Arctic freshwaters are facing multiple environmental pressures, including rapid climate change and increasing land___use activities. Freshwater plankton assemblages are expected to reflect the effects of these stressors through shifts in species distributions and changes to biodiversity. These changes may occur rapidly due to the short generation times and high dispersal capabilities of both phyto___ and zooplankton. Spatial patterns and contemporary trends in plankton diversity throughout the circumpolar region were assessed using data from more than 300 lakes in the U.S.A. (Alaska), Canada, Greenland, Iceland, the Faroe Islands, Norway, Sweden, Finland, and Russia. The main objectives of this study were: (1) to assess spatial patterns of plankton diversity focusing on pelagic communities; (2) to assess dominant component of __ diversity (turnover or nestedness); (3) to identify which environmental factors best explain diversity; and (4) to provide recommendations for future monitoring and assessment of freshwater plankton communities across the Arctic region. Phytoplankton and crustacean zooplankton diversity varied substantially across the Arctic and was positively related to summer air temperature. However, for zooplankton, the positive correlation between summer temperature and species numbers decreased with increasing latitude. Taxonomic richness was lower in the high Arctic compared to the sub___ and low Arctic for zooplankton but this pattern was less clear for phytoplankton. Fennoscandia and inland regions of Russia represented hotspots for, respectively, phytoplankton and zooplankton diversity, whereas isolated regions had lower taxonomic richness. Ecoregions with high __ diversity generally also had high __ diversity, and turnover was the most important component of __ diversity in all ecoregions. For both phytoplankton and zooplankton, climatic variables were the most important environmental factors influencing diversity patterns, consistent with previous studies that examined shorter temperature gradients. However, barriers to dispersal may have also played a role in limiting diversity on islands. A better understanding of how diversity patterns are determined by colonisation history, environmental variables, and biotic interactions requires more monitoring data with locations dispersed evenly across the circumpolar Arctic. Furthermore, the importance of turnover in regional diversity patterns indicates that more extensive sampling is required to fully characterise the species pool of Arctic lakes.

The Putorana Plateau (Krasnoyarsk Territory, Russia) is one of the largest mountainous regions of subarctic Eurasia. Studies of aquatic ecosystems of this are far from complete. In particular, microcrustaceans (Cladocera и Copepoda) of the Putorana Plateau are poorly investigated, although they are one of the main components of meiobenthic and zooplanktonic communities and a target for monitoring of the anthropogenic influence and climate change. An open question is a biogeographical status of the crustacean fauna of the plateau. Additionally, it is unknown which environmental factors significantly affect benthic and planktonic crustacean assemblages? Based on the samples collected in tundra and forest tundra ponds in the western and central parts of the plateau, analysis of the composition of crustacean fauna and factors regulating the assemblage structure was performed. In total, 36 Cladocera and 24 Copepoda species were found. Of these, 23 taxa are new for the region, and four are new to science. Species richness of Copepoda is higher in the central part and on the western slopes of the plateau than in foothills, while number of the Cladocera species in contrast decreases in mountainous areas. Variations in meiobenthic assemblages are due to the research area, type of water supply and less affected by altitude above sea level. For planktonic assemblages the size of the water body and, to a lesser degree, by macrophytes species composition was significant. Almost 12.8% of microcrustacean species of the Putorana Plateau can be attributed to glacial relics. Crustacean fauna of the Putorana Plateau has a high species richness and distinguishes significantly from the fauna of both western and eastern regions of the Arctic. The specifics of faunal composition of the region are connected to the climatic features of Middle Siberia and the retaining of the Pleistocene fauna in some glacial refugia.

