Showing papers in "Freshwater Biology in 2016"

Ecological research and management of intermittent rivers: An historical review and future directions

Abstract: Rivers and streams that do not flow permanently (herein intermittent rivers; IRs) make up a large proportion of the world's inland waters and are gaining widespread attention. We review the research on IRs from its early focus on natural history through to current application in management and policy. The few early studies of the ecology of IRs were largely descriptive. Nevertheless, in the 1970s, synthesis of this sparse research complemented work on temporary standing waters to found a powerful framework for much of the subsequent research on IRs. Research on the ecology and biogeochemistry of IRs continues to fuel our understanding of resistance and resilience to drying and flooding as disturbances. Syntheses of the growing literature, including cross-continental and cross-climate comparisons, are revealing the generality and individuality of ecological and ecosystem responses to flow cessation and surface water loss. Meanwhile, increasing numbers of experiments test the causality of these responses. Much of the increased consideration of IRs in research, management and policy is driven by the observed and projected shifts in flow regimes from perennial to intermittent associated with changes in land and water use and climate, superimposed on the high incidence of natural intermittency. The need to protect and better manage IRs is prompting researchers to develop new or modified methods to monitor flow status and assess the ecological condition of these systems. Intermittent river research and management will benefit from greater exploration of aquatic–terrestrial linkages, wet–dry cycling and temporal dynamics, more-detailed mapping and predictive modelling of flow intermittency and the application of metapopulation and metacommunity concepts alongside multiple-stressors and novel-ecosystems research. By building on existing knowledge, continuing to develop quantitative models and distribution maps and using experiments to test hypotheses and concepts, we can further ecological understanding and wise management of these ubiquitous ecosystems.

The importance of high-quality algal food sources in stream food webs - current status and future perspectives

Abstract: Summary While many streams and rivers are dominated by terrestrial inputs of organic carbon, algae are an important trophic base for stream food webs. However, the nutritional importance of algae for stream invertebrates only recently has been highlighted. Algae are acknowledged as higher quality food than terrestrial organic matter for the growth and reproduction of invertebrates. In part, this is because of higher algal polyunsaturated fatty-acid (PUFA) content. Here, we review the important influence of algal food quality, as assessed by PUFA, in stream food webs. Current field investigations have mainly focused on the fatty-acid dynamics of macroinvertebrates, and indicate that algal eicosapentaenoic acid (EPA), α-linolenic acid (ALA) and linoleic acids (LIN) are present in all macroinvertebrates. However, fungal and bacterial tracers have also been observed in a range of macroinvertebrates. The omega-3 (ω3)/omega-6 (ω6) ratio >1 in most macroinvertebrates strongly indicates that dietary energy of algae is highly retained in stream food webs. Interspecific differences in PUFA composition seem to be affected by dietary PUFA and consumer physiology. Some studies have suggested that besides dietary EPA, the shorter chain C18 PUFA LIN and ALA also can improve growth and reproduction of stream invertebrate consumers. Some macroinvertebrates may preferentially retain or synthesise long-chain PUFA from C18 PUFA when experiencing low-quality food. However, this process is controversial since other species have shown very limited ability to synthesise long-chain PUFA. Algal PUFA composition is strongly influenced by abiotic factors, particularly light, nutrients, and temperature. Human disturbance (i.e. riparian vegetation removal and nutrient inputs) on algal PUFA content and their consequent effects on macroinvertebrates and fish clearly warrant further scientific attention. Controlled feeding trials and manipulative studies are required to measure PUFA conversion capacities and reproductive investment of stream macroinvertebrates under different food quality conditions, which will provide insights into how freshwater species can cope with different nutritional food conditions due to human disturbance and climate change.

A landscape approach to advance intermittent river ecology

Abstract: Intermittent rivers are increasingly viewed as shifting mosaics of lotic (flowing water), lentic (standing water) and terrestrial (dry riverbed) habitats. The diversity, spatial arrangement, temporal turnover and connectivity of these habitats are controlled by the magnitude, frequency, duration and extent of drying and rewetting events, which maintain habitat heterogeneity and control biodiversity and biogeochemical processes in intermittent rivers. We consider intermittent rivers as spatiotemporal landscape mosaics to identify the implications such a view has for empirical and theoretical developments in landscape and river ecology. Using observational data of flow states collected by citizen scientists along 1400 km of river channels in western France, we used landscape metrics and ecologically scaled indices for four hypothetical, aquatic species (two fish and two insects) to describe the dynamics of intermittent river mosaics for five catchments. Dry patches dominated most observation dates but flowing patches had the longest average length and occupied the greatest proportion of channel length. At the start of each summer, catchments were almost entirely composed of flowing patches but lentic and dry patches could represented up to 80% of the catchments as summer progressed. Patch dynamics were typified by high levels of spatiotemporal variability. In contrast, ecologically scaled indices did not vary greatly among catchments within species. The ecologically scaled indices representing small fish were the most affected by habitat fragmentation. Such a landscape perspective could affect understanding of biodiversity patterns and biogeochemical processes in intermittent rivers. We outline the methodological developments required to integrate landscape approaches into intermittent river research, the associated challenges and current limitations in landscape ecology tools and models and the benefits of citizen science data sets. The continued quantification of shifting habitat mosaics in intermittent rivers will provide multiple opportunities to advance river and landscape ecology.

