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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 understanding the primary drivers of diversity.We examined benthic macroinvertebrate diversity using a circumpolar dataset of >1,500 Arctic lake and river sites. Rarefied α diversity within catchments was assessed along latitude and temperature gradients. Community composition was assessed through region‐scale analysis of β diversity and its components (nestedness and turnover), and analysis of biotic–abiotic relationships.Rarefied α diversity of lakes and rivers declined with increasing latitude, although more strongly across mainland regions than islands. Diversity was strongly related to air temperature, with the lowest diversity in the coldest catchments. Regional dissimilarity was highest when mainland regions were compared with islands, suggesting that connectivity limitations led to the strongest dissimilarity. High contributions of nestedness indicated that island regions contained a subset of the taxa found in mainland regions.High Arctic rivers and lakes were predominately occupied by Chironomidae and Oligochaeta, whereas Ephemeroptera, Plecoptera, and Trichoptera taxa were more abundant at lower latitudes. Community composition was strongly associated with temperature, although geology and precipitation were also important correlates.The strong association with temperature supports the prediction that warming will increase Arctic macroinvertebrate diversity, although low diversity on islands suggests that this increase will be limited by biogeographical constraints. Long‐term harmonised monitoring across the circumpolar region is necessary to detect such changes to diversity and inform science‐based management. [ABSTRACT FROM AUTHOR]

Abstract: Increasing rates of precipitation and higher air temperatures have increased the size and frequency of retrogressive thaw slumps—large depressions of thawed permafrost that form on the landscape—in north‐western Canada. Many of these thaw slumps flow into nearby stream systems, leading to increased sediment, solute and nutrient loads. We evaluated the impacts of retrogressive thaw slumps on measurements of algal biomass accumulation and decomposition of organic materials in streams in the Peel Plateau, Northwest Territories. We predicted that increased sediment loads from thaw slumps would decrease algal standing stock and decomposition in thaw slump‐impacted streams, overriding the potential positive effects of increased nutrient concentrations. Chl‐a measurements were obtained as a proxy for algal standing stock from sites upstream and downstream of thaw slumps by performing algae scrapes and deploying artificial substrates in 2014. Cotton strips were deployed at upstream and downstream sites in 2013 and 2014, and tensile strength was measured to assess breakdown. Grab water samples were taken to measure physical and chemical parameters at each site. Thaw slumps increased total suspended solids, but not dissolved nutrients at downstream sites. Our results indicated a significant negative relationship between Chl‐a and total suspended solids. Decomposition indicated a negative relationship with total suspended solids, but displayed much stronger positive relationships with temperature, pH and dissolved phosphorus. Our findings indicated that total suspended solids were a stronger driver of change in thaw slump‐impacted stream reaches than nutrients, although nutrients may be more influential during the initiation of thaw slump disturbances. Algal biomass accumulation was found to be more sensitive to thaw slump impacts than decomposition, which may lead to a functional and structural shift in favour of allochthonous‐based food webs over autochthonous ones at thaw slump‐impacted stream reaches. [ABSTRACT FROM AUTHOR]

Subsidy-stress responses can decouple density-dependent relationships between taxa within a community by increasing resources for some taxa while removing sensitive species. In this study, we examined the effects of a triad of subsidy-stress responses on predator-prey relationships in a benthic macroinvertebrate community under the action of insecticides, nutrients and predation pressure all at environmentally relevant doses., We stocked 72 outdoor artificial streams with benthic invertebrates to establish a fully factorial experiment to investigate the interactive effects of two anthropogenic factors: (i) three levels of nutrient enrichment (low, moderate and high); and (ii) three doses of imidacloprid, a common agricultural insecticide, at control, low (LOEC) and lethal doses (LC50). These factors were crossed with (iii) another, biotic stressor by stocking half of the streams with a predaceous perlid stonefly at near-natural densities., Fifty percent of the community-level responses were driven by the combined effect of nutrients and the insecticide. An additional 27% of the community variation was best explained by predation pressure, which we estimated as the gape width × density of Agnetina capitata at the end of the experiment., No negative effect of imidacloprid on A. capitata was detected ( P = 0.62). By the end of the 3-week experiment, the overall gape width and predation pressure of A. capitata had more than doubled (2.1-3.7x) in all moderate nutrient streams ( P < 0.01). Increased size and predation pressure of A. capitata covaried with the depressed density of all macroinvertebrate taxa as well as EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa and the mayfly Baetis in particular., Structural equation models (SEM) highlighted that nutrient enrichment was the main driver of individual and community responses in the artificial streams. Additionally, SEM showed that density but not body size of Baetis was severely hampered by A. capitata predation, suggesting that these stoneflies selectively foraged on other species in the first 10 days of the experiment., Our findings highlight the difficulties of assessing the risk of contaminants in aquatic communities where interacting species respond differently to subsidies and stresses. [ABSTRACT FROM AUTHOR]

