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Anthropogenic reductions in riverine connectivity can severely impede the migrations of anadromous species. In fragmented rivers, successful migrations depend on the ability of migrants to negotiate barriers or locate alternative passage routes. However, individual variation in the specific aspects of movement that determine migration success in fragmented rivers, is poorly characterised.Here, individual variation was investigated in the spawning migrations of 56 adult sea lamprey Petromyzon marinus, an anadromous, semelparous species that does not show fidelity to natal rivers. The variability and consequences of two key aspects of fish migration within fragmented rivers were tested: passage time (total time taken to pass a barrier) and retreats (exploratory downstream movements after unsuccessful passage attempts). These were tested using acoustic telemetry in the highly fragmented River Severn catchment, western England, U.K.Distinct unimodal, bimodal, and multimodal patterns of variation in passage times were displayed across the different barriers, potentially related to the physical characteristics of the barrier and prevailing river discharge conditions when the barriers were first approached, but were not related to lamprey body sizes. At the first three barriers encountered by upstream‐migrating sea lamprey in the study, between 30% and 46% of individuals made retreat movements, and between 5% and 100% of retreating individuals were able to locate alternative tributaries. Retreating individuals were highly variable in their distance and frequency of retreats; overall, retreat movements comprised 11% (lower‐upper quartiles 0%–52%, range 0%–76%) of the total distance moved before reaching spawning areas. Time‐to‐event analysis indicated that retreat rates reduced as river discharge increased.There was no evidence indicating that individual variation in passage time, or presence of retreat movements at barriers, influenced the subsequent upstream migration speed or final upstream extent of lampreys. While predictability in rank arrival timing was high within three unobstructed reaches, this predictability was disrupted at barriers due to individual variation in passage times.Anthropogenic barriers thus can both disrupt and reveal individual variation in the migration dynamics of anadromous species. Substantial variability in retreat behaviours can be displayed by anadromous species facing delays at barriers, with these behaviours also associated with environmental conditions and the availability of alternative migration routes. Individual variation in exploration and passage time of migrants strongly influence their eventual spawning distribution. [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]

Conservation management of freshwater ecosystems often focuses on mitigating or reversing the negative effects of altered patterns of river discharge. Assessments of management interventions frequently focus on direct, short‐term responses to discharge without consideration of underlying population trends that span multiple years.We sought to determine the effects on fish populations of annual variation in river discharge and water temperature after accounting for underlying population trends. We used data on five native and one non‐native fish species, collected over 7–20 years in seven rivers in the Murray‐Darling Basin, south‐eastern Australia.Population trends explained 3.4%–24.6% of the total variation in abundance and biomass of our six study species, while discharge and water temperature explained a further 1.2%–11.4% of this variation. However, population trends confounded the effects of discharge and water temperature, which suggests that the effects of annual discharge conditions may be contingent on past conditions and factors intrinsic to populations (e.g. age structure).Failing to account for population trends led to a combination of plausible and implausible associations with discharge and water temperature. Plausible associations included positive associations with the magnitude of spring discharge and negative associations with the number of days where discharge was below the long‐term 10th percentile. Determining whether estimated associations are real or artefactual requires a greater focus on the processes that underpin multiyear population trends.Our results highlight the potential for underlying trends in populations to confound the short‐term effects of discharge. Potential confounding of short‐ and long‐term changes in populations underscores the need to assess responses to river discharge in the context of overarching environmental conditions, including factors other than discharge. [ABSTRACT FROM AUTHOR]

Stream biofilms can improve water quality (e.g. excessive nitrogen loads) because their microorganisms such as microphytobenthos and bacteria, are involved in self‐depuration processes in the biofilm. In marine sediments, meiofauna can influence primary productivity of phototrophic biofilms, stimulate bacterial growth, and exert a significant role on nitrogen cycling. We hypothesised that in rivers biofilms, increased meiofauna abundance in combination with nutrient enrichment can lead to improve biofilm nutrient uptake ability.River biofilms (from the Garonne River, France) were incubated in flumes supplied with natural river water. The N - NO3‐ uptake rates of the biofilms as well as the response of microorganisms were studied under different conditions of meiofaunal density and N - NO3‐ concentration over a 10‐day experiment.Rotifers dominated the meiofauna community in all biofilms. Under high nitrate conditions, the N - NO3‐ biofilm uptake rate was significantly higher in meiofauna‐enriched than non‐enriched biofilms over the first 2 days. This suggests that under high nitrate concentration conditions, high densities of rotifers enhanced the nitrogen uptake capacity of phototrophic biofilm despite a lower biomass. Densities of rotifers and bacteria in biofilms were positively correlated under N - NO3‐ enriched conditions suggesting that these two communities interacted over the experimental period. Signature pigment concentrations show that, under low concentrations of N - NO3‐, biomasses of diatoms and green microalgae were higher in meiofaunal‐enriched than non‐enriched biofilms, indicating that rotifer activity favoured biomass accumulation of these algae.This study demonstrates that meiofauna inhabiting biofilms may contribute to the limitation of nitrogen loads in stream water and supports the thesis that the potential interactions between faunal groups and microbial communities of biofilms merits further investigation to improve our understanding of processes which regulate interactions between biofilms and the overlying water in rivers. [ABSTRACT FROM AUTHOR]

Climate change and progressive glacier loss are leading to rapid ecological shifts in alpine aquatic systems. Rock glaciers and paraglacial features such as proglacial lakes, moraines, and taluses can alter the gradients of glacial influence along alpine river networks, but the ecological responses are still understudied.We investigated benthic invertebrate communities of different stream types (upper kryal, lower kryal, glacio‐rhithral, krenal, and rock glacial) within a deglaciating area of the Italian Alps (Solda and Zay sub‐catchments). Alpha‐ (Shannon index and taxa richness) and β‐diversity (i.e. nestedness and turnover), production (abundance, biomass), average body mass of invertebrate taxa, and functional feeding groups were used to analyse the influence of habitat conditions on invertebrate communities in different microhabitats (mineral substrates, bryophytes, Hydrurus foetidus). We defined an index of habitat mildness based on water temperature, channel stability, turbidity, and organic detritus, to measure the difference in community metrics over a gradient of habitat amelioration, driven by the mixing of distinct stream types and their interactions with paraglacial features.While diversity proxies showed an asymptotic behaviour along the gradient, the quantitative metrics peaked at intermediate mildness. Peaks of invertebrate biomass/abundance were recorded at the glacio‐rhithral station below the talus. Highest average body mass occurred at Zay, specifically at the rock glacial stream, and at the lower kryal stations influenced by the lake/moraine and the rock glacier. The moraine outflow showed unexpected high biomass/abundance values of shredders and grazers, that were supported by a high epilithic production. Bryophytes acted as local scale multipliers of mildness as they stored more detritus, and hosted higher biomass/abundance of invertebrates and, in particular, of gatherer–collectors.Beta‐diversity, driven by the turnover component, was positively related to different habitat mildness of the sampling stations. Although influenced by distinct lentic/lotic conditions, rock glacial streams hosted several taxa typical of glacio‐rhithral and krenal stations, alongside large numbers of Diamesa (Chironomidae), a genus typical of kryal habitats.Under progressive deglaciation, rock glaciers and paraglacial features influence the invertebrate communities of alpine river networks. As they host a high number of taxa dwelling in non‐glacial locations, rock glacial streams may act as stepping stones facilitating invertebrate colonisation following glacier retreat. After glacier loss, rock glacial streams may represent climate refugia for cold adapted taxa and/or kryal specialists, because the slow thawing of their ice might sustain cold water conditions for a longer period of time. [ABSTRACT FROM AUTHOR]

The impacts of non‐native species are hypothesised to be proportional to the functional distinctiveness of invaders in their invaded ecosystems. Throughout the Patagonia region of southern South America, Chinook salmon (Oncorhynchus tshawytscha) have recently established non‐native populations, and their anadromous, semelparous life cycle could be functionally unique such that marine‐derived nutrients are delivered to streams which have historically lacked such a resource linkage with the ocean.We tested the hypothesis that salmon subsidise biofilm‐associated algae in streams throughout the Aysén province of southern Chile. Using spatial and temporal variation in the presence of salmon among multiple streams and across two spawning seasons, we found strong evidence of salmon‐subsidised algae in three out of four streams examined that have spawning salmon populations.The biofilm of subsidised streams had enriched stable isotopic ratios of nitrogen and carbon, indicating that marine‐derived nutrients were incorporated by biofilms. This nutrient uptake translated into increased algal biomass and percent of total biofilm biomass composed of algae, indicating that the incorporation of marine‐derived nutrients stimulated autotrophic production of biomass.In one stream, the incorporation of marine‐derived nutrients by biofilm occurred in only one of the two studied spawning seasons. Incorporation occurred in a year with low flows of water throughout salmon spawning (4.59 m3/s) and did not occur in a year with much higher flows (41.6 m3/s), suggesting that inter‐annual variation in discharge can mediate the subsidising effect of salmon.These results indicate that Chinook salmon have bridged the historical gap between productive marine ecosystems and nutrient‐poor stream ecosystems in Patagonia. Anadromous salmon can be a significant source of nutrients in nutrient‐limited catchments, and their ongoing expansion in southern South America is likely to entail ecological impacts in stream and riparian food webs. [ABSTRACT FROM AUTHOR]

Recent evidence shows that in lotic, physically‐dominated ecosystems like large rivers, zooplankton can develop spatially structured assemblages and fulfill functionally important roles. While refuting the long‐standing notion that riverine zooplankton communities are numerically depauperate and spatially homogeneous, this also shows that the importance of abiotic and biotic drivers and the consistency of ecological patterns are still poorly understood for these communities.We collected zooplankton along 300 km of the Illinois River for 5 years to test the influence of a suite of biotic and abiotic variables on zooplankton density, biomass, and diversity. We hypothesized abiotic variables such as temperature, turbidity, and velocity would be predominant predictors, with biotic variables like planktivory becoming important when physical factors do not constrain them.Results showed basin‐wide declines in zooplankton taxonomic richness as planktivory from the invading silver carp (Hypophthalmichthysmolitrix) increased. In contrast, density and biomass were explained by abiotic factors at the basin scale, with velocity, turbidity, and pH driving biomass and velocity driving density. We also found that both the driving factors and the plankton responses varied among the upper, middle, and lower sections of the Illinois River.We conclude that while zooplankton communities of the Illinois River are highly structured in space, the driving forces behind their distribution patterns are not simple. Instead, there is a complex spatio‐temporal template of biological, chemical, and hydraulic factors shaping these communities in the Illinois River. Along with emphasizing the importance of spatial heterogeneity in large river ecosystems, these results also support a growing view that invasive species interactions can supersede the well‐established role of abiotic factors in explaining global freshwater diversity losses. This also highlights the importance of including long‐term zooplankton sampling in ecological monitoring programs in diverse and productive large river systems like the Illinois. [ABSTRACT FROM AUTHOR]