Water‐level fluctuations regulate the structure and functioning of natural lakes

Abstract: Summary Despite becoming one of the main pressures on aquatic ecosystems globally, understanding of the ecological impacts of altered water-level regimes in lakes lags far behind that of other human disturbances (e.g. eutrophication, acidification). We employed a multifaceted approach to explore the potential importance of water-level fluctuations (WLF) for the structure and functioning of littoral zones and multiple trait responses of primary producers and benthic consumers across a range of natural lakes. We found that lakes with high WLF had significantly more coarse littoral substrata with less coverage of macrophyte vegetation in the shallows than in lakes with low WLF. Lakes with high WLF also had greater proportions of motile diatom species and omnivorous benthic invertebrates in shallow waters, altered taxonomic and trophic structure of benthic consumers and more homogeneous algal and benthic invertebrate assemblages. Variation along the littoral depth gradient needs to be examined when assessing the impacts of hydrological pressures in lakes. We found that depth interacted with WLF in its effects on habitat structure and mediated the response of both producer and consumer benthic assemblages to WLF. Our results indicate that amplified WLF significantly affect both the structure and functioning of lake ecosystems. Given the growing importance of WLF as an increasingly pervasive pressure on lake ecosystems worldwide, our findings have important implications for the conservation and management of global aquatic biodiversity. Inclusion of both biological traits and sampling along a depth gradient in existing and in future monitoring programmes could improve significantly the ability to detect and predict effects of altered patterns of WLF on lake ecosystems.

Fostering integration of freshwater ecology with ecotoxicology

Abstract: Summary Ecology and ecotoxicology have different historical roots, despite their similar names, but are slowly converging to meet the challenge of addressing the massive global proliferation and release of chemicals in the environment. The conceptual, methodological, review and standard research papers in this special issue reflect this emerging trend of blending ecological and ecotoxicological perspectives to assess impacts in freshwater ecosystems. Assessing community and ecosystem impacts of chemical contaminants is complex, however, and will require approaches that explicitly consider biological and chemical diversity as well as the natural variability of environmental factors at multiple spatial and temporal scales. Central themes of the papers in this issue are (i) the importance of indirect effects of chemical contaminants on species interactions and food webs; (ii) effects of multiple stressors, especially interactions between contaminants and environmental factors; (iii) consequences of chemical exposure on ecosystem processes such as primary production and litter decomposition; (iv) the need to account for context dependency and (v) potentially harmful community and ecosystem effects of emerging contaminants, among which nanoparticles are prominently represented. Collectively, these papers show that integrating ecological principles into the design and implementation of ecotoxicological research is essential for assessing and predicting contaminant impacts on biological communities and ecosystems. Conversely, applied ecology and bioassessment would benefit from concepts and approaches developed in ecotoxicology and from fully embracing chemical contaminants as key drivers of community structure and ecosystem processes.

Multiple-stressor effects on stream invertebrates: a mesocosm experiment manipulating nutrients, fine sediment and flow velocity

Abstract: Summary Land-use changes have degraded ecosystems worldwide. A particular concern for freshwater biodiversity and ecosystem function are stressors introduced by intensified agriculture. Typically several stressors affect freshwater ecosystems simultaneously. However, the combined effects of these multiple stressors on streams and rivers are still poorly understood, yet of critical importance to improve freshwater management. We investigated responses of benthic macroinvertebrates to three globally important agricultural stressors affecting streams (nutrient enrichment, fine sediment deposition and reduced current velocity), using 64 stream mesocosms (full-factorial 2 × 2 × 2 design, eight replicates of each treatment combination) established on the banks of the Breitenbach Stream (Hesse, Germany). The experiment ran for 1 month (16 days of colonisation, 14 days of manipulations), and all invertebrates in the mesocosms were collected at the end of this period. Fourteen of the 17 studied invertebrate response variables were affected by one or more stressors each. Negative effects on richness or abundance of pollution-sensitive Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa were particularly common. Overall, both sediment addition and stream flow velocity reduction had pervasive and strong effects. Responses to sediment addition were mostly negative, whereas decreased current velocity reduced several EPT metrics but increased the abundances of some of the other common taxa. Nutrient enrichment had few effects, but these were consistently negative. Combined stressor effects were mainly additive, with only two interactions found in total, both between reduced velocity and nutrients (on the crustacean Gammarus spp. and ceratopogonid midges). This finding implies that multiple-stressor responses may be predicted from knowledge of single-stressor effects in this stream community (unlike the often synergistic or antagonistic responses observed elsewhere). However, further taxon-specific responses and interactions among stressors may have been obscured by limited taxonomic resolution, especially for the numerically dominant Chironomidae. Genetic approaches are required to address this limitation in the future.

Effects of nanoparticles in fresh waters: risks, mechanisms and interactions

Abstract: Summary Ecotoxicology is often criticised for its simplistic approach, which does not normally consider the complexity of field conditions. Simple laboratory experiments can still be useful, however, especially for assessing effects of emerging stressors such as nanoparticles, which exhibit fates, exposure profiles and modes of action substantially different from those of traditional chemicals. Here we argue that it is important to understand the potential effects of environmental conditions (e.g. UV radiation, dissolved organic matter, chemical stressors) on the fate and ecotoxicological potential of nanoparticles by using simple and well-controlled experiments, while aiming to mimic realistic environmental conditions as closely as possible. The observation that increasingly complex test systems may yield lower effect thresholds for nanoparticles than standardised tests suggests that current approaches require modification. Specifically, research is encouraged on interactions among trophic levels, community composition and ecosystem and evolutionary processes, so that effects observed in complex environmental settings can be explained mechanistically. We highlight recent discoveries in ecotoxicology and ecology that suggest nanoparticle-induced consequences on evolutionary and ecosystem processes as well as their potential transfer across ecosystem boundaries. These insights may encourage further research on nanoparticle effects informed by ecological theory.