The vertical distribution within the substrate of three lotic mayflies (Rhithrogena sp., Paraleptophlebia sp. and Baetis tricaudatus Dodds) was examined in laboratory streams to determine if diel patterns of vertical movement across substrate surfaces existed and whether light intensity, temperature, substrate composition or food availability influenced the observed movement patterns. 2. Rhithrogena sp. and B. tricaudatus exhibited significant diel movement from lower substrate surfaces during the day to upper surfaces at night, although >64% of B. tricaudatus occupied the upper surfaces throughout the 24 h period. In contrast, Parakptophlebia sp. larvae did not change their vertical distribution over the 24 h period. 3. The proportion of mayflies occupying substrate surfaces was influenced by the amount of upper and lower surface area available in the substrate and, possibly, crevice sizes associated with lower surfaces. 4. The movement of Rhithrogena sp. from lower surfaces during the day to the upper surfaces during the night was influenced by changes in light intensity, but not by changes in food availability or water temperature. 5. The diel patterns of vertical movement exhibited by lotic mayflies in this investigation raise the possibility that at least some biotic interactions may vary over a diel cycle. These could include competitive interactions or temporal cycling of prey availability to predators. [ABSTRACT FROM AUTHOR]

Arctic landscapes are characterised by their numerous ponds and lakes. With increased climate warming and consequent changes in hydrological connectivity, the ecology of many of these waterbodies is expected to change.We sampled 13 ponds and 22 lakes for zooplankton and various limnological and physical environmental variables in the vicinity of Cambridge Bay, Nunavut (69.1169° N, 105.0597° W), with the aim of testing how well the degree of hydrological connectivity explained patterns in species composition and abundance and contributed to crustacean production. Ponds were hydrologically isolated while the lakes were arranged in four lake chains and ranged from the headwaters to highly inter‐connected lakes receiving water from one to 768 upstream lakes.In all sites combined, 77 zooplankton species were found, including 56 rotifers, six copepods, 11 cladocerans, two fairy shrimp species, a mysid, and a tadpole shrimp. We show that the zooplankton communities differed between hydrologically isolated (ponds) and connected (lakes) systems, with 17 species unique to ponds and 20 to lakes. Furthermore, the communities were more similar within lake chains and hence in lakes closer to each other. In ponds, distances between waterbodies had no impact on community similarity.Zooplankton abundance was higher in lakes (255 ind/L) than in ponds (65 ind/L) due to the higher number of rotifers that accounted for nearly 80% of the zooplankton abundance in lakes. In ponds, rotifers, cladocerans, and copepods were equally abundant. In terms of biomass, cladocerans represented 65% of total biomass in all waterbodies except for a chain of deep lakes that had abundant fish communities. In these lakes, cladoceran abundance and biomass were low and probably limited by fish predation.Zooplankton production was higher in ponds (4.6 mgC m−3 day−1) than in lakes (1.7 mgC m−3 day−1), and within lakes was lowest in the chain composed of large lakes (1.3 mgC m−3 day−1). The high production in ponds was closely linked to high zooplankton biomass and further explained by high gross primary production supported by relatively high nutrient concentrations in these shallow systems.Our study emphasises that hydrological connectivity is key to shaping zooplankton communities, although fish presence also appears to affect them. It shows that isolated ponds (that currently account for c83% of all waterbodies on southern Victoria Island) are hotspots of aquatic biological productivity that probably play an essential role in Arctic freshwater landscapes. Finally, our study provides critical baseline information on the current composition of Arctic zooplankton communities important for estimating climate change impacts. [ABSTRACT FROM AUTHOR]

1. Laboratory experiments were performed to determine whether flightinitiation by lotic mayfly larvae of Baetis tricaudatus exposed to the longnose dace (Rhinichthys cataractae) is affected by an individual's size and its physiological state. 2. We used a three factorial ANOVA design to test whether flight initiation distances (FIDS) were affected by Baetis size (small, large), starvation level (low, high), and the length of a predator-free foraging period (short, long). 3. FIDs were significantly affected by the interaction between Baetis sizeand starvation level, and the main level effects of starvation and Baetis size. FIDs of small larvae were unaffected by starvation level and the length of the predator-free feeding period, whereas starvation reduced FIDs of large larvae 2-fold. 4. Subsequent experiments showed that size-dependent differences in FIDs could not be attributed tosize-specific differences in the economics of rates of food intake or gut evacuation. For both small and large larvae, food intake rates declined with increasing time spent foraging and both small and largelarvae consumed the majority (70-- 75 percent) of food within the first 3 h of the 12-h feeding period. Similarly, gut evacuation declined with increasing starvation time and rates of evacuation did not differ between small and large larvae. These data suggest that size-dependent differences in flight initiation by Baetis larvae do not involve the energetics of food intake or processing. [ABSTRACT FROM AUTHOR]

