic systems. Here, we review findings from observational field studies on metacommunity organisation in aquatic systems., Species sorting (i.e. species are 'filtered' by environmental factors and occur only at environmentally suitable sites) prevails in aquatic systems, particularly in streams and lakes, but the degree to which dispersal limitation interacts with such environmental control varies among different systems and spatial scales. For example, mainstem rivers and marine coastal systems may be strongly affected by 'mass effects' (i.e. where high dispersal rates homogenise communities to some degree at neighbouring localities, irrespective of their abiotic and biotic environmental conditions), whereas isolated lakes and ponds may be structured by dispersal limitation (i.e. some species do not occur at otherwise-suitable localities simply because sites with potential colonists are too far away). Flow directionality in running waters also differs from water movements in other systems, and this difference may also have effects on the role of dispersal in different aquatic systems., Dispersal limitation typically increases with increasing spatial distance between sites, mass effects potentially increase in importance with decreasing distance between sites, and the dispersal ability of organisms may determine the spatial extents at which species sorting and dispersal processes are most important., A better understanding of the relative roles of species sorting, mass effects and dispersal limitation in affecting aquatic metacommunities requires the following: (i) characterising dispersal rates more directly or adopting better proxies than have been used previously; (ii) considering the nature of aquatic networks; (iii) combining correlative and experimental approaches; (iv) exploring temporal aspects of metacommunity organisation and (v) applying past approaches and statistical methods innovatively for increasing our understanding of metacommunity organisation. [ABSTRACT FROM AUTHOR]

Beta diversity modelling has received increased interest recently. There are multiple definitions of beta diversity, but here, we focus on variability in species composition among sampling units within a given area. This facet can be described using various approaches. Some approaches ignore the spatial scale of the area considered (i.e. region limits), while some consider different region limits as a starting point for the analysis of beta diversity., We focused specifically on the beta diversity-environmental heterogeneity relationship in running waters. First, we present two conceptual models, which assume either (1) strong environmental control among localities (riffle sites in our case) within each region unit (a region unit encompasses a species pool and can be a stream or a basin or an ecoregion) or (2) that the spatial level of a region unit affects the relative importance of mechanisms affecting variability in species composition among localities (i.e. among riffle sites) within each region unit. Second, we compared three recent studies that used similar methods to examine the beta diversity-environmental heterogeneity relationship, but which were based on different region units, comprising sets of streams or sets of basins or sets of ecoregions., Our conceptual framework assumes that environmental control is not likely to be the sole mechanism affecting variability in community composition among localities within each region unit, but it is likely to be most important when dispersal rates are intermediate (i.e. among localities within a basin). In contrast, if dispersal rates are very high (i.e. among localities within a stream) or very low (i.e. among localities within an ecoregion), environmental control is in part masked by high dispersal rates or is prevented from occurring because not all species can reach all localities, respectively. Such scale dependency in the relative strength of environmental control might therefore transcend spatial scales from individual region units to the strength of the beta diversity-environmental heterogeneity relationship. We emphasise that the beta diversity-environmental heterogeneity relationship can only be tested across multiple region units. The results of three case studies are consistent with these predictions. Specifically, the beta diversity-environmental heterogeneity regression was highly significant across multiple basins, but not across multiple streams or across multiple ecoregions., We suggest that researchers take spatial scale and region unit level explicitly into account when inferring the mechanisms structuring ecological communities and mapping variation in beta diversity. We also propose a unified terminology for studies examining the beta diversity-environmental heterogeneity relationship in running waters because inconsistent terminology is likely to hamper the progress of our science. [ABSTRACT FROM AUTHOR]