Context dependency in biodiversity patterns of central German stream metacommunities

Abstract: Summary Context dependency is an emerging topic that is challenging our understanding of the factors shaping biodiversity in metacommunities. River networks and other dendritic systems provide unique systems for examining variation in the processes shaping biodiversity between different metacommunities. We examined biodiversity patterns in five benthic invertebrate data sets, from two catchments in central Germany, with the aim of exploring context dependency in these systems. We used variance partitioning to disentangle the variation explained in three biodiversity metrics: taxonomic richness, Simpson's diversity and local contribution to beta diversity (LCBD; a measure of the uniqueness of a site). As explanatory variables, we used proxies of network position (i.e. catchment size and altitude) and habitat conditions. Contrary to our expectation, we found no evidence of a decline in LCBD downstream in our study. Local habitat conditions and catchment land use played a much stronger role than catchment size and altitude in explaining variation in the three biodiversity metrics. Observed patterns were highly variable between different data sets in our study. These findings suggest that factors shaping biodiversity patterns in these systems are highly context dependent and less related to their position along the river network than local habitat conditions. Given the clear context dependency between data sets, we urge researchers to focus on disentangling the factors driving the high levels of variability between individual systems through the study of a number of replicate, rather than single, metacommunities.

Is drift the primary process promoting the resilience of river invertebrate communities? A manipulative field experiment in an intermittent alluvial river

Abstract: Summary In river systems, aquatic invertebrate communities are surprisingly persistent over time and generally recover quickly from disturbances. Drift has long been viewed as the primary process promoting this resilience, and it plays a important role in predictive models of community composition and concepts in lotic ecology. More recently, other processes such as vertical migration from the hyporheic zone, aerial oviposition from distant refuges and the use of resistance forms (e.g. diapause) have received greater recognition and support for their importance. In this study, the view that drift is the primary process promoting invertebrate community resilience was challenged in an intermittent alluvial river using reach-scale flow manipulations. First, six treatment channels were completely dried for 1 week, while three others were left flowing to be used as controls. Second, flow was re-established in channels and drift was either allowed or blocked for a 4-week period. Third, during this period the resilience of community structure, composition and function was compared between treatments, and the potential for colonisation from the drift, hyporheic zone, aerial oviposition and resistance forms was measured. Communities recovered after only 2 weeks in all of the previously dried channels, and contrary to our hypotheses, invertebrate community structure, composition and functional trait composition were not altered by blocking drift, indicating it was not the primary process promoting resilience in this river. Three lines of evidence suggested colonisation from the hyporheic zone and not aerial oviposition nor resistance forms promoted resilience following rewetting including the following: (i) finding all common benthic taxa in the hyporheic zone during the drying event, (ii) a distinct decrease in invertebrate size upon rewetting in all treatment channels and (iii) a negative correlation between resilience and water table depth. This experiment highlighted the potential importance of the hyporheic zone as a key source of colonisation in alluvial rivers and emphasises the need for a three-dimensional perspective when considering community resilience in rivers. Adaptive management approaches are needed to direct attention to sources (e.g. hyporheic zone) that are essential to promoting community resilience in rivers facing increased pressures due to climate change, water abstraction and flow regime alteration.

Riparian plant guilds of ephemeral, intermittent and perennial rivers

Abstract: Plant functional types (or guilds) increasingly are being used to predict vegetation response to global changes. Continued human population growth coupled with projected warmer and drier climate will alter the hydrologic regimes of many arid‐zone rivers, including intermittent rivers. We aimed to identify (i) woody plant guilds associated with distinct stream types of an arid region and (ii) plant traits indicating adaptation to the selective pressures of water availability and fluvial disturbance. We used hierarchical clustering to identify 11 plant guilds from floodplains, terraces and uplands of eight Arizona rivers that vary in surface flow permanence, depth to ground water and intensity of fluvial disturbance. Six guilds were riparian pioneers with small, wind‐dispersed seeds, three guilds were late‐seral, shade‐tolerant riparian taxa with large animal‐dispersed seeds, and two guilds were composed of desert xerophytes. Within the riparian pioneer and seral groups, guilds varied in water acquisition and productivity traits including wood density and rooting depth. The community‐weighted traits varied or covaried with water availability and fluvial disturbance. Root: shoot ratio, canopy height and leaf area were influenced strongly by water availability, with the latter two showing a nonlinear response to changes in water table depth. Leaf length increased, and wood density decreased, as sites become wetter and more fluvially disturbed. Community‐weighted seed mass, seed dispersal and spinesence varied most strongly with elevation above thalweg (an indicator of decreasing fluvial disturbance). These analyses will enable prediction of changes in the relative abundance of plant types and plant traits in response to changes in stream flow regimes, such as shifts towards greater intermittency. The distribution patterns of guilds among riparian habitat types emphasise the importance of focusing conservation efforts not only on the limited number of perennial rivers remaining in arid regions, but also on intermittent and ephemeral rivers with shallow water tables.

Habitat engineering by beaver benefits aquatic biodiversity and ecosystem processes in agricultural streams

Abstract: Summary Small-scale discontinuities, formed by accumulations of wood, are recognised as a key feature of functionally intact forested streams because they promote organic matter retention, increase habitat complexity and provide flow refugia. Re-establishing such features in physically degraded streams is therefore a common priority for restoration schemes. Ecosystem engineering by beavers in the form of dam building might offer a natural mechanism for restoring degraded streams. Despite an increase in beaver reintroductions globally, the ecosystem engineering concept has rarely been applied to restoring biodiversity and ecosystem function, especially within degraded freshwater systems. By comparing multiple beaver-modified and unmodified sites on headwater streams draining 13 ha of pastureland in eastern Scotland, U.K., we investigated if hydromorphological changes caused by reintroduced beavers (Castor fiber) translate into desirable biological responses when there is a long history of physical degradation and contraction of the regional species pool due to agricultural land use. Beaver modified in-stream habitat by constructing 10 dams, thus creating a series of interconnected dam pools. Organic matter retention and aquatic plant biomass increased (7 and 20 fold higher respectively) in beaver ponds relative to unmodified channels, consistent with the lower fluctuation in stream stage observed below a series of dams. Growing season concentrations of extractable P and NO3 were on average 49% and 43% lower respectively below a series of dams than above, although colour and suspended solids concentrations increased. Macroinvertebrate samples from beaver-modified habitats were less taxon rich (alpha diversity on average 27% lower) than those from unmodified stream habitat. However, due to significant compositional differences between beaver versus unmodified habitats, a composite sample from all habitats indicated increased richness at the landscape scale; gamma diversity was 28% higher on average than in the absence of beaver-modified habitat. Feeding guild composition shifted from grazer/scraper and filter feeder dominance in unmodified habitats to shredder and collector-gatherer dominance in beaver-created habitats. Dam building by beaver in degraded environments can improve physical and biological diversity when viewed at a scale encompassing both modified and unmodified habitats. By restoring ecosystem processes locally, it may also offer wider scale benefits, including greater nutrient retention and flood attenuation. These benefits should be evaluated against evidence of any negative effects on land use or fisheries.

