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River flow regimes face increasing pressure from human activities including water resource management operations and climate change. Consequently, extreme hydrological events are becoming more severe and commonplace, and there is a pressing need to understand and manage their ecological effects. Extreme low-flows (ELFs) – those displaying significantly greater magnitudes and durations than typical low-flow conditions – are being increasingly experienced globally. Fish and macroinvertebrate responses to ELFs have been more widely researched relative to other organism groups in riverine environments, although such studies have employed variable methodological techniques. In this perspective piece, we identify field-based assessments and controlled experiments as two key research paradigms used to examine riverine faunal responses to ELFs. Field-based assessments are often explorative and can benefit from utilising large-scale and long-term datasets. Alternatively, controlled experiments typically employ more hypothesis-driven approaches and can establish strong cause and effect linkages through high replication and control over potentially confounding parameters. Each paradigm clearly possesses their respective strengths, which we highlight and discuss how these could be better harnessed to optimise scientific advancements. To date, studies examining faunal responses to ELFs in these two research paradigms have largely been undertaken in parallel. Here, we argue that future research should seek to develop closer synergies to optimise the quality and quantity of evidence to better understand riverine faunal responses to ELFs. Such scientific advances are of paramount importance given the vulnerability of riverine fauna, and the ecosystems they comprise, as they face a new era of ELFs in many global regions.

Summary Extreme climatic events are a natural feature of climate variability and a major organising force in running waters. Climate change is shifting the occurrence of extremes, and understanding the far-reaching consequences for river and stream ecosystems is a research priority. The aim of this special issue is to unite research on the wide array of contrasting extreme events affecting river and stream ecosystems globally, including heat waves and hot days, fires, droughts, heavy rainfall and floods, tropical cyclones, storm surges and coastal flooding. This issue contains reviews as well as observational and experimental case studies that assess the ecological impacts of extreme events at multiple levels of biological organisation, from individuals to ecosystems and food webs, in river and stream ecosystems from the poles to the tropics. Together, these papers synthesise knowledge of extreme events and their effects, identify research gaps and propose new hypotheses and approaches to guide future research. The contributions reveal that impacts of single events such as catastrophic floods, droughts and heat waves are highly context dependent, ranging from deleterious to beneficial, and are contingent upon event magnitude, extent and timing relative to life cycles of constituent species. Not all extreme events generate extreme ecological impacts, but combinations of events that are cumulative, compound or cross key biological or physical thresholds may have the most adverse ecological consequences. Long-term monitoring programmes and sensor networks are essential in describing rare and usual events. Other approaches, including experiments, will be needed to gain a stronger mechanistic understanding of extreme events, and to predict and manage future events that may be more frequent and intense.

Summary The frequency and intensity of droughts and stream drying are likely to increase with climate change, but how freshwater macroinvertebrate populations respond to such disturbances is poorly understood. Theory predicts that stochastic disturbances should generate density-independent mortality, but density dependence is possible if mortality arises indirectly through changes in consumer–resource dynamics, or if mortality is mediated by disturbance refugia. Empirical evidence for density-dependent or density-independent mortality in freshwater invertebrates is scarce, but important to predict how population sizes will alter with changing climate. In a stream mesocosm experiment, we tested whether short-term drying events (5–6 days) resulted in population losses of macroinvertebrates, and whether any losses were density-dependent or density-independent. On each of 10 replicate drying events over a year and over a naturally occurring range of invertebrate densities, we measured pre- and post-disturbance densities. From these data, we calculated per capita mortality rates, tested statistically for associations between mortality rates and pre-disturbance densities, and assessed whether mortality differed seasonally. Invertebrate assemblages in the channels were derived from a chalk stream in south-west England. Analyses focussed on 12 core taxa that were common in the community, which included a taxonomically diverse group of worms, Gastropoda, Gammaridae, Diptera, Ephemeroptera and Trichoptera. Responses to drying were species-specific and three different response types were observed. Half of the taxa were resistant to drying and there was no apparent mortality, even in the near-total loss of free water (Potamopyrgus antipodarum, Bezzia sp., Chironomini, Radix balthica and two groups of Naididae). Four taxa showed density-independent mortality (Polycentropus flavomaculatus, Tanypodinae, Gammarus pulex, Baetis scambus), but differed in vulnerability with per capita loss rates ranging from 57 to 100%. Strong evidence of density-dependent mortality was observed in Orthocladiinae and suggested also for Tanytarsini. Mortality varied seasonally for only one taxon (Tanytarsini), with higher mortality in warm versus cool months. Particular response types were not necessarily associated with particular taxonomic groups or body shapes. This range of responses to drying by different, naturally co-occurring, populations suggests that community-level impacts of drying are underpinned by diverse, taxon-specific responses. The experiment provides some support for theoretical predictions of density-independent mortality, but also some of the first evidence that disturbance refugia can result in density-dependent mortality.