Floods can directly affect riverine organisms by displacing them, and population‐level responses to floods can vary depending on flood magnitude and organism mobility. Benthic organisms can resist displacement until substrates become unstable, whereas mobile organisms are generally more resilient. Freshwater mussels are benthic organisms with low mobility, and there is limited research on their population‐level responses to floods. This study provides novel insights to population‐level responses of mussels to large floods (>500 m3/s).Population dynamics (i.e. abundance, survival, and site fidelity) and sampling efficiency (i.e. detection probability) were estimated in a robust design framework for four freshwater mussel species (Cyclonaias petrina, Cyclonaias pustulosa, Amblema plicata, and Tritogonia verrucosa) from 2017 to 2019 at two sites (upper and lower sites) within riffle habitats in the Colorado River, Texas, U.S.A. Individuals of each species were affixed with shellfish tags, with C. petrina and C. pustulosa individuals also being affixed with passive integrated transponder tags. Changes in population dynamics related to the flood event at each site were directly tested. During sampling, a major flood occurred at each of the two study sites; the floods differed in magnitude but were in the 99th percentile of historical flows at their respective gages.There were site‐ and species‐specific differences in estimated abundances, survival, and site fidelity during periods with the floods. Estimated abundances of C. petrina, C. pustulosa, and T. verrucosa were reduced 40–78% by the lesser flood magnitude (1,283 m3/s) at the upper site. Estimated abundances of C. petrina, C. pustulosa, and A. plicata were reduced 93–95% by the greater flood magnitude (4,332 m3/s) at the lower site. There was a reduction in survival of C. petrina at the upper site, while initially high survival at the lower site was reduced during the interval with the flood for all species. Finally, there was a reduction in site fidelity of C. pustulosa at the lower site.Floods reduced the abundance of all species within riffle habitats at the two sites. Large floods, therefore, affect population dynamics of mussels, but the fate of the displaced mussels is unknown, and with limited inference, reach‐scale effects are unknown. This study adds to the growing body of knowledge about responses of aquatic organisms to large floods, although quantification of recolonisation and fate of displaced mussels are needed to fully understand long‐term effects of large floods on mussel communities. [ABSTRACT FROM AUTHOR]

Glacial retreat, accompanied by shifts in riparian vegetation and glacier meltwater inputs, alters the energy supply and trophic structure of alpine stream food webs. Our goal in this study was to enhance understanding of dietary niches of macroinvertebrates inhabiting different alpine streams with contrasting glacial and non‐glacial (groundwater, precipitation, snowmelt) water inputs in conjunction with seasonal and habitat‐specific variation in basal resource availability.We measured a range of stream physico‐chemical attributes as well as carbon and nitrogen isotopes (δ13C, δ15N) of macroinvertebrates and primary food sources at seven sites across seasons within a Swiss glaciated catchment (Val Roseg) undergoing rapid glacial retreat (1–2 km between 1997 and 2014). Sampling sites corresponded to streams used in a previous (1997/1998) study within the same alpine catchment.Physico‐chemical attributes showed wide variation in environmental conditions across streams and seasons. Significant correlation among physico‐chemical proxies of glacier meltwater (phosphate‐P, total inorganic carbon, conductivity, turbidity) and macroinvertebrate δ13C, δ15N, and size‐corrected standard ellipse area (a proxy for feeding niche width) values showed that the extent of glacial water input shapes the energy base among alpine streams. Feeding niche differences among common alpine stream insect taxa (Chironomidae, Baetidae, Heptageniidae) were not significant, indicating that these organisms probably are plastic in feeding behaviour, opportunistically relying on food resources available in a particular stream and season.Seasonal trends in macroinvertebrate δ13C largely followed patterns in periphyton δ13C values, indicating that autochthonous resources were the main consumer energy source within the stream network, as shown previously. The overall range in macroinvertebrate δ13C (−33.5 to −18.4‰) and δ15N (−6.9 to 6.7‰) values also corresponded to values measured in the previous study, suggesting that macroinvertebrates altered diets in line with changes in environmental conditions and food resources during a period of rapid glacial retreat. Our results suggest that environmental changes brought on by rapid glacial retreat have not yet caused a profound change in the trophic structure within these fluvial networks. [ABSTRACT FROM AUTHOR]

The importance of flow‐related factors to benthic organisms, as well as the role of habitat conditions in shaping aquatic communities during low‐flow periods, have been recognised. Despite this, the preferences of macroinvertebrates to the ratio of lentic to lotic habitats at the reach scale have not been accurately quantified in most instances.Aquatic invertebrates and habitat features in a range of temporary rivers in Sardinia were investigated. The investigation focused on the flow‐related characteristics that contribute to defining the lentic–lotic condition of the river reaches. The relation of habitat features to benthic taxa distributions was assessed using multidimensional scaling. The main aim of the paper was to quantify the responses of taxa to the different lentic and lotic habitat conditions by applying hierarchical logistic regressions. Finally, taxon optima were aligned along the lentic–lotic gradient and the responses of different taxonomic groups compared.Unbroken waves and imperceptible flow were correlated with benthic taxa variability, suggesting local hydraulics and turbulence have a major role in regulating community composition. The overall lentic–lotic character of the river reaches was also clearly related to the benthic taxa distribution. More than 80% of taxa were significantly related to the lentic–lotic gradient, and an asymmetrical response curve was the predominant model.Benthic groups showed taxon optima clustered in different ranges of the lentic–lotic gradient. Odonata, Coleoptera, Hemiptera, and Mollusca preferred clearly lentic conditions. Diptera mainly ranged on the lotic side of the gradient, while Trichoptera were relatively uniformly distributed across the gradient. Ephemeroptera taxa clustered in intermediate lentic–lotic conditions, with two species preferring extremely lentic habitats. In general, optima converged at intermediate and extremely lentic conditions, presumably due, respectively, to the coexistence of different lentic and lotic features and to the highly diverse environmental characteristics under extremely lentic situations.These results support the conclusion that dissimilar ecological factors act on benthic taxa along the lentic–lotic range and species favouring different lentic–lotic conditions are subjected to pressures of different nature. This should not be ignored when defining species preferences and studying community structure or relationships between species in Mediterranean rivers, which cyclically vary their habitat composition. In addition, the uneven distribution of optima of different groups along the lentic–lotic gradient might affect macroinvertebrate metrics when assessing ecological status or establishing reference conditions under variable climatic conditions. [ABSTRACT FROM AUTHOR]

Changes in climate are causing floods to occur more often and more intensely in many parts of the world, including agricultural landscapes of southern Wisconsin (U.S.A.). How flooding and greater flood frequency affect stream carbon dioxide (CO2) and methane (CH4) fluxes and concentrations is not obvious. Thus, we asked how diffusive fluxes of CO2 and CH4 varied over time, particularly in response to floods, in agricultural streams, and what were likely causes for observed flood responses.We measured concentrations and diffusive fluxes of CO2 and CH4 at 10 stream sites in mixed agricultural and suburban catchments in southern Wisconsin (U.S.A.) during the growing season (March–November) in a year that experienced multiple floods. Habitat, hydrologic, and water chemistry attributes were also quantified to determine likely drivers of changes in gas concentrations and fluxes.Habitat and water chemistry, as well as CO2 and CH4 concentrations and fluxes were temporally erratic and lacked any seasonality. Carbon dioxide and CH4 concentrations and fluxes were higher during floods along with increased water velocity, turbidity, and dissolved organic carbon and decreases in dissolved oxygen, soft sediment depth, and macrophyte cover.Increased gas concentrations and fluxes were probably due to flushing of gases from soils, respiration of organic matter in the channel, and increased gas exchange velocities during floods.Flooding alleviated both supply and transfer limits on CO2 and CH4 emissions in these agricultural streams, and frequent and prolonged flooding during the growing season led to sustained high emissions from these streams. We hypothesise that such persistent increases in emissions during floods may be a common response to high precipitation periods for many agricultural streams. [ABSTRACT FROM AUTHOR]

Dispersal, defined as the movement of individuals among local communities in a landscape, is a central regional determinant of metacommunity dynamics in ecosystems. Whereas both natural and anthropogenic ecosystem fragmentations can limit dispersal, previous attempts to measure such limitations have faced considerable context dependency, due to a combination of spatial extent and associated environmental variability, the wide range of dispersal modes, and abilities of organisms or variation in network topologies. Therefore, the role dispersal plays compared to local environmental filtering in explaining metacommunity dynamics remains unclear in fragmented dendritic ecosystems.We quantified α‐ and β‐diversity components of invertebrate metacommunities across 10 fragmented headwater stream networks and tested the hypothesis that dispersal is the primary determinant of biodiversity organisation in these dynamic and spatially constrained ecosystems.Alpha‐diversity was much lower in intermittent than perennial reaches, even long after rewetting, indicating an overwhelming effect of drying including a legacy effect on local communities.Beta‐diversity was never correlated with environmental distances but predominantly explained by spatial distances accounting for river network fragmentation. The nestedness proportion of β‐diversity was considerable and reflected compositional differences where communities from intermittent reaches were subsets of perennial reaches.Altogether, these results indicate dispersal as the primary process shaping metacommunity dynamics in these 10 headwater stream networks, where local communities recurrently undergo extinction and recolonisation events. This challenges previous conceptual views that local environment filtering is the main driver of headwater stream metacommunities.As river networks become increasingly fragmented due to global change, our results suggest that some freshwater ecosystems currently driven by local environment filtering could gradually become dispersal‐limited. In this perspective, shifts from perennial to intermittent flow regimes represent ecological thresholds that should not be crossed to avoid jeopardising river biodiversity, functional integrity, and the ecosystem services they provide to society. [ABSTRACT FROM AUTHOR]

The dispersal of aquatic plant propagules is highly facilitated in streams due to flow. As many aquatic plants predominantly spread through vegetative propagules, the specific retention and thus drift distance of dispersed plant fragments largely contribute to the rapid spread along the course of a stream.We determined fragment retention for four aquatic plant species (Elodea canadensis, Myriophyllum spicatum, Ceratophyllum demersum, Salvinia natans; representing four different common morpho‐structural groups) in sections of small to medium‐sized German streams with different levels of stream sinuosity.The number of fragments showed a logistic decline over drift distance. In two small streams, 90% of drifting fragments were retained at distances (D90) of only 5–9 m and 19–70 m, while higher D90 values of 116–903 m and 153–2,367 m were determined for sections of a medium‐sized stream. The likelihood of retention thereby decreased significantly with increasing stream size and was reduced in straightened stream sections.Differences in retention were more strongly related to fragment buoyancy rather than fragment size and morphology. Increasing buoyancy significantly lowered the likelihood of fragment retention over drift distance by a factor of 3–8, whereas contrasting effects were documented for size and morphology of fragments.The relevance of different obstacles was highly stream section‐specific and depended on obstacle abundance, distribution, and the degree of submergence/emergence.Our findings elucidate the dynamic retention patterns of plant fragments and highlight the strong interplay between extrinsic (stream) and intrinsic (fragment) properties. We conclude that straightened lowland streams of intermediate size promote the rapid dispersal of invasive aquatic plants and are particularly prone to invaders producing large amounts of small and highly buoyant plant fragments. Information on the species‐specific fragment colonisation dynamics in the field is further required to improve our understanding of the vegetative dispersal capacity of invasive aquatic plants in stream ecosystems. [ABSTRACT FROM AUTHOR]