Pollution-induced community tolerance (PICT): towards an ecologically relevant risk assessment of chemicals in aquatic systems

Abstract: A major challenge in environmental risk assessment of pollutants is establishing a causal relationship between field exposure and community effects that integrates both structural and functional complexity within ecosystems. Pollution-induced community tolerance (PICT) is a concept that evaluates whether pollutants have exerted a selection pressure on natural communities. PICT detects whether a pollutant has eliminated sensitive species from a community and thereby increased its tolerance. PICT has the potential to link assessments of the ecological and chemical status of ecosystems by providing causal analysis for effect-based monitoring of impacted field sites. Using PICT measurements and microbial community endpoints in environmental assessment schemes could give more ecological relevance to the tools that are now used in environmental risk assessment. Here, we propose practical guidance and a list of research issues that should be further considered to apply the PICT concept in the field.

Contribution of organic toxicants to multiple stress in river ecosystems

Abstract: Summary River ecosystems are threatened by multiple stressors, including habitat degradation, pollution and invasive species. However, freshwater ecologists have largely disregarded the contribution of toxicants to stress in rivers, whereas ecotoxicologists have primarily examined toxicant effects in artificial systems. As a result, there is a paucity of information on the co-occurrence of organic toxicants with other stressors and on the relative importance of toxicants for overall ecological risk in rivers. We used monitoring data for German rivers to analyse the individual and joint occurrence of four stressors: habitat degradation, invasive species, nutrient pollution and organic toxicants. All stressors were examined for ecological risks in terms of whether they exceeded low- and high-risk thresholds derived from published studies and regulatory thresholds. Nutrients and habitat degradation exceeded low and high risk thresholds at c. 85% of the sites and invasive species and organic toxicants at c. 50% of the sites. At least one stressor exceeded thresholds at all sites for which data on all four stressors were available. Toxicity showed weak positive correlations with nutrients and habitat degradation (0.2 < Spearman's ρ < 0.34, 0.009 < P < 0.08). The risks of ecological effects arising from habitat degradation and invasive species were higher in lowland rivers, particularly for invasive species. Our assessment shows that organic toxicants contribute notably to risks of ecological effects in rivers, to a similar extent as invasive species, although habitat degradation and nutrients are the dominant stressors. Exposure to multiple stressors is the typical situation prevailing in rivers. Consequently, mitigation measures focusing on individual stressors may not be effective at reducing ecological risks. This suggests that integrating concepts and data from freshwater ecology and ecotoxicology is essential to meet the challenge of managing multiple stressors in river ecosystems.

Dammed rivers: impoundments facilitate fish invasions

Abstract: Summary River damming and other anthropogenic disturbances of natural habitats are among the main drivers of species loss through a range of direct and indirect effects. While the effects of river damming on aquatic species are relatively well studied, particularly with regard to their impacts on diadromous species and stenotopic riverine specialists, there is a paucity of studies quantifying the effects of dam construction on whole communities. We conducted a global meta-analysis focussed on fish communities, comparing species richness, abundance and proportion of alien species between dammed and undammed rivers. Both longitudinal and cross-sectional studies were examined. We found that construction of dams did not have a noticeable effect on fish richness and abundance, but the increase in proportion of alien species was significant (mean effect size of 0.62). Our findings suggest that the conversion of lotic waterbodies into lentic habitats result in the extirpation of species unable to withstand a drastic change in environmental conditions, but the loss is compensated by colonising lacustrine or eurytopic species taking advantage of reduced competition and the availability of new niches specific to lentic habitats. However, when eurytopic natives are absent from waterbodies connected to the newly constructed reservoirs, vacant niches are instead exploited by alien species, resulting in impoverishment of native species richness although overall species richness may be maintained.

Revealing the pathways by which agricultural land-use affects stream fish communities in South Brazilian grasslands

Abstract: Summary Understanding mechanisms by which agricultural practices affect freshwater ecosystems helps to inform land-use policies and management strategies aimed at mitigating effects of agriculture on biodiversity. Land-use activities in the catchment, riparian and local scales likely influence stream fish communities via multiple pathways, for instance, by modifying the instream habitat. We investigated the mechanisms driving local stream fish taxonomic richness and functional diversity in South Brazilian grasslands by testing a theoretical path model in which we specified a priori relationships predicting how land use at multiple scales affects instream habitat and fish communities. Agricultural activities adjacent to streams (i.e. local impact on the streambank) and catchment-scale cropland area were positively related to macrophyte cover and negatively associated with coarse particulate organic matter (CPOM, i.e. woody debris and leaf litter). Local impact also increased substrate siltation and homogenisation. Riparian vegetation in the upstream buffer ameliorated instream habitat condition by dampening macrophyte proliferation and providing CPOM. Fish species richness increased with both macrophyte cover and CPOM, revealing multiple pathways of agricultural influence. However, functional diversity decreased only with substrate siltation, revealing a response to agricultural activities adjacent to sample sites. Agricultural streams showed a replacement of benthic and lithophilic species by a larger number of morphologically similar and macrophyte-associated nektonic fish. Our study indicates that land-use adjacent to streams and upstream riparian zones is critical for maintaining taxonomically and functionally diverse fish communities due to their strong effects on instream habitat. Protection and recovery of riparian zones from land-use change can mitigate the effects of agriculture on fish communities in South Brazilian grasslands.