Climate change is predicted to have dramatic effects on Arctic freshwater ecosystems through changes to the abiotic template that are expected to influence biodiversity. Changes are already ongoing in Arctic systems, but there is a lack of coordinated monitoring of Arctic freshwaters that hinders our ability to assess changes in biodiversity. To address the need for coordinated monitoring on a circumpolar scale, the Arctic Council working group, Conservation of Arctic Flora and Fauna, established the Circumpolar Biodiversity Monitoring Program, which is an adaptive monitoring program for the Arctic centred around four ecosystem themes (i.e., Freshwater, Terrestrial, Coastal, Marine). The freshwater theme developed a monitoring plan for Arctic freshwater biodiversity and recently completed the first assessment of status and trends in Arctic freshwater biodiversity. Circumpolar Biodiversity Monitoring Program–Freshwater has compiled and analysed a database of Arctic freshwater monitoring data to form the first report of the state of circumpolar Arctic freshwater biodiversity. This special issue presents the scientific analyses that underlie the Circumpolar Biodiversity Monitoring Program–Freshwater report and provides analyses of spatial and temporal diversity patterns and the multiple‐stressor scenarios that act on the biological assemblages and biogeochemistry of Arctic lakes and rivers. This special issue includes regional patterns for selected groups of organisms in Arctic rivers and lakes of northern Europe, Russia, and North America. Circumpolar assessments for benthic diatoms, macrophytes, plankton, benthic macroinvertebrates, and fish demonstrate how climate change and associated environmental drivers affect freshwater biodiversity. Also included are papers on spatial and temporal trends in water chemistry across the circumpolar region, and a systematic review of documented Indigenous Knowledge that demonstrates its potential to support assessment and conservation of Arctic freshwaters. This special issue includes the first circumpolar assessment of trends in Arctic freshwater biodiversity and provides important baseline information for future assessments and studies. It represents the largest compilation and assessment of Arctic freshwater biodiversity data to date and strives to provide a holistic view of ongoing change in these ecosystems to support future monitoring efforts. By identifying gaps in monitoring data across the circumpolar region, as well as identifying best practices for monitoring and assessment, this special issue presents an important resource for researchers, policy makers, and Indigenous and local communities that can support future assessments of ecosystem change.

Climate warming and subsequent landscape transformations result in rapid ecological change in Arctic freshwaters. Here we provide a synthesis of the diversity of benthic diatoms, plankton, macrophytes, macroinvertebrates, and fish in Arctic freshwaters. We developed a multi‐organism measure of α diversity to characterise circumpolar spatial patterns and their environmental correlates, and we assessed ecoregion‐level β diversity for all organism groups across the Arctic. Alpha diversity was lowest at high latitudes and elevations and where dispersal barriers exist. Diversity was positively related to temperature, and both temperature and connectivity limited diversity on high latitude islands. Beta diversity was highly variable among ecoregions for most organism groups, ranging from 0 (complete similarity) to 1 (complete dissimilarity). The high degree of dissimilarity within many ecoregions illustrates the uniqueness of many Arctic freshwater communities. Northward range expansion of freshwater taxa into Arctic regions may lead to increased competition for cold‐stenothermic and cold‐adapted species, and ultimately lead to the extinction of unique Arctic species. Societal responses to predicted impacts include: (1) actions to improve detection of changes (e.g., harmonised monitoring, remote sensing) and engagement with Arctic residents and Indigenous Peoples; and (2) actions to reduce the impact of unwanted changes (e.g., reductions of CO₂ emissions, action against the spread of invasive species). Current Arctic freshwater monitoring shows large gaps in spatial coverage, while time series data are scarce. Arctic countries should develop an intensified, long‐term monitoring programme with routine reporting. Such an approach will allow detection of long‐term changes in water quality, biodiversity, and ecosystem services of Arctic freshwaters.

We studied the taxonomic composition and the ecological characteristics of freshwater copepods collected from two lakes and 41 water bodies of the European northeast tundra and from 39 lakes in the northern taiga of Central Siberia. Both regions are situated beyond the Arctic Circle (Russia). Up to seven planktonic and 13 benthic species were recorded in the European tundra lakes and 27 species occurred in the small tundra water bodies. The taiga lakes of the Siberian region harboured 31 species. We confirmed the exclusively European distribution of Bryocamptus zschokkei komi Borutzky and the restricted distribution of Leptodiaptomus angustilobus (Sars) and Neutrodiaptomus pachypoditus (Rylov) in Asian ponds. Overall, we found a higher diversity of calanoids in the Siberian taiga than in the European tundra region. In addition, our results show that the biomass of particular copepod species, but not the total copepod biomass, is related to the surface area of lakes, size of small water bodies, the ecological category, pH, temperature, oxygen concentration and metal content.