Intermittent streams (IS) comprise a large proportion of the drainage network in many parts of the world. The non‐flow period of IS are known to impact stream biota because aquatic habitats dry out. However, less well understood are the relative effects of the temporal component of these drying events including their duration and frequency.Here, we characterised effects of temporal component of drying events on abundant and species‐rich meiofauna. The effects were assessed in 22 streams in the north‐eastern Iberian Peninsula. The duration and frequency of non‐flow events was characterized over a period of 250 days prior to sampling the sediment‐dwelling meiofauna in riffle zones that completely dried out.Overall, meiofauna abundances were amongst the highest ever reported for streambeds. Most meiofaunal taxa correlated positively with the frequency of drying events and correlated positively with the length of dry periods recorded shortly before sampling, suggesting that the community was able to recover quickly. Tardigrades were the only group to correlate positively with the longest dry periods, suggesting that they had the best resilience capabilities in streams that had experienced the longest droughts.On average, nematodes made up half of the meiofauna. We identified a total of 113 different nematode species. The nematode community was more taxonomically diverse in IS, with a smaller proportion of bacterivores and a higher proportion of fungivore species such as Filenchus vulgaris. Thereby resembling the trophic structure commonly observed in soil ecosystems.Our results show that most meiofauna were positively influenced by drying disturbance, that is being able to quickly recover after them. This suggests outstanding resilience capabilities, and points out meiofaunal organisms as key players for kick‐starting stream food webs and functions once flow returns. [ABSTRACT FROM AUTHOR]

The coastal rivers of the São Paulo State in SE Brazil have different lengths and seawater influence. We evaluated whether: (1) environmental heterogeneity (EH) is associated with the species and life‐form richness of aquatic macrophytes; and (2) EH and geographical distance influence species composition in these coastal rivers.We recorded the macrophyte species and life form occurrence and collected explanatory variables characterising the water, sediment, and river channel at 100 sampling sites over 8 rivers. We applied a principal component analysis to the explanatory variables and calculated the rivers' EH using the range of principal component 1 scores. We also determined the position of each river mouth along the coastline to measure the distance between the rivers. We used quasi‐Poisson generalised linear models to evaluate the effects of EH on richness of species and life forms. To determine the effect of EH and geographical distance (Euclidean distance matrices) on the variation in species composition (Jaccard dissimilarity matrix) among the rivers, we applied multiple regressions on distance matrices.The most heterogeneous river had heterogeneity score about five times greater than the least heterogeneous river. Sediment salinity, river width, total phosphorus concentration of water and distance from river mouth were the most important variables contributing to the rivers' EH. We found that EH did not explain variation in species richness; however, it had a significant positive relationship with life‐form richness. The effect of EH was greater than that of the geographical distance on the variation in species composition among the rivers. The pairs of rivers with the most similar EH were the most similar in species composition, but not all of them were geographically close.We conclude that EH influences life‐form richness but does not influence species richness of aquatic macrophytes in the coastal rivers we studied; however, EH does influence species composition regardless of geographical distance among rivers. [ABSTRACT FROM AUTHOR]

This review summarises knowledge on the ecology, toxin production, and impacts of toxic freshwater benthic cyanobacterial proliferations. It documents monitoring, management, and sampling strategies, and explores mitigation options.Toxic proliferations of freshwater benthic cyanobacteria (taxa that grow attached to substrates) occur in streams, rivers, lakes, and thermal and meltwater ponds, and have been reported in 19 countries. Anatoxin‐ and microcystin‐containing mats are most commonly reported (eight and 10 countries, respectively).Studies exploring factors that promote toxic benthic cyanobacterial proliferations are limited to a few species and habitats. There is a hierarchy of importance in environmental and biological factors that regulate proliferations with variables such as flow (rivers), fine sediment deposition, nutrients, associated microbes, and grazing identified as key drivers. Regulating factors differ among colonisation, expansion, and dispersal phases.New ‐omics‐based approaches are providing novel insights into the physiological attributes of benthic cyanobacteria and the role of associated microorganisms in facilitating their proliferation.Proliferations are commonly comprised of both toxic and non‐toxic strains, and the relative proportion of these is the key factor contributing to the overall toxin content of each mat.While these events are becoming more commonly reported globally, we currently lack standardised approaches to detect, monitor, and manage this emerging health issue. To solve these critical gaps, global collaborations are needed to facilitate the rapid transfer of knowledge and promote the development of standardised techniques that can be applied to diverse habitats and species, and ultimately lead to improved management. [ABSTRACT FROM AUTHOR]

Freshwater crabs are the largest macroconsumers in many neotropical headwater streams, but few studies have examined their roles in ecosystem processes such as leaf litter breakdown. As omnivorous macroconsumers, freshwater crabs affect multiple trophic levels. They may directly increase leaf breakdown through fragmentation and consumption or indirectly decrease breakdown by consuming other macroinvertebrates, including shredders and detritivores.In a headwater stream in Monteverde, Costa Rica, we conducted an in‐stream experiment with 40 enclosures to quantify the effects of pseudothelphusid crabs on both leaf breakdown and macroinvertebrate colonisation of leaves. Half of the enclosures were randomly selected to contain two crabs (mean carapace width = 30 mm) and half were controls without crabs. We sampled mixed leaf packs from the enclosures on days 11, 19, 28, 34, and 42. We found the leaves of one species (Koanophyllon pittieri) almost completely decomposed by day 28 in both treatments (crab versus no crab). The other two leaf species (Meliosma idiopoda, Quercus brenesii) composed the remaining leaf mass at the end of the experiment.At 42 days, enclosures with crabs had faster rates of leaf breakdown than those without crabs (with crabs: k = −0.020; without crabs: k = −0.016; p = 0.034). This suggests that the magnitude of direct leaf breakdown by crabs, due to fragmentation, consumption, or manipulation of leaves, was greater than any indirect effects on leaf breakdown via crab consumption of other leaf‐consuming species.Macroinvertebrate composition based on taxa abundances or biomasses did not significantly differ between treatments (ANOSIM; p = 0.73 and p = 0.65, respectively). Shredder and detritivore abundances and biomasses increased significantly through time (ANOVA; p ≤ 0.001), but there was no evidence of an effect of crab presence (p > 0.2), nor were there significant interactions between crab presence and time (p > 0.3).This is one of the first studies to quantify the effects of pseudothelphusid freshwater crabs on leaf breakdown rates. Our results suggest that these crabs can play a significant role in detrital processing in neotropical headwater streams. This study has also demonstrated that short‐term enclosure experiments are useful in measuring in‐stream effects of crab activity on leaf breakdown. [ABSTRACT FROM AUTHOR]

Temperature is known to stimulate metabolism with cascading effects on multiple biological processes. These effects may, however, vary across processes, types of organisms or levels of biological organisation. They can also vary with nutrient availability, with potentially stronger temperature effects when nutrients are not limiting. This context dependence of temperature effects on processes challenges our ability to anticipate their consequences on ecosystems in a changing world.In headwater streams, the decomposition of allochthonous leaf litter, driven by both microbial decomposers and invertebrates, is known to respond to both temperature and nutrient availability. These food webs are highly tractable and a useful model system to investigate the variations of temperature effects on processes across types of organisms (microbes versus invertebrates), resource availability levels (nutrient concentration), and levels of biological organisation (from individual to ecosystem).In a microcosm experiment, we measured the effects of temperature and nitrogen availability (four levels each) on respiration rates of litter-consuming microbes and invertebrates and their decomposition activity in different contexts of food web complexity. The latter included one treatment without invertebrate detritivore (microbial decomposers only), three single invertebrate taxa (Gammarus, Potamophylax, and Sericostoma) treatments, and one mixed invertebrate taxa treatment (three‐species altogether).Microbial processes increased nearly exponentially with temperature (Arrhenius model, activation energy (± 95% confidence interval) = 0.56 ± 0.53 and 1.00 ± 0.23 eV for litter decomposition and respiration), while invertebrate‐driven processes increased (activation energy from 0.47–1.15 eV) up to a maximal value at an intermediate temperature (c. 11–15°C depending on species and process), above which process rates decreased. By contrast, litter consumption in mixed invertebrate species treatments was not significantly influenced by temperature, because of a negative effect of species mixing occurring above 12°C. Nitrogen had a weaker influence, only slightly stimulating litter consumption by mixed‐species invertebrates, which limited the scope for synergies with temperature effects.Our results raise issues about how aquatic litter consumers meet their energy requirements at high temperature and suggest that a general consequence of warming could be loss of carbon through mineralisation in headwater stream food webs. In several aspects, our results deviate from expectations based on universal relationships between temperature and individual metabolism (e.g. metabolic theory of ecology), suggesting that we may need to develop less simplistic assumptions to predict the consequence of warming on ecosystem processes. [ABSTRACT FROM AUTHOR]

The hyporheic zone is an important habitat for benthic invertebrates and early‐developmental stages of gravel spawning fish. However, the eutrophication of running waters and, in turn, the excessive periphyton biomass leads to its biological clogging. The result of these processes is oxygen depletion and a reduction in the habitat quality of the hyporheic zone.This study assessed whether top‐down effects of two important European river fish species, the large herbivorous cypriniform common nase (Chondrostoma nasus, L.) and the large omnivorous cypriniform European chub (Squalius cephalus, L.), can reduce eutrophication effects in the hyporheic zone. A 4‐week mesocosm‐based field experiment in a eutrophic river was conducted using cage enclosures stocked or not with either nase or chub.The top‐down control of periphyton was expected to reduce biological clogging and thereby increase oxygen availability in the hyporheic zone. Accordingly, we hypothesised that in enclosures stocked with either fish the concentrations of dissolved oxygen in the hyporheic zone would be higher and the periphyton biomass would be lower than in enclosures without fish stocking.Hyporheic oxygen concentrations were significantly higher in enclosures stocked with either nase or chub than in enclosures without fish stocking. However, periphyton ash‐free dry mass was significantly reduced only in enclosures stocked with nase, not in those stocked with chub. Thus, the positive effects of nase and chub on hyporheic oxygen availability were caused by different mechanisms.Our results demonstrate that nase and chub can reduce eutrophication effects in the hyporheic zone of running waters. Hence, protecting and enhancing stocks of herbivorous and omnivorous fish will contribute to restoring the hyporheic zone in efforts to preserve biodiversity in eutrophic rivers. [ABSTRACT FROM AUTHOR]