Gene-to-ecosystem impacts of a catastrophic pesticide spill: testing a multilevel bioassessment approach in a river ecosystem

Abstract: 1. Pesticides can have strong deleterious impacts in fresh waters, but understanding how these effects cascade through natural ecosystems, from microbes to apex predators, is limited because research that spans multiple levels of biological organisation is rare. 2. We report how an accidental insecticide spill altered the structure and functioning of a river across levels ranging from genes to ecosystems. We quantified the impacts on assemblages of microbes, diatoms, macroinvertebrates and fish and measured leaf-litter decomposition rates and microbial functional potential at upstream control and downstream impacted sites 2 months after the spill. 3. Both direct and indirect impacts were evident across multiple levels of organisation and taxa, from the base of the food web to higher trophic levels. At the molecular level, differences in functional gene abundance within the impacted sites reflected a combination of direct and indirect effects of the pesticide, via elevated abundances of microbial populations capable of using chlorpyrifos as a resource (i.e. direct effect) and oxidising ammonia released by decaying macroinvertebrate carcasses (i.e. indirect effect). 4. At the base of the food chains, diatom taxa found only in the impacted sites were an order-of- magnitude larger in cell size than the largest comparable taxa in control communities, following the near extirpation of their consumers. Population biomass of the key detritivore Gammarus pulex was markedly lower, as was the rate of litter decomposition in the impacted sites. This was partially compensated for, however, by elevated microbial breakdown, suggesting another indirect food-web effect of the toxic spill. 5. Although many species exhibited population crashes or local extirpation, total macroinvertebrate biomass and abundance were largely unaffected due to a compensatory elevation in small tolerant taxa such as oligochaetes, and/or taxa which were in their adult aerial life stage at the time of the spill (e.g. chironomids) and thus avoided contact with the polluted waters and were able to repopu- late the river quickly. Mass–abundance scaling of trophic links between consumers and resources revealed extensive restructuring within the food web. 6. This case study shows that pesticides can affect food-web structure and ecosystem functioning, both directly and indirectly across levels of biological organisation. It also demonstrates how an integrated assessment approach, as adopted here, can elucidate links between microbiota, macroinvertebrates and fish, for instance, thus improving our understanding of the range of biological consequences of chemical contamination in natural ecosystems.

Environmental heterogeneity among lakes promotes hyper β-diversity across phytoplankton communities

Abstract: The extent to which stochastic and deterministic processes influence variations in species communities across space and time remains a central question in theoretical and applied ecology. Despite their high dispersal ability, the composition of phytoplankton communities displays striking spatial variations among lakes even at small spatial scale. To investigate the mechanisms underlying the distribution of phytoplankton species, we evaluate the contribution of stochastic, spatial and environmental processes in determining β-diversity patterns of phytoplankton at a regional scale. Phytoplankton communities were surveyed in 50 different lakes from north-central France, a region characterised by strong environmental heterogeneity. The regional species pool was characterised by extremely high β-diversity levels, which were mainly explained by species replacement (i.e. turnover) rather than by differences in species richness (i.e. nestedness). Null models of random species distribution and spatial processes failed to explain observed β-diversity patterns. At the opposite, local environmental conditions strongly influenced the degree of uniqueness of local phytoplankton communities, with the most contrasted environments, including human-dominated areas, promoting highly distinct phytoplankton communities. Our results suggest that species-sorting mechanisms that arise from variations in local environmental conditions drive high species turnover at the region scale. Thus, in a landscape strongly impacted by cultural eutrophication, further anthropogenic impacts on aquatic ecosystems would likely induce regional homogenisation of phytoplankton communities. Overall, our study supports the fact that the management of lakes and reservoirs in anthropic landscapes should aim at maintaining environmental heterogeneity while preventing further eutrophication in order to favour the maintenance of high phytoplankton β- and γ-diversity.

Metacommunity patterns across three Neotropical catchments with varying environmental harshness

Abstract: Summary Most metacommunity studies indicate that dispersal processes play a minor role compared with species sorting in explaining metacommunity organisation, in particular, in stream systems. However, the role of dispersal could vary with environmental harshness, as a result of frequent resetting of community succession by disturbances and the selection of generalist species from regional species pools. The importance of dispersal may also be mitigated by species dispersal ability. In this study, we explored how species sorting and dispersal shaped invertebrate and fish metacommunities across streams in three tropical headwater catchments in Bolivia with contrasting environmental harshness, including flow regime, altitude and climate conditions. We addressed the hypothesis that the relative roles of dispersal and species sorting vary with environmental harshness: we predicted that the role of species sorting would predominate in benign conditions, whereas that of dispersal would predominate under moderate environmental harshness, and that neither dispersal nor species sorting would be relevant to explain metacommunities under high environmental harshness. We also hypothesised that the role of dispersal would decrease with increasing species dispersal ability. Although there was little or no spatial autocorrelation of environmental distances (i.e. environmental differences) across the headwater catchments, community similarity correlated more strongly with environmental than spatial distances among headwater sites that had low environmental harshness, but the opposite pattern was observed among sites with moderate environmental harshness. Under high environmental harshness, neither environmental harshness nor spatial distances between sites explained community similarity. Under moderate environmental harshness, the correlation between community similarity and spatial distances was the strongest for moderate dispersers of both invertebrates and fish. Yet, in contrast to fish, strongly dispersing invertebrate taxa were spatial structured, suggesting that they were not able to reach all sites as predicted. Our results suggest the role of dispersal might be underestimated, notably in systems prone to environmental harshness. Better proxies for dispersal, along with the use of spatial distances to account for resistance to animal movements in river systems and that account for flow magnitude and directionality, slope, riparian vegetation, wind and streambed roughness, may promote a more realistic integration of dispersal processes in basic and applied metacommunity research.