The biotic resistance hypothesis provides one of several explanations for the limited biological recovery of streams recovering chemically from acidification. The hypothesis proposes that acidification has changed the presence, abundance and interactions among species in acidified streams to the extent that acid-sensitive colonists cannot re-invade even where acidity has ameliorated. As a first step in testing for biotic resistance in streams, we conducted a field experiment to determine whether the success (growth rate) of acid-sensitive recolonists (mayfly nymphs, Baetis rhodani) is reduced by competition with abundant acid-tolerant residents (stonefly nymphs, Leuctra inermis) in a chemically recovering Welsh stream (UK). Gut contents analysis revealed a marked overlap in resource use between the two species. However, when Baetis was exposed to several (0, 0.25, 0.5 and 1 times ambient) densities of its putative competitor, Leuctra, growth rates of the colonist were not affected by the residents at any of the densities tested. These results do not support the hypothesis that resident species constrain colonist populations by affecting growth rates through competition for limited resources or interference. Further work is required to assess whether independent and/or interactive ecological effects of other common residents might affect colonists in ecosystems recovering from past stressors.

Ecological realism is an important yet rarely reported feature of model ecosystems. In this case study, we assess the realism of four outdoor artificial stream mesocosms (4 m2) bordering a chalk river in southern England. Comparisons of physiochemical conditions and benthic macroinvertebrate assemblages were made between the mesocosm units and the parent water body, a side arm of the River Frome. Physicochemistry of the mesocosm replicates was similar to that of the source stream, with congruent temporal variation evident between the real system and each of the models. The high realism of the mesocosms was explained by the outdoor location and close physical proximity of the array to the source stream, and the short mesocosm residence time of water sourced from the parent feeder system. Mesocosms supported a diverse array of benthic macroinvertebrates (60 families from 14 taxonomic orders), including all macroinvertebrate families in the source stream. Individual mesocosms contained a mean of 89% of source stream biota. We conclude that once-through mesocosms can be satisfactory analogues of natural systems, particularly where model and natural scales overlap.

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.

The relationship between mobility and diet in eight common stream insects was investigated in a New Zealand mountain stream. Propensity to drift, distance drifted (return rate to the streambed) and propensity to colonize patches of stony substrata were used as measures of mobility, while diet was assessed by gut contents analysis. The eight insect species formed two well-defined groups on the basis of diet; leaf-eaters (shredders) with homogeneous diets, and grazer-collectors with more heterogeneous gut contents dominated by elements of epilithon and fine particulate detritus. With the exception of the cased caddisfly Olinga feredayi, shredders (two trichopterans, a plecopteran and a scirtid beetle) had a high propensity to drift, settled from the drift slowly, and were strong colonizers of substrate patches. In contrast, grazer-collectors (two ephemeropterans and a plecopteran) exhibited a much lower propensity to drift, returned to the streambed rapidly and colonized stony patches less readily. Despite being very similar in size, body form and mouthpart morphology, the two stoneflies had very different diets and mobility characteristics. By comparison, the two mayflies differed strongly in body form and general behaviour (crawler v swimmer) but had very similar diets and levels of mobility. We propose that the contrasting mobilities of the two groups of insects reflect differences in the distribution (patchiness) of their main foods (leaf detritus and epilithon) on the streambed.