Environmental factors such as nutrient and light availability may play important roles in determining the magnitude and direction of microbial priming and detrital decomposition and, therefore, the relative importance of microbial priming in carbon (C) dynamics in freshwater ecosystems.We integrated light availability with an existing conceptual model predicting the magnitude of the priming effect (PE) along a dissolved nutrient gradient (i.e. nutrient PE model). Our modified light‐nutrient PE model hypothesises how light may mediate priming at any given nutrient concentration and provides a calculation method for quantitative PE values (i.e. light effect size at a given nutrient concentration).We used recirculating stream mesocosms with Quercus stellata (post oak) leaf litter as an organic matter (OM) substrate in a 150‐day experiment to test our model predictions. We manipulated light levels [ambient (full light), shaded (c. 19% of ambient)] and phosphorus (P) concentration (10, 100, 500 µg PO4‐P/L) in a fully factorial design. We also supplied all mesocosms with 500 µg/L dissolved inorganic nitrogen. Microbial biomass, water column dissolved organic C, and leaf litter dry mass and recalcitrant OM [i.e. the fibre (cellulose + lignin) component of post oak substrate] were measured. Recalcitrant OM (ROM) k‐rates (day−1) were used to calculate the light effect size within P treatments as a log response ratio (ln[ambient k‐rate/shade k‐rate]) to ascertain PE magnitude and direction (positive or negative).Light was an important driver of dissolved organic C, a potential source of additional labile organic matter essential for priming heterotrophic microbes. There were weak PEs in total leaf litter dry mass remaining, but PEs were more pronounced in leaf litter ROM remaining. The strongest positive PEs (specific to litter ROM pools) occur in the highest P treatment, presumably due to a change in which nutrient, nitrogen versus P, was a limiting factor for microbes based on nutrient ratios rather than P concentration alone. These results illustrate the importance of considering light levels, nutrient ratios (rather than individual nutrients), and detrital ROM components in further PE model development. [ABSTRACT FROM AUTHOR]

Temperate headwater streams traditionally have been considered heterotrophic and brown food web dominated with little primary production. Recent work, however, suggests algae on leaves in these streams may play a greater role than previously thought through interactions with microbial decomposers like fungi. Algae also may be important for macroinvertebrates colonizing leaves in streams. Algae are a more nutritious food resource for shredders than fungi and bacteria and provide a food resource for non‐shredder macroinvertebrates.In a field experiment, we manipulated light in three low‐nutrient and three high‐nutrient streams using leaf bags filled with red maple leaves in winter and spring. After four weeks we measured algal and fungal biomass, leaf stoichiometry, and macroinvertebrate abundance and biomass associated with the leaf bags. We also identified the macroinvertebrate community and examined differences in functional feeding guilds and taxa under ambient‐ and shaded‐light treatments and low‐ and high‐nutrient concentrations in relation to measured leaf characteristics.Algal biomass on leaves was greatest in high‐nutrient streams and ambient‐light treatments in both seasons. Fungal biomass on leaves was greatest in high‐nutrient streams and showed a moderate marginally significant positive correlation with algae during the winter. Leaf C:N was negatively correlated to algae in winter and fungi in both seasons, while leaf N:P and C:P were negatively correlated to fungi in winter and algae in spring. Interactions between fungi and algae on leaves and the nutritional importance of each for macroinvertebrates likely change across seasons, potentially impacting macroinvertebrate community composition.Macroinvertebrate diversity did not differ, but biomass was significantly greater in shaded‐light treatments during spring. Abundance was highest in the high‐nutrient ambient‐light conditions in both seasons, corresponding to greatest algal biomass. Functional feeding guild biomass and abundance were related to different leaf characteristics by season and guild. Higher algal biomass was an important factor for colonization of certain macroinvertebrates (e.g., Ephemerella (Ephemeroptera: Ephemerellidae) and Stenonema (Ephemeroptera: Heptageniidae)), while others were more abundant under shaded treatments with lower algal biomass (e.g., Tipula (Diptera: Tipulidae)), indicating taxa‐specific responses.Leaf‐associated algae may be an important factor mediating macroinvertebrate communities associated with leaves in temperate headwater streams. Our results demonstrate that green and brown food webs intersect within leaf packs, and they cannot be easily disentangled. We therefore should consider both autochthonous and allochthonous resources within headwater streams when examining their communities or developing water management strategies. [ABSTRACT FROM AUTHOR]

1. Forested headwater streams are generally considered to be light‐limited ecosystems where primary production is reduced, and the main source of energy and nutrients is composed of allochthonous detritus. We hypothesised that in these ecosystems, the development of primary producers might also be limited by (1) competition for nutrients with leaf‐litter decomposers (e.g. bacteria and fungi), and (2) leaf‐litter leachates or allelopathic compounds produced by aquatic fungi.2. To test these hypotheses, a 48‐day mesocosm experiment was performed in 12 artificial streams containing stream water inoculated with epilithic biofilm suspensions collected from a forested headwater stream. Three different treatments were applied: control without leaf litter (C), microbially conditioned leaf litter added at the beginning of the experiment and left to decompose throughout the experiment (L), or leaf litter renewed three times during the experiment (RL).3. We predicted that (1) the presence of litter, through microbial nutrient immobilisation and allelopathy, would reduce primary production and that (2) this effect would be amplified by litter renewal. We also predicted that nutrient competition would mean that (3) leaf‐litter decomposers will alter primary producer community composition and physiology. These predictions were tested by analysing biofilm development, physiology, stoichiometry, and benthic algal community structure. To distinguish between the effects of nutrient immobilisation and allelopathy, the biofilm responses to leaf‐litter leachates collected after different microbial conditioning durations were also measured in a parallel laboratory experiment.4. Contrary to our expectations, by day 28, primary producer growth was higher in the mesocosms containing leaf litter (L and RL) despite the rapid decrease in dissolved nutrients when leaf litter was present. After 48 days, the lowest phototrophic biofilm development was observed when leaf litter was renewed (RL), whereas phototrophic biofilm development was similar in the C and L treatments. Biofilm stoichiometry indicated that this effect was most probably related to greater nitrogen limitation in the RL treatment. The presence of leaf litter also affected primary producers' photophysiology, which could be attributed to changes in taxonomic composition and to physiological adjustments of primary producers.5. Laboratory measurements showed that despite a strong inhibition of primary producer growth by unconditioned leaf‐litter leachates, microbially conditioned leaf litter had either low or no effects on the development of primary producers.6. These results reveal that leaf‐litter decomposers can have both positive and negative effects on primary producers underlining the need to consider microbial interactions when investigating the functioning of forested headwater streams. [ABSTRACT FROM AUTHOR]

Land‐use change is a leading driver of biodiversity loss, especially in tropical fresh waters where the conversion of natural forest to monoculture plantations impacts freshwater fish assemblages. The environmental pathways underpinning shifts in fish assemblages, however, are poorly understood, but could potentially be inferred from trait–environment relationships.We addressed this knowledge gap using eco‐morphological traits to explain fish occurrences in oil palm‐impacted streams of the Endau drainage in Peninsular Malaysia. We also investigated how traits relate to differences in environmental conditions associated with land‐use change. We then integrate findings from the above to test how potential pathways of land‐use driven environmental changes can impact species occurrences through effects on life history, feeding habits, and mobility.Mixed‐effects models show that fishes with superior (upward‐facing) mouths and low body mass were more likely to occur in oil palm streams than forest streams, and these traits were associated with grass‐dominated riparian zones and reduced woody debris in oil palm streams, respectively. Structural equation models show that mouth positions statistically mediated the effect of riparian vegetation on fish species occurrences in oil palm streams. Specifically, fishes with superior mouths were more likely to feed on terrestrial invertebrates.Our analysis of easily measurable traits revealed pathways of land‐use impact that are potentially more widely applicable than conventional taxa‐based approaches. Fishes with superior mouths tended to occur in oil palm streams as they were able to more effectively exploit inputs of terrestrial invertebrates that are potentially associated with grass‐dominated riparian vegetation.Moreover, shifts in traits may suggest land‐use driven changes in stream ecosystem functioning (e.g. in terms of the role of terrestrial subsidies), thereby informing targeted management actions in land‐use impacted habitats (e.g. retention/restoration of riparian trees). [ABSTRACT FROM AUTHOR]

Macrophytes play important functional roles in river ecosystems, providing habitat and food, as well as influencing flow, water chemistry, and sediment dynamics. They also represent an important component of river biodiversity.Artificial river barriers have the potential to disrupt macrophyte dispersal, and compromise their distribution and persistence, but little information is available compared to barrier impacts on fish and macroinvertebrates. Here, we review the mechanisms supporting dispersal of river macrophytes in rivers and evaluate the nature of barrier impacts on macrophytes.Hydrochory (dispersal of propagules by water) is the principal mechanism of downstream dispersal, while zoochory (dispersal of propagules by animals) is likely to be the most important vector of upstream dispersal and inter‐catchment transport.Most studies have focused on the impact of large structures such as dams, and the findings indicate the impact is highly context dependent. Slow‐flowing habitats upstream of dams can act as traps to drifting propagules and thereby interrupt hydrochory. However, the consequences of interrupted hydrochory for downstream populations are unclear. River regulation can result in lower macrophyte diversity, although the lentic habitats associated with reservoirs can also favour an increase in the abundance and richness of macrophyte communities.Instream barriers are unlikely to affect zoochory by birds directly, but barriers are well known to restrict fish movements, so there is considerable potential for barriers to disrupt zoochory by fish, although no empirical study has specifically examined this possibility.There is a paucity of studies examining the impacts of low‐head barriers on macrophyte dispersal. Given the influence of macrophytes on river processes, we call for further research into barrier impacts on macrophyte population dynamics in order to gain a better understanding of the consequences of river fragmentation for fluvial communities and ecosystem functioning. [ABSTRACT FROM AUTHOR]