An ecological and ecotoxicological perspective on fine particulate organic matter in streams

Abstract: Summary Fine particulate organic matter (FPOM) provides a key longitudinal link within stream networks, and is the predominant food source for filter- and deposit-feeding invertebrates, collectively classified as ‘collectors’. Organisms involved in producing and using FPOM are sensitive to chemical and other anthropogenic stressors, but information on such impacts, and on FPOM dynamics in general, is limited. Here, we review information on the ecological role of FPOM in streams, and discuss potential impacts on FPOM dynamics of organic and inorganic chemical stressors, including metals and pesticides. Emphasis is placed on faecal particles produced within the leaf-litter processing chain. Key biological factors controlling the resource quality of FPOM for collectors include the identity of the invertebrates producing FPOM, and the nutritional quality of their food resources. FPOM nutrient content is also strongly influenced by microbial colonisation and activity, and FPOM processing rates are thus likely to be sensitive to the impacts of stressors affecting microbes, including nutrients and antimicrobial chemicals. The potential for FPOM to bind and subsequently transport chemical stressors is high, particularly for hydrophobic compounds, but the extent of such effects and impacts on collectors consuming contaminated particles has attracted only limited attention. Combining concepts and research approaches from ecotoxicology and basic stream ecology would facilitate development of a common integrated framework for understanding the role of FPOM, and assessing anthropogenic impacts on FPOM dynamics in stream networks.

Challenges, developments and perspectives in intermittent river ecology

Abstract: Although more than half the world's river networks comprise channels that periodically cease to flow and dry [intermittent rivers (IRs)], river ecology was largely developed from and for perennial systems. Ecological knowledge of IRs is rapidly increasing, so there is a need to synthesise this knowledge and deepen ecological understanding. In this Special Issue, we bring together 13 papers spanning observational case studies, field and laboratory experiments and reviews to guide research and management in this productive field of freshwater science. We summarise new developments in IR ecology, identify research gaps and needs, and address how the study of IRs as highly dynamic ecosystems informs ecological understanding more broadly. This series of articles reveals that contemporary IR ecology is a multifaceted and maturing field of research at the interface between aquatic and terrestrial ecology. This research contributes to fresh water and general ecology by testing concepts across a range of topics, including disturbance ecology, metacommunity ecology and coupled aquatic-terrestrial ecosystems. Drying affects flow continuity through time and flow connectivity across longitudinal, lateral and vertical dimensions of space, which aligns well with the recent emphasis of mainstream ecology on meta-system perspectives. Although most articles here focus on the wet phase, there is growing interest in dry phases, and in the terrestrial vegetation and invertebrate assemblages living in and along IR channels. We encourage interdisciplinary studies on IRs to further blur the demarcation between aquatic and terrestrial ecosystems and develop more integrated perspectives. As a result of climate change and human modification of landscapes and waterways, flooding and drought are expected to become more extreme and widespread. Shifts in streamflow regimes from perennial to intermittent may exacerbate the duration and frequency of dry phases in IRs with serious implications for river ecosystems and the quality and diversity of services they provide.

Macroinvertebrate seedbank composition in relation to antecedent duration of drying and multiple wet‐dry cycles in a temporary stream

Abstract: 1. The aquatic invertebrate ‘seedbank’ comprises life stages that remain viable in the bed sediments of temporary freshwaters during dry phases. This seedbank promotes persistence of temporary-stream macroinvertebrates, but how its inhabitants respond to extended dry phases or repeated transitions between wet and dry phases remains unknown. 2. We rehydrated samples collected from the dry bed of a temperate-zone stream during a supra-seasonal drought, to examine the seedbank assemblage. Samples were first collected in autumn, from 12 sites along the ephemeral (4 sites), intermittent (2) and near-perennial (6) reaches, which had been dry for up to 8 months. Our first hypothesis was that assemblage composition would be related to the dry-phase duration preceding sampling, with longer dry phases reducing abundance and richness. 3. We revisited the same sites in three subsequent seasons, collecting and rehydrating sediments from all dry sites: five sites in early spring, three in late spring and four in late summer. Unpredictable flow resumption and redrying occurred between sampling dates. Our second hypothesis was that repeated wet-dry cycles would not degrade the assemblage because temporary-stream taxa would be adapted to fluctuating hydrological conditions. 4. Multiple individuals of only Chironomidae, Oligochaeta and Pisidium were present at sites that experienced the longest dry phases, providing some support for our first hypothesis. An additional 21 taxa were recorded across the remaining (shorter dry phase) sites in autumn, indicating that such sites act as refuges and potential recolonist sources following flow resumptions. 5. Although several insect orders first recorded in early spring were absent in later seasons, taxon-specific life cycles indicated that these absences were probably seasonal and not due to repeated wet-dry cycles. 6. We recorded 38 taxa in total, highlighting the seedbank as a dry-phase resistance mechanism for many temporary-stream macroinvertebrates. Our results also suggest that seedbank diversity may be threatened by increases in drought extent and duration.