Freshwater chemistry across the circumpolar region was characterised using a pan‐Arctic data set from 1,032 lake and 482 river stations. Temporal trends were estimated for Early (1970–1985), Middle (1986–2000), and Late (2001–2015) periods. Spatial patterns were assessed using data collected since 2001.Alkalinity, pH, conductivity, sulfate, chloride, sodium, calcium, and magnesium (major ions) were generally higher in the northern‐most Arctic regions than in the Near Arctic (southern‐most) region. In particular, spatial patterns in pH, alkalinity, calcium, and magnesium appeared to reflect underlying geology, with more alkaline waters in the High Arctic and Sub Arctic, where sedimentary bedrock dominated.Carbon and nutrients displayed latitudinal trends, with lower levels of dissolved organic carbon (DOC), total nitrogen, and (to a lesser extent) total phosphorus (TP) in the High and Low Arctic than at lower latitudes. Significantly higher nutrient levels were observed in systems impacted by permafrost thaw slumps.Bulk temporal trends indicated that TP was higher during the Late period in the High Arctic, whereas it was lower in the Near Arctic. In contrast, DOC and total nitrogen were both lower during the Late period in the High Arctic sites. Major ion concentrations were higher in the Near, Sub, and Low Arctic during the Late period, but the opposite bulk trend was found in the High Arctic.Significant pan‐Arctic temporal trends were detected for all variables, with the most prevalent being negative TP trends in the Near and Sub Arctic, and positive trends in the High and Low Arctic (mean trends ranged from +0.57%/year in the High/Low Arctic to −2.2%/year in the Near Arctic), indicating widespread nutrient enrichment at higher latitudes and oligotrophication at lower latitudes.The divergent P trends across regions may be explained by changes in deposition and climate, causing decreased catchment transport of P in the south (e.g. increased soil binding and trapping in terrestrial vegetation) and increased P availability in the north (deepening of the active layer of the permafrost and soil/sediment sloughing). Other changes in concentrations of major ions and DOC were consistent with projected effects of ongoing climate change. Given the ongoing warming across the Arctic, these region‐specific changes are likely to have even greater effects on Arctic water quality, biota, ecosystem function and services, and human well‐being in the future. [ABSTRACT FROM AUTHOR]

Comprehensive assessments of contemporary diatom distributions across the Arctic remain scarce. Furthermore, studies tracking species compositional differences across space and time, as well as diatom responses to climate warming, are mainly limited to paleolimnological studies due to a lack of routine monitoring in lakes and streams across vast areas of the Arctic.The study aims to provide a spatial assessment of contemporary species distributions across the circum‐Arctic, establish contemporary biodiversity patterns of diatom assemblages to use as reference conditions for future biomonitoring assessments, and determine pre‐industrial baseline conditions to provide historical context for modern diatom distributions.Diatom assemblages were assessed using information from ongoing regulatory monitoring programmes, individual research projects, and from surface sediment layers obtained from lake cores. Pre‐industrial baseline conditions as well as the nature, direction and magnitude of changes in diatom assemblages over the past c.200 years were determined by comparing surface sediment samples (i.e. containing modern assemblages) with a sediment interval deposited prior to the onset of significant anthropogenic activities (i.e. containing pre‐1850 assemblages), together with an examination of diatoms preserved in contiguous samples from dated sediment cores.We identified several biotypes with distinct diatom assemblages using contemporary diatom data from both lakes and streams, including a biotype typical for High Arctic regions. Differences in diatom assemblage composition across circum‐Arctic regions were gradual rather than abrupt. Species richness was lowest in High Arctic regions compared to Low Arctic and sub‐Arctic regions, and higher in lakes than in streams. Dominant diatom taxa were not endemic to the Arctic. Species richness in both lakes and streams reached maximum values between 60°N and 75°N but was highly variable, probably reflecting differences in local and regional environmental factors and possibly sampling effort.We found clear taxon‐specific differences between contemporary and pre‐industrial samples that were often specific to both ecozone and lake depth. Regional patterns of species turnover (β‐diversity) in the past c.200 years revealed that regions of the Canadian High Arctic and the Hudson Bay Lowlands to the south showed most compositional change, whereas the easternmost regions of the Canadian Arctic changed least. As shown in previous Arctic diatom studies, global warming has already affected these remote high latitude ecosystems.Our results provide reference conditions for future environmental monitoring programmes in the Arctic. Furthermore, diatom taxa identification and harmonisation require improvement, starting with circum‐Arctic intercalibrations. Despite the challenges posed by the remoteness of the Arctic, our study shows the need for routine monitoring programmes that have a wide geographical coverage for both streams and lakes. [ABSTRACT FROM AUTHOR]