European rivers experience increased loading of total organic carbon (TOC) from terrestrial sources due to factors involving changes in land use, climate and soil acidity. However, little is known about how increased TOC is linked to changes in the bioreactivity of organic matter in these rivers on a continental scale.We compiled paired measurements of TOC and biological oxygen demand in 5‐day 20°C dark incubations from 3,486 EU monitoring rivers. Assuming first‐order decay and a fixed respiratory quotient, annual average TOC and biological oxygen demand values were used to calculate 11,060 values of the decay coefficient k.The k decreased by two orders of magnitude as a power function of increasing TOC. This relationship could partly be explained by carbon quality, as the C:N ratio of the organic matter was the lowest in high‐reactivity low‐TOC rivers, and vice versa. A trend analysis showed that TOC increased by 18% from 1996 to 2012, while k decreased by as much as 50%. As a consequence, the biological oxygen demand in the water decreased over time in spite of the water browning trend (increased TOC).Together, these results suggest that reactivity is low near terrestrial hot spots for TOC export, and low during years with high terrestrial TOC loading, but increases in rivers with low TOC concentrations where internal processes in the water have high relative influence on bulk TOC quality. Thus, browning of European freshwaters is linked to strong decreases in TOC reactivity on a continental scale, suggesting that the impacts of browning on microbial water deoxygenation and greenhouse gas production are less severe than previously thought. [ABSTRACT FROM AUTHOR]

Energy pathways in stream food webs are often driven by allochthonous basal resources. However, allochthonous dissolved organic carbon (DOC) is generally viewed as a minor if not insignificant basal resource because much of the DOC pool comprises high molecular weight, recalcitrant compounds and is inefficiently incorporated into biomass. Nevertheless, there is increasing evidence that the relatively small, labile fraction of DOC may indeed fuel microbial activity to a level that stimulates productivity across multiple trophic levels, resulting in a trophic upsurge. Here, we tested the trophic upsurge hypothesis by subsidising the labile DOC pool of an Alaskan boreal stream that had relatively high nutrient availability but low levels of naturally occurring DOC.We continuously added ecologically relevant (0.250 mg C/L, c. 10% increase above ambient bulk DOC) concentrations of labile DOC (acetate‐C) for 62 days to a treatment reach that was statistically indistinguishable in its channel form and chemistry from an upstream reference reach. We measured responses of periphyton production and biomass, whole reach metabolism and nutrient uptake, benthic invertebrate abundances, and juvenile salmonid (Dolly Varden, Salvelinus malma) abundance and growth.Measurements of basal ecosystem responses collectively indicated increased energy mobilization at the base of the food web in response to labile DOC addition. Periphyton bacterial production in the treatment reach was generally >1.5× reference reach values, and periphyton ash‐free dry mass, chlorophyll‐a, and chlorophyll‐a:ash‐free dry mass were all greater in the treatment reach by the end of the study. Throughout dosing, ecosystem respiration was 1.3× greater in the treatment reach and dissolved inorganic nitrogen uptake was greater in the treatment reach on eight out of nine measurements.Benthic invertebrate counts, dominated by Baetis spp. and Chironomidae, were c. 4× greater after 28 dosing days and c. 8× greater after 56 days in the upstream portion of the treatment reach. Abundance generally declined with increasing distance from the dosing station. Dolly Varden fry and parr age classes were nearly 2× more abundant in the upstream portion of the treatment reach than in any section of the reference reach and also declined with increasing distance from the dosing station. Further, Dolly Varden tagged with passive integrated transponders prior to the experiment had significantly higher instantaneous growth rates in the treatment reach than those recaptured in the reference reach.The strong consumer responses to small quantities of labile DOC mirrored significant treatment reach increases in basal ecosystem function and therefore demonstrated a response consistent with a trophic upsurge. Terrestrial DOC has historically been viewed as contributing little to metazoan consumers, instead modulating the influence of nutrients and being respired out of a disconnected microbial loop. Because we dosed the treatment reach with a relevant concentration of labile DOC, based on measurements in nearby peatland‐draining streams, we suggest that terrestrial DOC deserves more attention as a basal resource for whole food webs, akin to nutrients fuelling green (autochthonous) pathways. [ABSTRACT FROM AUTHOR]

It is often assumed that invertebrate consumers in small tropical streams are dependent on allochthonous sources, although recent studies indicate that algae can form the base of food webs in tropical streams. Fish in tropical streams can feed across several trophic levels and the origin and path of energy and nutrient flow is uncertain for many species.We collected fish, insects, periphyton, and leaf litter from 20 streams across four Atlantic Forest catchments. We analysed stomach contents of fish to define trophic guild and fish dietary trophic position. We also analysed stable isotopes of carbon and nitrogen of fish and their resources to identify the main basal resources of the food web and to estimate trophic positions and identify the path of energy flow.We found that autochthonous sources were the primary resource base for fish communities. Trophic positions estimated from diet and isotopes were similar and correlated for insectivore and algivore–insectivore fish, but not for algivore–detritivore or omnivore fish. Using path analysis, fish classified as algivore–detritivores appear to have derived their biomass through a diet of primary consumer insects and periphytic algae and thus, are more likely to play a trophic role as algivore–insectivores in these streams. However, omnivores probably derived much of their biomass from aquatic insects.Our findings support other studies of tropical systems in which the main basal resource is autochthonous, even in small streams. We also show that the assignment to a specific trophic guild for some fish species, based on gut contents, does not reflect what they assimilate into their bodies. In some species, food sources that are uncommon can make a disproportionately important contribution to their biomass.This study affirms the important role of inconspicuous algal resources in aquatic food webs, even in small forested streams, and demonstrates the effectiveness of taking a combined approach of diet analysis, isotopic tracing, and modelling to resolve food web pathways where the level of omnivory is high. [ABSTRACT FROM AUTHOR]

Studying the geographical distribution of species can reveal conditions and processes that may drive species presence and abundance. Organism distribution has frequently been explained by climate, but the relative role of local environmental predictors is not fully understood. Moreover, in the freshwater realm, intrinsic differences existing between different categories of water bodies can lead to significant differences in species–environment relationships. Here, we tested the relative importance of broad‐scale climate and local environmental predictors in explaining plant species distributions in freshwater lakes and streams.We built species distribution models to investigate which predictors best explain aquatic plant distribution in two categories of water bodies. We used species inventories and records of three climate and eight local environmental predictors for 150 lakes and 150 streams in Finland.We found that sets of predictors that explain the distribution of macrophyte species are unique depending on if species are in a lake or a stream. Overall, air temperature and ecosystem size were essential to predict aquatic plant species presence in both water body categories. Broad‐scale climate predictors were always very important in explaining species distribution, while local environmental conditions such as water chemistry were of variable influence, depending on species and water body category.These results are probably due to high spatial and temporal variability and range of water physico‐chemical parameters, especially in streams. Nonetheless, despite a lower relative importance than climatic factors, local environmental predictors also strongly affected species distributions.Our findings highlight that incorporating local environmental conditions to species distribution models in addition to climate predictors is necessary to improve predictions, particularly for distribution of stream flora. Considering the species‐specific responses of aquatic plants to their environment, studying species individually with species distribution models represents a useful analysis. [ABSTRACT FROM AUTHOR]

Dams and their associated impoundments may restrict dispersal and gene flow among populations of numerous freshwater species within stream networks, leading to genetic isolation. This can reduce effective population sizes and genetic diversity, increasing the risk of local extinction.We studied crayfishes from multiple up‐ and downstream sites in three impounded and two unimpounded streams in the Bear Creek and Cahaba River drainages, Alabama, U.S.A. Using mitochondrial DNA (cytochrome oxidase subunit I gene) sequence data generated from population‐level sampling of two abundant native crayfishes, Faxonius validus and Faxonius erichsonianus (Decapoda: Cambaridae), we assessed species' spatial genetic structure and genetic diversity, estimated the magnitude and directionality of gene flow, and compared results between the species.For both species, levels of genetic diversity (number of haplotypes, and haplotypic and nucleotide diversity) were the same or higher in impounded compared to unimpounded streams. Conversely, crayfish populations in up‐ and downstream sections of unimpounded streams displayed high genetic similarity and bidirectional gene flow, whereas in impounded streams, crayfish populations typically had greater up‐ and downstream genetic differentiation and predominantly unidirectional, downstream gene flow.Although impoundments were associated with lower connectivity between up‐ and downstream sections for F. validus and F. erichsonianus, the magnitude of genetic effects was species‐specific, with greater differentiation between F. validus populations up‐ and downstream of impoundments.In an ecologically short timeframe, impoundments appear to have fragmented stream crayfish populations, and even species with relatively high abundances and large ranges had lower gene flow among populations in impounded streams compared to unimpounded streams. In addition, feedbacks between genetic and demographic effects on fragmented populations may decrease the probability of long‐term persistence. [ABSTRACT FROM AUTHOR]

Streams draining forested landscapes are fuelled by terrestrial plant litter, which can be transported downstream or retained and broken down locally. However, fluxes of plant litter in streams can vary at multiple spatio‐temporal scales, affecting the availability of this key resource in heterotrophic stream food webs.To explore this question we quantified several processes related to litter dynamics (i.e. litter inputs, storage, losses by transport and losses by breakdown) by sampling litter at multiple sites in three streams of the Brazilian Cerrado biome (which has a tropical wet–dry climate) for 2 years. We assessed the relative contribution of different spatial (among and within streams) and temporal scales (annual, seasonal and monthly) to total variability of these processes (hereafter fluxes).Spatial and temporal variability of fluxes were both high, but spatial variation was 1.67‐fold greater than temporal variation (61 versus 37%, respectively), especially at the within‐stream scale (50% overall); an exception was litterfall, which varied less spatially than temporally (24 versus 76%). Temporal variation of litter storage (and hence availability to consumers) was mostly seasonal and due to differences in net transport.Inputs and transport were higher in the wet than the dry season (wet versus dry season, 1.45 versus 0.92 and 1.43 versus 0.06 g litter m−2 day−1), while breakdown was similar between both seasons (0.88 versus 0.94 g litter m−2 day−1). Storage (i.e. accumulation) rate was positive and negative in the dry and wet season, respectively, indicating that litter was stored in the dry season and exported in the wet season. The transitional dry–wet season showed the highest inputs, breakdown and storage (3.21, 1.63 g litter m−2 day−1 and 145 g litter m−2), while the wet–dry season showed lower inputs (as in the dry season), higher transport (as in the wet season) and lower breakdown and storage than the other seasons (0.93, 0.65, 0.31 g litter m−2 day−1 and 24 g litter m−2).Our results underscore the role of variation in biophysical drivers of litter fluxes within streams (e.g. pool–riffle configuration, substrate features, biological communities), and suggest that high within‐stream replication is necessary to study litter fluxes at larger scales and over time. The seasonal patterns suggested potential changes in litter dynamics under future climate scenarios in the tropics, including increased storage due to reduced transport in a drier climate. [ABSTRACT FROM AUTHOR]