Terrestrial and aquatic invertebrates in the riverbed of an intermittent river: parallels and contrasts in community organisation

Abstract: Summary Although intermittent rivers (IRs) are coupled aquatic and terrestrial ecosystems, their constituent communities are generally considered separately by ecologists. Yet, both communities are disturbed by alternating flowing and dry phases. IRs are thus good systems in which to explore the parallels and contrasts in community organisation in response to disturbance, and gain understanding in the drivers of these responses. We compared the structure and composition of aquatic and terrestrial communities in the Albarine River, France. We collected aquatic and/or terrestrial invertebrates in the riverbed and adjacent riparian zone at four intermittent reaches and aquatic invertebrates in the riverbed at three perennial reaches over 1 year. A total of 175 taxa were collected at intermittent reaches, including 62 aquatic and 113 terrestrial taxa, contrasting with the low biodiversity generally associated with IRs. Aquatic and terrestrial communities were mainly subsets of those in perennial reaches and in the riparian zone, respectively, indicating that the riverbed of intermittent reaches is strongly linked to perennial reaches and the riparian zone by dispersal. Total taxonomic richness in the riverbed (i.e. γ diversity) of intermittent reaches was mainly due to among-reach variation for terrestrial invertebrates, but to within-reach variation for aquatic invertebrates. For both communities, among-reach dissimilarity was mainly due to spatial turnover rather than community nestedness. Spatial turnover was higher for terrestrial communities than for aquatic communities. Our results indicate that aquatic and terrestrial communities have different organisation patterns in this intermittent river; probably in response to the contrasting spatial arrangements of colonist sources. Heterogeneous sources of terrestrial colonists along the riparian zone compared to more homogeneous sources of aquatic colonists from perennial reaches could explain the relative contribution of nestedness versus turnover.

Go with the flow: the movement behaviour of fish from isolated waterhole refugia during connecting flow events in an intermittent dryland river

Abstract: In many intermittent, dryland rivers, fish are confined to isolated waterholes for much of the year. It is only during brief flow events, which typify the hydrology of these systems, that fish are able to move between waterholes and explore surrounding habitat. Because most of the river channel will dry afterwards, there is a strong advantage for selection of persistent waterholes. Two hundred and fifteen individual fish of three common large-bodied species were tagged in two isolated waterholes in the Moonie River (Queensland, Australia) over 3years. Their movements were monitored to identify the flow events that trigger fish movement between waterholes, differences in response among species and size classes and refuge selection preferences. Some individuals of all species moved during flow events and others remained within the same waterhole. There was no clear upstream or downstream preference, and most individuals used a reach of up to 20km, although some individuals ranged over more than 70km in only several days. Above a threshold flow of 2m above commence-to-flow level, timing of flow was more important than magnitude, with most movement occurring in response to the first post-winter flow event, independent of its magnitude and duration. Many of the fish that moved displayed philopatry and subsequently returned to their starting waterhole either by the end of a flow event or on subsequent events, suggesting ability to navigate and a preference for more permanent refuge pools. Maximising survival in a highly variable environment provides a plausible mechanism for maintaining these behaviours. Modifications to both flow regime and hydrological connectivity may reduce movement opportunities for fish in intermittent rivers. Our findings show that fish in intermittent systems use networks of waterholes and that management and conservation strategies should aim to maintain movement opportunities at large spatial scales to preserve population resilience.

Dieback of riparian alder caused by the Phytophthora alni complex: projected consequences for stream ecosystems

Abstract: Alder trees (Alnus spp.) are key nitrogen-fixing riparian species in the northern hemisphere. Inputs of nitrogen-rich leaf litter from alder into stream food webs can contribute significantly to nitrogen dynamics at local and landscape scales. Alder trees also provide habitats for terrestrial and aquatic organisms, and help stabilize river banks. Recently, substantial declines in alder stands have occurred along streams in Europe, with damages observed in some parts of North America also. A major driver has been the invasive oomycete pathogen Phytophthora alni species complex, which can spread rapidly along stream networks. This review synthesises information on the pathogen, processes of spread and infection, and its impacts on alder. We further address the potential ecosystem-level and management consequences of the decline of alder, and highlight research needs. The alder dieback caused by P. alni is associated with reductions in shade and quality and quantity of leaf litter. A decline in the structural integrity of branches and roots further threatens bank stability. Stream banks dominated by other tree species or no trees at all will result in ecosystem-level changes both above and below the waterline. The P. alni taxonomic complex includes different species with varying phenotypes. An improved understanding of their environmental tolerances, virulence and evolution, along with the processes regulating the spread and impacts of the pathogen, would assist in identification of the riparian and stream systems most vulnerable not only to invasion but also to the heaviest disease outbreaks and ecosystem-level impacts. Within the P. alni complex, the highly pathogenic hybrid species P. x alni is favoured by mild winters and warm, but not excessively hot summer temperatures suggesting possible changes in distribution and level of impact under future global climate change.

Strong land‐use effects on the dispersal patterns of adult stream insects: implications for transfers of aquatic subsidies to terrestrial consumers

Abstract: The dispersal of terrestrial adults of freshwater insects is a key process regulating the transfer resources from aquatic habitats to terrestrial consumers, including high‐quality lipids synthesised in the aquatic environment. However, the efficiency of these transfers depends strongly on both subsidy production in the aquatic source habitat and permeability of the aquatic–terrestrial boundary. We assessed how land use (agriculture versus forest) affects stream riparian habitats and the dispersal of adult aquatic insects. Flying insects were sampled alongside eight streams (four agricultural, four forested) in central Sweden using sticky traps, placed from 1 up to 100 m from the stream edge. Environmental variables including temperature, wind, vegetation structure and canopy cover were also quantified. Abundances of adult aquatic insects were greater at agricultural than at forested sites, but most (64%) were caught close to the stream edge. In contrast, catches of adult aquatic insects declined relatively little with increasing distance from the forest streams. Overall, dispersal of the dominant aquatic insect order (Diptera) was positively associated with greater tree density and canopy shading and negatively associated with higher wind speeds and soil temperatures, more open habitats and steeper topographies. Taxonomic differences among assemblages, reflecting differing dispersal capacities, were also important, but less so than environmental factors. Our results indicate that adult aquatic insects in our forested landscapes are more likely to subsidise near‐ground food webs at a greater distance from stream channels, whereas the abundant subsidy emerging from the agricultural sites is likely to be most influential near the stream channel. A key question for future research is whether the deposition of large quantities of aquatic subsidy in agricultural riparian habitats exceeds the resource requirements and processing capacities of terrestrial consumers.