Climate change poses a significant threat to Arctic freshwater biodiversity, but impacts depend upon the strength of organism response to climate‐related drivers. Currently, there is insufficient knowledge about Arctic freshwater biodiversity patterns to guide assessment, prediction, and management of biodiversity change.As part of the Circumpolar Biodiversity Monitoring Program's first freshwater assessment, we evaluated diversity of diatoms, benthic macroinvertebrates, and fish in North American Arctic rivers. Alpha diversity was assessed in relation to temperature, water chemistry, bedrock geology, and glaciation history to identify important environmental correlates. Biotic composition was compared among groups to evaluate response to environmental gradients.Macroinvertebrate α‐diversity declined strongly with increasing latitude from 48°N to 82°N, whereas diatom and fish diversity peaked around 70°N without a clear latitudinal decline. Macroinvertebrate diversity was significantly positively related to air temperature. Diatom diversity was related to bedrock geology and temperature, whereas fish diversity was related to glaciation history.Fish and macroinvertebrate assemblages differed between sites in western Canada, where invertebrate composition was more variable, and Alaska, where fish composition was more variable. In sites with both diatom and macroinvertebrate data, diatom composition was distinct in Alaska, where richness was highest in former glacial refugia. Macroinvertebrate composition was distinct in lowest‐latitude eastern and high‐latitude western Canadian sites where temperature was highest.Temperature, precipitation, geology, calcium, and substrate size were important environmental correlates for diatoms and macroinvertebrates, although the relative importance of each correlate differed. Diatom taxa were most strongly associated with water chemistry, whereas benthic invertebrate composition related most strongly to precipitation and temperature.This large‐scale study provides the most substantial integration and analysis of river diatom, macroinvertebrate, and fish data from the North American Arctic to date. Findings suggest that macroinvertebrates will show the strongest response to climate‐related shifts in temperature, whereas diatoms and fish are more likely to respond to climate‐induced shifts in nutrients and hydraulic connectivity. However, significant gaps in data coverage limited our ability to reliably evaluate spatial patterns and detect change. These gaps could be reduced by improving collaborative efforts between the U.S.A. and Canada to harmonise future monitoring. [ABSTRACT FROM AUTHOR]

1. Employing field-deployed mesocosms, we examined the effects of 12-h pulse and 20-day press (continuous) exposures of the common agricultural insecticide, imidacloprid, on nymph abundance, emergence patterns and adult body size of Epeorus spp. (Heptageniidae) and Baetis spp. (Baetidae). 2. In press exposures, reduced nymph density was driven by reduced survivorship; in pulse exposures, reduced nymph density may reflect increased emergence because of stress. 3. Once exposed to imidacloprid, Epeorus and Baetis mayflies developed less and emerged smaller than their control counterparts. Concentrations as low as 0.1 μg L−1 (12-h pulse) reduced head length in Baetis and thorax length in Epeorus. 4. In all of the Baetis and Epeorus examined, effects were only found in males. Male survivorship and body size can affect population dynamics. Sublethal doses of this widely applied agricultural insecticide have the potential to reduce reproductive success of mayfly populations. [ABSTRACT FROM AUTHOR]

1. Laboratory experiments were completed to identify the mechanisms by which the predatory flatworm, Dugesia tigrina, imposes mortality on its Aedes aegypti and Daphnia magna prey. Feeding trials were completed in glass microcosms which contained one of three - nine densities of small or large individuals of each prey species. 2. Mortality by Dugesia on small and large Aedes followed a type II functional response, whereas the mortality of Daphnia resembled a type III functional response. Prey mortality imposed by Dugesia consisted of consumptive and non-consumptive elements. Non-consumptive mortality occurred when prey individuals trapped in mucus trails subsequently died but were not ingested. 3. Additional experiments were conducted to quantify consumptive (capture followed by ingestion) and non-consumptive mortality (death not followed by ingestion). 4. Consumptive mortality followed a type II functional response for small and large individuals of both prey species, whereas non-consumptive mortality increased with prey density, although the relationships differed with prey species and size. The nonconsumptive mortality of large Daphnia increased at an accelerating rate with prey density and exceeded consumptive mortality at intermediate prey abundances. In contrast, non-consumptive mortality of small Aedes and small Daphnia was lower than consumptive mortality and either increased with prey density at a decelerating (small Aedes) or accelerating (small Daphnia) rate. 5. These results suggest that the importance of consumptive and non-consumptive mortality to total prey mortality needs to be considered when modelling predator-prey dynamics. [ABSTRACT FROM AUTHOR]