Understanding changes in macroinvertebrate communities is important because they play a large role in stream ecosystem functioning, and they are an important food resource for fish. Beaver‐induced changes to stream morphology could alter macroinvertebrate communities, which in turn could affect food webs and ecosystem function. However, studies investigating the effects of North American beaver activities on macroinvertebrates are rare in the inter‐mountain west, an area with high potential for beaver‐assisted restoration.The aim of this study was to quantify differences in the macroinvertebrate community between unaltered segments of streams and within beaver ponds in north‐eastern Utah, U.S.A. We assessed macroinvertebrate species richness, biomass, density, functional feeding group composition, mobility group composition, and macroinvertebrate habitat characteristics to test the hypothesis that macroinvertebrate communities will differ among habitat types (undammed stream segments and beaver ponds) in beaver‐occupied streams.Beaver pond communities significantly differed from lotic reach communities in many ways. Beaver ponds were less diverse with 25% fewer species. Although there was variability among streams, in general, beaver ponds had 75% fewer individuals and 90% lower total macroinvertebrate biomass compared to lotic reaches.Regarding functional feeding groups, beaver ponds contained more engulfers, while lotic reaches contained more scrapers, filterers, and gatherers. For mobility groups, beaver ponds had more sprawlers, while lotic reaches had more clingers. Swimmers were also more prevalent in lotic reaches, although this is probably due to the abundance of Baetis within lotic reaches. More beaver pond taxa were classified as lentic‐dwelling insects, while more lotic reach taxa were categorised as preferring lotic habitats.The creation of ponds by beavers fundamentally altered the macroinvertebrate community in north‐eastern Utah streams. Such changes to stream macroinvertebrate communities suggest that recolonisation of beavers across North America may be altering stream functioning and food webs. Our study highlights the need to further investigate the effects of beaver recolonisation on stream communities. [ABSTRACT FROM AUTHOR]

In some regions, climate change is increasing the variability of rainfall and the frequency of extreme events such as drought. Consequently, non‐flow periods have grown in length and frequency, both in temporary and in formerly permanent streams. Water abstraction for human use may further prolong these dry periods.We analysed the resistance and resilience of biofilms from permanent and temporary streams to non‐flow conditions. This was achieved by exposing cobbles (collected from permanent and temporary streams) with intact biofilm to 31 days of non‐flow, followed by 20 days of stream flow in artificial stream channels. Biofilm resistance and resilience were assessed at a structural (algal biomass, pigment composition, and algae and cyanobacteria composition) and functional level (photosynthetic efficiency and community metabolism).Algal taxa in biofilms from permanent and temporary streams differed throughout the experiment. Biofilms from permanent streams were less resistant to non‐flow than those from temporary streams at structural level. Permanent stream biofilms also presented lower resilience at a structural level, but responded similarly to temporary stream biofilms at a functional level.Our investigation shows how the non‐flow period disturbed permanent stream biofilms, and suggests that temporary stream biofilms will have greater adaptive capacity as hydroperiod becomes shorter due to climate change. [ABSTRACT FROM AUTHOR]

Stream hydro‐morphology refers to the heterogeneous distribution of hydrologic conditions that occur above a complex benthic surface such as a streambed.We hypothesised that hydro‐morphological conditions will influence the retention, re‐distribution, and microbial‐driven decomposition of leaf litter inputs in stream ecosystems because each process varies with overlying water velocity.We tested this hypothesis using: (1) the spatial distribution of water velocity within a stream reach as a surrogate of stream hydro‐morphology; (2) leaf tracer (i.e. Ginkgo biloba L.) additions with serial recovery to examine the relationship between benthic retention and overlying velocity; and (3) measurements of leaf litter decomposition (i.e. Alnus glutinosa [L.] Gaertn.) under different water velocity conditions.Results demonstrate that water velocity exerts a significant influence on the retention and re‐distribution of leaf litter inputs within the reach. The observed range of water velocity (from c. 0 to 0.92 cm/s) also strongly influences the range of leaf litter decomposition rates (0.0076–0.0222/day).Our findings illustrate that water velocity influences leaf litter dynamics in streams by controlling leaf litter transport, retention and re‐distribution as well as how leaves decompose within recipient stream reaches. Ultimately, the results show that the efficiency of leaf litter inputs in supporting stream ecosystem function is dependent on the hydro‐morphological characteristics of the receiving stream ecosystems. [ABSTRACT FROM AUTHOR]

We used a 27‐year record of Dreissena populations in the freshwater tidal Hudson River to describe interannual variation in population density, body size, and body condition; estimate long‐term variation in recruitment, survivorship, and shell growth; and assess possible controls on the populations.Dreissena populations in the Hudson have been highly variable, with interannual ranges of c. 100‐fold in abundance and biomass, and 7‐fold in mean body mass. This large interannual variation arises from both long‐term trends and 2–5‐year cycles.Long‐term trends include the 2008 appearance of the quagga mussel (Dreissenarostriformis), which still forms a small part (<10%) of the dreissenid community, and a decline in zebra mussel body size. The decline in body size was caused by a long‐term decline in adult survivorship rather than a decline in rates of shell growth. We could detect no long‐term trends in adult abundance or spread of Dreissena onto soft sediments in the Hudson.We observed persistent, strong cycles in adult abundance and body size. These were driven by the appearance and decay of eight dominant year classes over the 27 years of our study, and were a result of temporal variation in recruitment rather than temporal variation in survivorship. The observed strongly irregular recruitment appears to arise from strong adult–larval interactions, and is consistent with previous simulation model results showing that interactions between adults and larvae can drive persistent cycling.We found evidence that negative density dependence affects recruitment, somatic growth, and body condition of Dreissena in the Hudson. Warm summers may also cause high adult mortality.We put our results into the context of a conceptual model of Dreissena population dynamics, and argue that neither the dynamics nor the controls of populations of these important invaders is known satisfactorily. [ABSTRACT FROM AUTHOR]

Despite years of attention, the dynamics of species constrained to disperse within riverine networks are not well captured by existing metapopulation models, which often ignore local dynamics within branches.We develop a modelling framework, based on traditional metapopulation theory, for patch occupancy dynamics subject to local colonisation–extinction dynamics within branches and regional dispersal between branches in size‐structured, bifurcating riverine networks. Using this framework, we investigate whether and how spatial variation in branch size affects species persistence for dendritic systems with directional dispersal, including one‐way (up‐ or downstream only) and two‐way (both up‐ and downstream) dispersal.Variation in branch size generally promotes species persistence more obviously at higher relative extinction rate, suggesting that previous studies ignoring differences in branch size in real riverine systems might overestimate species extinction risk.Two‐way dispersal is not always superior to one‐way dispersal as a strategy for metapopulation persistence especially at high relative extinction rate. The type of dispersal that maximises species persistence is determined by the hierarchical level of the largest, and hence most influential, branch within the network. When considering the interactive effects of up‐ and downstream dispersal, we find that moderate upstream‐biased dispersal maximises metapopulation viability, mediated by spatial branch arrangement.Overall, these results suggest that both branch‐size variation and species traits interact to determine species persistence, theoretically demonstrating the ecological significance of their interplay. [ABSTRACT FROM AUTHOR]

Our project sought to determine ecological effects of adding low‐head dams and levees to large rivers by examining potential changes to aquatic food webs over a 70‐year period in the Lower Ohio River (LOR) and Upper Mississippi River (UMR).We employed museum collections of fish and compound specific stable isotope analysis of amino acids to evaluate long‐term changes in primary food sources for multiple species of fish in each river.Fishes in both rivers depended more on autochthonous than allochthonous carbon sources throughout the 70‐year period (based on measurements of isotopic signatures of algae, C3 plants, C4 plants, cyanobacteria, and fungi), but the relative use of different carbon sources differed between the UMR and LOR. Significant but opposite shifts in trophic positions (TP) between rivers over time (higher TP in the UMR; lower in the LOR) were correlated with major anthropogenic changes to habitat structure (e.g. slight decrease in abundance of side channels in the UMR; increase in pool water depth in the LOR) resulting from low‐head dam construction. They may also have been influenced by likely increased primary productivity in the UMR from agricultural nitrogen inputs and by possible shifts in the importance of phytoplankton versus benthic algae in the LOR from changes in water depth. Shifts in trophic position and reliance on various food sources were not correlated with variation in discharge, gage height, or temperature.Although these two rivers have contrasting hydrogeomorphic complexity (UMR is an anastomosing river, while the LOR is a constricted channel river) and different discharge patterns (seasonal versus yearly operation in some cases), both differ substantially from rivers having hydrogeomorphic changes resulting from construction of high dams (>15 m). It is not surprising, therefore, that factors controlling trophic position and reliance on different carbon sources vary among different types of dams and river structures. [ABSTRACT FROM AUTHOR]

In low‐gradient, macrophyte‐rich rivers, we expect that the significant change in macrophyte biomass among seasons will strongly influence both biological activity and hydraulic conditions resulting in significant effects on nutrient dynamics. Understanding seasonal variation will improve modelling of nutrient transport in river networks, including annual estimations of export, which could optimise decision‐making and management outcomes.We explored the relationships among seasonal differences in reach‐scale nutrient uptake, macrophyte abundance, solute transport and transient storage in the River Gudenå (Denmark), a large macrophyte‐rich river. We used the minimal pulse addition technique to measure uptake of ammonium, nitrate, soluble reactive phosphorus, as well as reach‐scale metabolism, and surface transient storage in spring, summer, and autumn.We found that riverine uptake changed among seasons and was linked to macrophyte biomass via both biological activity, reflected in reach‐scale metabolism, and through physical processes, as solute transport was influenced by longitudinal dispersion. In this macrophyte‐rich river, seasonal changes in macrophyte biomass affected contact time between the water and biota, which influenced ammonium and soluble reactive phosphorus uptake. Using stoichiometric scaling of reach‐scale metabolism, we found that seasonal variation also influenced the relative contributions of autotrophic and heterotrophic biota in assimilatory uptake.In summary, riverine nutrient uptake was not static, highlighting the importance of seasonality, with significant implications for modelling of nutrient export in river networks. Moreover, current management strategies that remove macrophyte biomass (i.e. weed cutting and dredging) will short‐circuit the positive effects of enhanced nutrient uptake resulting from abundant macrophytes in rivers. [ABSTRACT FROM AUTHOR]

Passive acoustic monitoring is gaining momentum as a viable alternative method to surveying freshwater ecosystems. As part of an emerging field, the spatio‐temporal replication levels of these sampling methods need to be standardised. However, in shallow waters, acoustic spatio‐temporal patchiness remains virtually unexplored.In this paper, we specifically investigate the spatial heterogeneity in underwater sounds observed within and between waterholes of an ephemeral river at different times of the day and how it could affect sampling in passive acoustic monitoring.We recorded in the Einasleigh River, Queensland in August 2016, using a linear transect of hydrophones mounted on frames. We recorded four times a day: at dawn, midday, dusk, and midnight. To measure different temporal and spectral attributes of the recorded sound, we investigated the mean frequency spectrum and computed acoustic indices.Both mean frequency spectrum and index analyses revealed that the site and diel activity patterns significantly influenced the sounds recorded, even for adjacent sites with similar characteristics along a single river. We found that most of the variation was due to temporal patterns, followed by between‐site differences, while within‐site differences had limited influence.This study demonstrates high spatio‐temporal acoustic variability in freshwater environments, linked to different species or species groups. Decisions about sampling design are vital to obtain adequate representation. This study thus emphasises the need to tailor spatio‐temporal settings of a sampling design to the aim of the study, the species and the habitat. [ABSTRACT FROM AUTHOR]