On the importance of trait interrelationships for understanding environmental responses of stream macroinvertebrates

Abstract: Summary Biological traits of organisms are expected to provide increased mechanistic understanding of species–environment relationships. Linking traits to environmental conditions is, however, not straightforward, as traits are interconnected within species and can affect the adaptive value of each other. The aim of our study was to evaluate the importance of these trait interrelationships for understanding environmental responses of freshwater macroinvertebrates. To this end, we investigated whether environmental responses of macroinvertebrates sharing a given trait were consistent or differed according to their taxonomy or to their other traits. We divided the macroinvertebrates into groups based on single traits (49 single-trait modalities), on taxonomy (10 orders) and on their overall trait profile (10 trait profile groups [TPGs], defined using self-organising maps clustering). Abundances of each of these 69 groups were related to 24 environmental variables using boosted regression tree (BRT) modelling, to assess the environmental responses of single traits, orders and TPGs. Cross-validated predictive power (R2) of the BRT models ranged from < 1% to 38%. Environmental responses of macroinvertebrates sharing a given trait were inconsistent and varied according to order and/or TPG. Single-trait responses often reflected the responses of the most abundant taxonomic group expressing the trait, suggesting that analysis of trait responses simply revealed patterns in habitat use by the most abundant species and not necessarily mechanistic relationships. Further, taxa from the same TPG (hence showing large overlap in their traits) but belonging to different orders showed different environmental responses. This indicates that the order a taxon belongs to confers unique information related to its evolutionary history that was not captured by our 49 trait modalities. However, groupings by orders cannot replace trait-based approaches, since TPGs also revealed differences in trait profiles within some orders, which were associated with different environmental responses. Our results highlight the importance of considering multiple rather than single traits when linking macroinvertebrates to environmental variables, including the potential information conveyed by evolutionary history.

Lake-type-specific seasonal patterns of nutrient limitation in German lakes, with target nitrogen and phosphorus concentrations for good ecological status

Abstract: Summary Eutrophication is a global environmental problem that leaves many lakes with impaired ecological status. Human activity has increased the total concentrations of both nitrogen and phosphorus in aquatic systems, but their relative influence on phytoplankton biomass is uncertain. Their action as alternative limiting resources complicates assessment of their relative influence and disagreement may be in part due to seasonal shifts and lake-type-specific differences in the prevalence of limitation by nitrogen versus phosphorus. Debate continues as to whether measures to reduce nitrogen would be beneficial in addition to controls placed on phosphorus. We used a piecewise model to test whether total nitrogen (TN) concentrations, in addition to total phosphorus (TP), influence phytoplankton biomass in 369 lowland German lakes. The piecewise model predicts biomass from TN for low N : P ratio lakes, and from TP for high N : P ratio lakes. We tested three N : P mass ratios to divide lakes: dissolved inorganic nitrogen to TP (DIN : TP), DIN to dissolved reactive phosphorus (DIN : DIP) and TN : TP. TN was a better predictor of biomass than TP when either the DIN : TP ratio was below 1.6, DIN : DIP was below 8.4, or TN : TP below 29; predictions were most accurate when using the DIN : TP ratio. To investigate seasonal and lake-type-specific patterns of N and P limitation, we used the DIN : TP ratio, together with absolute concentrations of DIN and DIP, to predict the limiting nutrient at each lake in each month of the vegetation period. N limitation was much more common in polymictic than stratified lakes. While a high proportion of both stratified and polymictic lakes were P limited in early spring (60–70%), for polymictic lakes, we found a strong shift from P limitation to N limitation in summer: more than 50% of polymictic lakes were N limited between June and September and only 15–30% were P limited. To obtain lake-type-specific nutrient targets we estimated the average TN and TP concentrations at which lakes of different types achieved good ecological status according to EU water framework directive criteria. Stratified lakes achieved good ecological status at concentrations of 400–500 μg L−1 TN or 20–35 μg L−1 TP, while for polymictic lakes values of 500–1000 μg L−1 TN, or 35–75 μg L−1 TP were required. We estimate that nitrogen has an important influence on phytoplankton biovolume, and thus ecological status, for many polymictic lakes in Germany. While there is some uncertainty in the nutrient targets required to achieve good ecological status, this uncertainty is small compared with the range of concentrations currently observed, and lakes with moderate or worse status have concentrations of both TN and TP that are far above these current target estimates.

Imperfect detection and the determination of environmental flows for fish: challenges, implications and solutions

Abstract: Summary Relationships between river flow characteristics and fish community/population dynamics (i.e. flow–ecology relationships) underpin methods to determine and monitor environmental water allocations. Quantifying these relationships can be difficult, and consequently, most environmental flow strategies for fish conservation in Australian rivers are based on general flow–ecology relationships as opposed to statistical predictions. Of those studies that have investigated relationships between flow and fish, most have not accounted for incomplete and variable detection of fish by the sampling methods, thus making the implicit assumption that sampling efficiency is invariant. This important assumption is rarely met, leading to inconsistent research findings and spurious results, and a reliance on generic flow–ecology principles for defining flow management strategies. We illustrate how and when detection probability varies when sampling freshwater fish and the consequences to scientific inference about fish–flow relationships. Methods for accounting for imperfect detection of fish are identified and tools to increase the efficiency of experimental designs while reducing sampling cost are discussed. These tools include methods for borrowing information among experimental components and simulation techniques to optimise sampling designs. We argue that, due to the very nature of sampling designs to quantify flow–ecology relationships (e.g. sampling at different flow magnitudes/regimes), the challenge of imperfect detectability is particularly relevant to environmental flow science. We encourage the broader adoption of methods that account for imperfect detection to improve inference about fish–flow relationships and increase the successful application of environmental flows for managing fish communities.