SUMMARY 1. The apparent absence of a specialist herbivorous grazer guild from many acid streams suggests that algae-grazer linkages in acid-stream food webs are weak or absent. It has been hypothesized that the absence of herbivores is a consequence of the low quality and/ or quantity of biofilms in acid streams. 2. We compared the taxonomic composition, biomass and potential nutritional quality of epilithic biofilms from four acid and four circumneutral streams, and examined whether nymphs of a herbivore‐detritivore, the stonefly Nemurella pictetii (Plecoptera: Nemouridae), could grow equally well when fed on eight biofilms from four acid and four circumneutral streams. 3. Biofilms from acid and circumneutral streams differed strongly in algal composition, the former having relatively more coccoid green algae but fewer diatoms and filamentous green algae. Diatom floras differed with stream water pH. 4. The quantity (i.e. area-specific chlorophyll content, algal numbers and AFDM) and quality (biomass-specific protein and soluble carbohydrate content) of biofilms differed significantly, both among sites of similar pH, and overall between the groups of acid and circumneutral streams. 5. Nymphs of N. pictetii grew successfully on biofilms for 8‐10 weeks up to emergence. However, no systematic differences in growth rate were found between the two groups of acid and circumneutral streams. Differences in the digestibility of benthic algae from different sources, and the adjustment of nymphal feeding rates, are discussed in the light of a lack of a clear relationship between growth and food quality.

Many common primary consumers in New Zealand streams are thought to be opportunists that have broad trophic requirements. To test this contention, larval growth of six commonly occurring insect species (Pycnocentria evecta, Pycnocentrodes aureola, Beraeoptera roria, Deleatidium sp. (nr vernale), Nesameletus ornatus and Zelandobius confusus) was compared when fed three contrasting diets (epilithic biofilm, Nothofagus leaves (CPOM) and fine sediment (FPOM)) for 28 days. Larvae of all six species grew well (increasing in dry weight by 60-550% in 28 days) when fed on each of the three diets. Growth of four species (Z. confusus, P. aureola, Deleatidium sp. and B. roria) was affected by diet. Where differences existed, higher mean growth was observed on biofilm than on CPOM and/or FPOM, suggesting that the former resource was of higher quality. Survivorship of larvae was high on all diets (70-100 %), although slightly reduced survival of N. ornatus and B. roria was observed on FPOM. Variation in the growth responses of the six species is discussed in relation to mouthpart morphology, habitat preferences and larval mobility. Our results are consistent with the view that some New Zealand stream invertebrates are trophic generalists.

Summary 1. The extent of spatial and temporal variation, and the taxonomic and biochemical composition of epilithic biofilm were determined in a first-order, acidic stream in the Ashdown Forest, southern England. 2. The biofilm had an autotrophic component consisting of acidobiontic diatoms, small coccoid green algae, zygnematacean filamentous green algae, and a heterotrophic component of bacteria. Fungi were absent. 3. Diatoms were the dominant algal group in spring. These were succeeded by coccoid green algae in summer. Synchronous variation existed between algae and bacteria, suggesting a strong link between the two groups. 4. Quantitative community descriptors (algal and bacterial densities, chlorophyll, ash-free dry matter) and biochemical descriptors (protein, lipid and carbohydrate content) of biofilm presented a consistent temporal pattern in which biofilm biomass peaked in spring and early summer and decreased subsequently. 5. Stepwise multiple regression revealed that temporal variation in the quantity of biofilm coating substrata was primarily associated with photosynthetically active radiation. The quantity of biofilm coating the upper surfaces of substrata increased with substratum particle size (small stones, large stones and bedrock). Smothering of algae by fine particulate organic matter, proximal shading by bed sediments, and physical disturbance are proposed as mechanisms that brought about the pattern.