Nerites (Gastropoda: Neritidae) are prominent members of tropical marine and freshwater gastropod faunas and rich assemblages can be found in many streams of islands in the Indo‐Pacific. For example, the streams of Fiji and New Guinea each support at least 23 species of freshwater neritimorphs, with representatives in the genera: Clithon, Neripteron, Neritilia, Neritina, Neritona, Neritodryas, Septaria, and Vittina.The striking diversity of this group in the small coastal streams of Pacific Islands contrast with a paucity of taxa in tropical Australia, despite northern Australia occupying a similar latitude. Just four taxa have been reported from Australia and only two can be considered common. These patterns are in marked contrast to the wide distribution of many marine nerites in the Pacific and conflicts with Island Biogeography Theory.Strikingly, many of these stream taxa have adopted an amphidromous lifestyle; adult gastropods feed and reproduce in freshwater, whereas larvae are swept to the ocean and undergo a marine dispersive phase before settling near the entrance to creeks and re‐entering these freshwater systems as crawling juveniles.Rapid transit of larvae to the ocean via short, steep, fast‐flowing streams may offer an explanation for this biogeographic conundrum. Larvae that do not reach the ocean within a few days may starve or exhibit poor survival. Hence, the disruption of stream–ocean connectivity may explain the low diversity of these taxa in northern Australia. Sea level rise in northern Australia in the current interglacial has further weakened stream–ocean connectivity with the development of vast flood plains and slow‐moving rivers.We contend that: (1) poor stream–ocean connectivity is not conducive to the maintenance of populations of nerites in northern Australia; and (2) new records of freshwater nerites may be revealed by surveys in short, steep coastal streams of northern Australia. [ABSTRACT FROM AUTHOR]

Network connectivity is a key feature of rivers that affects patterns and processes in lotic ecosystems. Few studies have considered how changes in river reach connectivity might affect ecosystem attributes at a whole river network scale. The use of population dynamics models of keystone species at a river network scale allows exploration of how the effects of altered natural connectivity patterns might propagate through a river network.In this study, we present a metapopulation model to estimate the spatial distribution of the population density of brown trout (Salmo trutta), an ecologically and socioeconomically important top predator, in a river network in northern Spain. The model accounts for the presence of barriers that limit longitudinal connectivity in upstream and downstream directions. The model estimates the spatial distribution of densities of three age‐classes (young‐of‐the‐year, juveniles, and adults) in all river reaches that make up the network based on topology, connectivity and population dynamics (e.g. age‐class specific mortality, spawning, age‐class dispersal, and spawning migration patterns).Seventy‐five percent of the modelled population densities fell within the 95% confidence intervals of the empirical data (84.6% for young‐of‐the‐year, 69.2% for juveniles, and 69.2% for adults) collected in 13 reaches. The simulated removal of all longitudinal barriers to migration in the river network (re‐naturalisation of the whole catchment) produced a modelled increase in brown trout density in the most downstream reaches of the river network and lowered fish densities in the upstream portion of the network when bias in juvenile and adult movement direction was assumed. Furthermore, the simulated removal of a single obstacle affected fish density even in distant tributaries.The proposed model is an appropriate tool for the evaluation of spatial patterns of brown trout density at a river network scale and for the assessment of the impact of altered connectivity. This might help simulate the results of management strategies regarding river connectivity and show where population decreases or increases could be expected, although empirical knowledge of overall trout movement in the studied river networks is required for drawing realistic scenarios. [ABSTRACT FROM AUTHOR]

Intermittently freezing and drying lotic systems are common in many parts of the world. These ecosystems provide dynamic habitats for biota, as both freezing and drying processes result in the loss of water flow along stream and river channels. However, research into the ecological effects of intermittent freezing on stream biota has remained relatively scarce compared to the recent increase in studies exploring the ecology of intermittent streams that dry.Climate change is predicted to alter the distribution and dynamics of intermittently freezing stream types. A better understanding of the responses of stream biota to intermittent freezing would inform predictions of the effects of ongoing climate change on these ecosystems.In this review, we compare the effects of both freezing and drying events on stream biota, ranging from individual‐level adaptations to population effects, community‐level biodiversity variation and food webs.Despite major contrasts in environmental conditions (e.g. the drying stream channels are exposed to a direct interaction with the riparian zone during the dry phases and the freezing stream channels are enclosed during the freezing phases), we show that both freezing and drying phenomena have pervasive effects on the structure and dynamics of aquatic stream biota.At the individual organism level, aquatic taxa use a range of physiological, morphological, life‐history, and behavioural responses to persist despite changing habitat conditions in both freezing and drying streams. The variety of organisms' responses can result in both biodiversity increases and decreases, which may affect population and community dynamics in both ecosystem types.Future research should further explore the population and community‐level effects of freezing in stream ecosystems. Due to the ecological parallels between intermittently drying and freezing streams, approaches developed in research into drying streams may also be applied in freezing stream ecosystems. This could improve our understanding of climate change effects on freezing stream ecosystems. [ABSTRACT FROM AUTHOR]

Ecological shifts that enhance the efficiency of resource acquisition by consumers can affect the fate of resource subsidies in recipient ecosystems. To date, findings have been mixed about whether plant litter breakdown by stream decomposers is faster in locations where the litter originates from (i.e. home region) compared with other locations (away region). This phenomenon, known as home‐field advantage (HFA), may be influenced by litter quality and particularly decomposer groups (shredders versus microbes), rather than being an inherent consequence of the breakdown of locally native litter.We studied the effects of HFA on litter breakdown and litter‐associated communities in streams within mid‐ to late‐successional forests. Two pairs of riparian broadleaf and one pair of conifer litter species with contrasting chemical quality were reciprocally incubated across two temperate regions (˜3,000 km apart). Species in each pair are congeneric and allopatric. The HFA was assessed using within‐pair litter comparisons in home and away regions. Coarse‐ and fine‐mesh litterbags were used to separate the contributions to litter breakdown by microbial decomposition and shredder feeding.Conifer litter species used in this study were more recalcitrant than broadleaf litter species. Consequently, there was a significant positive HFA for the microbial decomposition of conifer litter. In contrast, there was no HFA for either microbial or shredder‐mediated breakdown of broadleaf litter, which might have been overridden by catchment‐scale differences in biophysical variables affecting breakdown. The effects of HFA on shredder colonisation and feeding on broadleaf litter were more variable among streams. Numerically abundant shredder taxa tended to have higher rates of colonisation and consumption on higher‐quality litter than on low‐quality litter, irrespective of litter origin.Our results suggest that prior (evolutionary and/or contemporary) litter exposure could strongly influence the litter‐processing capacity of microbial decomposers, but not shredders, at the community level. In particular, microbial decomposers specialised in degrading conifer litter probably had lower resource‐use plasticity than those processing broadleaf litter. Inter‐stream differences in riparian vegetation and habitat conditions could influence the HFA through altering the litter exposure history of decomposer communities. Our findings highlight the importance of prior litter–microbe interactions in determining the rates of microbial decomposition of native and exotic litter species. [ABSTRACT FROM AUTHOR]

Understanding how different food resources sustain stream food webs is fundamental towards increasing our knowledge on trophic structure and energy flow pathways in fluvial ecosystems.Food webs in small mountain streams are sustained by autochthonous (instream primary production) and allochthonous (inputs from the terrestrial ecosystem) organic resources, with their relative importance highly dependent on catchment land cover.This study aimed to understand how catchment land cover determines food resource type (autochthonous, allochthonous) and quantity in mountain streams, and how this affects energy flow pathways and food web structure. We hypothesised that food resource type and quantity would reflect catchment land cover. Thus, changes in food resources would lead to shifts in macroinvertebrate assimilation of autochthonous and allochthonous food resources and consequently in dominant energy flow pathways. We further hypothesised that changes in food resources will have strong effects on dominant feeding groups and community biomass distribution among taxa in food webs.Energy flow pathways were quantified by combining macroinvertebrate biomass measures and assimilation of food resources estimated from δ2H and δ15N in 10 streams along a forest cover gradient, located in the Cantabrian Mountains (northern Spain).Results showed that grassland/shrub dominated streams had a higher proportion autochthonous food resources and a lower proportion of allochthonous food resources, whereas forested streams showed the opposite pattern. Changes in food resources with forest cover resulted in shifts in food resource assimilation and dominant energy flow pathways. Forested streams were mainly sustained by allochthonous resources, while streams flowing through grassland/shrub landscapes were mostly sustained by autochthonous resources.Food resource assimilation differed between feeding groups. Detritivores showed a fixed assimilation of allochthonous resources independent of resource quantity, while omnivore assimilation was determined by the dominant food resource. This was reflected in food‐web structure. There was an increase in detritivore biomass and conservation of omnivore biomass with increasing forest cover, leading to a more equal distribution of community biomass among macroinvertebrates comprising individual food webs.The dependence of stream food webs on dominant food resources highlights the importance of catchment land cover in determining energy flow pathways and food web structure in low order mountain streams. These findings will improve our predictions on the effects of land cover change on the functioning of mountain stream ecosystems. [ABSTRACT FROM AUTHOR]

Biotic interactions affect species distributions, and environmental factors that influence these interactions can play a key role when range shifts in response to environmental change are modelled.In a field experiment using enclosures, we studied the effects of the thermal habitat on intra‐ versus inter‐specific competition of juvenile Dolly Varden Salvelinus malma and white‐spotted charr Salvelinus leucomaenis, as measured by differences in specific growth rates during summer in allopatric and sympatric treatments. Previous laboratory experiments have shown mixed results regarding the importance of temperature‐dependent competitive abilities as a main driver for spatial segregation in stream fishes, and no study so far has confirmed its existence in natural streams.Under natural conditions in areas where the two species occur in sympatry, Dolly Varden dominate spring‐fed tributaries (cold, stable thermal regime), whereas both species often coexist in non‐spring‐fed tributaries (warm, unstable thermal regime). Enclosures (charr density = 6 per m2) were placed in non‐spring‐fed (10–14°C) and spring‐fed (7–8°C) tributaries.In enclosures placed in non‐spring‐fed tributaries, Dolly Varden grew 0.81% per day in allopatry and had negative growth (−0.33% per day) in sympatry, whereas growth rates were similar in allopatry and sympatry in spring‐fed tributaries (0.68 and 0.58% per day). White‐spotted charr grew better in sympatry than in allopatry in both thermal habitats. In non‐spring‐fed tributaries, they grew 0.17 and 0.79% per day and in spring‐fed tributaries 0.46 and 0.75% per day in allopatry and sympatry, respectively.The negative effect of inter‐specific competition from white‐spotted charr on Dolly Varden thus depended on the thermal habitat. However, there was no strong evidence of a temperature‐dependent effect of intra‐ and inter‐specific competition on white‐spotted charr growth.Multiple factors may shape species distribution patterns, and we show that temperature may mediate competitive outcomes and thus coexistence in stream fish. These effects of temperature will be important to incorporate into mechanistic and dynamic species distribution models. [ABSTRACT FROM AUTHOR]

Animal movement at localised scales is often modulated by competing pressures such as avoiding predators while acquiring resources and mates. The relative magnitude of these trade‐offs may affect males and females differently, often resulting in sex‐specific differences in movement.Sex‐biases in movement have been linked to mating systems (e.g. monogamy or polygamy) in birds and mammals; however, this relationship has received less attention among fishes. Using passive integrated transponder tags and a series of stationary antennas, we evaluated the movement dynamics of a small‐bodied, sexually dimorphic stream fish Fundulus olivaceus over a 30‐day period in a fourth‐order tributary to the Pascagoula River in Mississippi (U.S.A.).We documented dissimilar sex‐specific movement behaviours at different spatial scales that were likely to be facilitated by differential resource demands and competitive pressures. Females exhibited an increased propensity to engage in longer, exploratory moves (>30 m); whereas most males remained active within an established territory, making few long‐distance longitudinal movements.Local activity levels (proportion of individuals moving) were positively related to density (manipulated during the study), and density was found to affect the magnitude of sex‐specific movement. In contrast to females, males increased local activity and movement distance at the reduced density, presumably to expand territory size or mate‐searching behaviours, suggesting local mate competition may suppress the movement distance of males.Despite some evidence substantiating a relationship between movement and mating system, our results suggest that the documented sex‐specific differences may be related to traits that co‐evolve with mating systems, rather than the mating system per se. Our findings also highlight the importance of spatial scale when evaluating patterns of sex‐biased movement tendencies. [ABSTRACT FROM AUTHOR]

Understanding the relative importance of abiotic factors and biotic interactions in determining species co‐occurrence is a fundamental goal in ecology. Most studies of ecology of freshwater fishes have focused on negative biotic interactions (competition and predation), and much less is known about more positive interactions. The goal of this study was to quantify the relative importance of habitat features and biotic interactions (positive and negative) on co‐occurrence of three pairs of feeding‐associative Leporinus (L.) fishes in rivers of central Brazil (L. friderici–L. octomaculatus, L. striatus–L. octomaculatus, L. friderici–L. striatus).We sampled 146 sites on two rivers of central Brazil and used two‐species occupancy models to examine the relative importance of habitat features and commensalistic feeding association for determining co‐occurrence of the three species pairs. Using an information–theoretic framework, we compared weight‐of‐evidence for models containing effects of habitat covariates only, biotic interactions only, and their combination.Model evidence supported the hypothesis that feeding activity of L. friderici facilitates occurrence of L. octomaculatus and that the interaction is mediated by habitat covariates. In contrast, most evidence suggested that occurrence of L. striatus is independent of L. friderici, and is instead determined entirely by habitat features. Evidence for facilitation of L. octomaculatus by L. striatus was mixed—there is evidence supporting the habitat‐only hypothesis as well as habitat‐mediated facilitation.Our results suggest that positive interactions can affect co‐occurrence among freshwater fishes, at least for species that interact strongly. More facultative interactions, such as those between L. friderici and L. striatus, may be less important, at least in certain abiotic contexts. This study indicates that biotic interactions between species of freshwater fish can be evident beyond the microhabitat scale to positively influence species co‐occurrence and adds to evidence from other groups of freshwater organisms indicating the importance of biotic interactions in facilitating species co‐occurrence. [ABSTRACT FROM AUTHOR]

Flow modification of lotic ecosystems is one of the main threats to global freshwater biodiversity. Commonly, and in the river studied here, modification results from hydroelectric dam installation.We evaluated the impacts of damming on zooplankton communities in the Amazonian floodplain of the Madeira River (Porto Velho, Rondônia, Brazil) following construction in 2012 of the run‐of‐river dam of Jirau Hydroelectric Power Plant. Using data sampled between 2009 and 2015, we tested for discontinuities in zooplankton community composition attributable to damming and the naturally occurring flood pulse.The flood pulse remained the main predictor explaining variation in zooplankton community structure even with the installation of the dam on the Madeira River. Despite this, discontinuities for the entire zooplankton community and for the main compositional groups (testate amoebae, rotifers, cladocerans, and copepods) were detected in relation to the dam (pre‐/post‐dam periods), mainly in ebb and low water, and with weaker evidence of dam effects during flood and highwater hydrological periods.A multivariate regression tree explained 9.6% of the variation in zooplankton communities and identified four groups: (1) flood and high‐water periods; (2) low water post‐dam; (3) low water pre‐dam; and (4) ebb hydrological periods. The deviance in each multivariate regression tree node was attributable to variation in eight rotifer, three testate amoeba, and three copepod taxa.Our study demonstrates that the flood pulse, dam construction, and interaction between both of these factors affect zooplankton community structure in the Madeira River. While for many zooplankton community variables, effects occurred mainly during ebb and low‐water periods, some effects were also observed during high water and flood periods. We thus recommend the establishment of a permanent environmental monitoring programme during all hydrological periods in tropical floodplain rivers and the addition of sampling sites downstream from dams.Many rivers in the world are increasingly disrupted by multiple dams, yet little is known of their effects, especially for run‐of‐river dams. Our study identified short‐term impacts of only one run‐of‐river dam on zooplankton communities. More research is needed on the effects of multiple run‐of‐river dams on zooplankton and other biota, especially in tropical floodplain rivers, so that negative effects can be understood and ameliorated. [ABSTRACT FROM AUTHOR]

Running waters in Mediterranean regions are strongly regulated by dams, which produce significant alterations to natural flow regimes. Climate change will reduce discharge and increase flow intermittence in Mediterranean streams, which will lead to an intensified flow regulation to meet water demands. Very little is known about how the functional features of aquatic communities vary along combined anthropogenic flow alteration and natural intermittence gradients.As intermittent streams are subjected to natural stress (droughts and flash floods), the flow regime alteration effect may differ from that observed in perennial rivers. Consequently, studies that aim to determine the effects of flow regulation on the functioning of aquatic communities in a global change context are crucial.By applying linear mixed‐effect models and null models to the macroinvertebrate communities from 65 stream sites in the Segura River Basin (south‐east Spain), we assessed the separate effects of natural flow intermittence and flow regulation, as well as their interaction, on biological traits and functional diversity indices.Natural flow intermittence and flow regulation were mainly associated with loss of taxa with semivoltine or univoltine cycles and more sensitive aquatic stages (i.e. eggs), and with the replacement by taxa with multivoltine cycles and more resistant aquatic stages (i.e. adults). Flow regulation intensified the impact of natural flow intermittence on some biological traits, producing synergistic effects (i.e. decreasing interstitial taxa and tegument breathers and increasing taxa with aquatic adult stages). At the same time, antagonistic (life cycle) and opposing (shredders) interaction effects were also observed. Functional diversity, functional dispersion, and functional redundancy underwent a non‐random decrease as the flow regime alteration increased, and a significant antagonistic interaction was also found between both stressors for functional redundancy.In general, flow regulation effects were stronger in perennial than in intermittent streams because natural intermittence imposes a previous eco‐evolutionary pressure on aquatic biota by selecting those resistant or recovery traits that confer resilience to anthropogenic flow regime alterations. Thus, the natural flow regime influences the functional sensibility of communities to anthropogenic flow alteration. [ABSTRACT FROM AUTHOR]

We studied benthic macroinvertebrate communities in 12 mid‐elevation mountain streams in the Sierra Nevada, California. Data were collected at nine times (seven springs and two autumns) over a 14‐year period of variable hydrological conditions, including years of flood and extreme unprecedented drought.The taxonomic and trait structures of communities were similar during wet and average springs but became increasingly different as drought continued and smaller streams became intermittent.The density of total invertebrates, primarily chironomids, increased during the drought years, but the richness and abundance of mayfly, stonefly, and caddisfly (EPT) taxa declined during the late drought. During the late drought, the proportions of rheophilic, semivoltine taxa with clinger or swimmer behavioural habits decreased, whereas small, stress‐tolerant taxa with burrower and climber habits increased. Collector–gatherers dominated in all periods, but during the severe drought the relative abundance of micropredators increased and filterers, shredders, and grazers decreased.We also found greater changes in community structure between spring runoff and autumn base flow conditions in an average than a wet year. However, spring to autumn population growth rates were much greater during the wet than average year.Invertebrate richness and EPT abundance decreased and community structure showed large changes when stream discharge fell below 1–10 L/s, but remained relatively constant across a range of higher flows. During the severe drought, there were significant declines in the densities of 40% of the common EPT taxa compared to average and wet years, but chironomid, mite, and other invertebrate taxa showed variable responses.Invertebrate diversity, community structure, and ecosystem functions in small, headwater streams are especially vulnerable to drought conditions, which are expected to increase in frequency and intensity with climate change in the Sierra Nevada and other mountainous regions. [ABSTRACT FROM AUTHOR]

Predicted trends towards more intense droughts are of particular significance for running water ecosystems, as the loss of critical stream habitat can provoke sudden changes in biodiversity and shifts in community structure. However, analysing ecological responses to the progressive loss of stream habitat requires a continuous disturbance gradient that can only be generated through large‐scale manipulations of streamflow.In the first experiment of its kind, we used large artificial stream channels (mesocosms) as analogues of spring‐fed headwaters and simulated a gradient of drought intensity that encompassed flowing streams, disconnected pools, and dry streambeds. We used breakpoint analysis to analyse macroinvertebrate community responses to intensifying drought, and identify the taxa and compositional metrics sensitive to small changes in drought stress.We detected breakpoints for >60% of taxa, signalling sudden population crashes or irruptions as drought intensified. Abrupt changes were most pronounced where riffle dewatering isolated pools. In the remnant wetted habitat, we observed a shift to larger body sizes across the community, primarily driven by irruptions of predatory midge larvae and coincident population collapses among prey species (worms and smaller midges).Our results suggest that intense predation in confined, fragmented stream habitat can lead to unexpected changes in body sizes, challenging the conventional wisdom that droughts favour the small. Pool fragmentation might thus be the most critical stage of habitat loss during future droughts, as the point at which impacted rivers and streams begin to exhibit major shifts in fundamental food web properties. [ABSTRACT FROM AUTHOR]