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Human activities have often altered sediment dynamics in rivers, leading to aquatic habitat degradation. The dominant paradigm in fish–sediment interactions focuses on excessive fine sediments as indicators of habitat and water quality degradation. In contrast, geomorphologists have provided frameworks linking altered sediment dynamics to much broader and more complex changes to channel morphology and stream habitats across spatial scales.In this paper we use an interdisciplinary approach with historical and contemporary channel morphology and fish community datasets to examine how altered sediment dynamics have affected stream fish community evolution in the Bayou Pierre, Mississippi, U.S.A.Erosional process regimes have advanced upstream consistent with models of knickpoint propagation and channel evolution, leaving most of the catchment in a state of increased sediment transport and local sediment deficits. Fish communities in modern samples have less α and β diversity and fewer habitat specialist taxa than in historical samples.Whole‐community analysis via non‐metric multidimensional scaling found a strong gradient of homogenisation towards a fauna comprised of small‐bodied taxa adapted to large rivers.Comparisons of community and geomorphic change on this gradient found that reaches that transitioned between process regimes had more community change, and that local habitat factors relating to advanced channel evolution were strongly positively related to total community change.These results demonstrate that a catchment‐scale geomorphic analysis provided a strong tool for understanding fish community change on a decadal scale. Our findings further demonstrate concordant ecological responses to a complex sequence of disturbances and suggest that a broader view of sediment dynamics offers a rich toolkit for ecological analyses. [ABSTRACT FROM AUTHOR]

1. Differing responses in riparian species richness and composition to disturbance have been reported as a possible explanation for the differences along and between rivers. This paper explores the role of physical disturbance in shaping landscape-scale patterns of species distribution in riparian vegetation along a free-flowing river in northern Sweden. 2. To test whether sensitivity to disturbance varies across large landscapes, we experimentally disturbed riparian vegetation along an entire, free-flowing river by scouring the soil and the vegetation turf, cutting vegetation, applying waterborne plant litter, and after a period of recovery we measured vegetation responses. The experiment was repeated for two consecutive years. 3. We found no significant effect of disturbance on species composition, but all three forms of disturbance significantly reduced species richness. There was no downstream variation in community responses to disturbance but morphological groups of species responded differently to different kinds of disturbance. Graminoids were most resistant, suppressed only by litter burial. All forms of disturbance except cutting reduced the density of herbaceous species, and species density of trees + shrubs and dwarf shrubs was negatively affected by both scouring and cutting. We also evaluated the effects of disturbance in relation to varying levels of species richness. In nearly all cases, responses were significantly negatively correlated with control plot species richness, and relative responses indicated that species-rich plots were less resistant to scouring and cutting. 4. Our results suggest that although all disturbance treatments had an effect on species richness, variation in sensitivity to disturbance is not the most important factor shaping landscape-scale patterns of riparian plant species richness along rivers. [ABSTRACT FROM AUTHOR]

Although most lotic ecosystems experience frequent and sometimes large disturbances, opportunities are uncommon to study primary succession in streams. Exceptions include new stream channels arising from events such as glacial retreat, volcanism, and catastrophic landslides. In 1980, the eruption and massive landslide at Mount St. Helens (WA, U.S.A.) created an entire landscape with five new catchments undergoing primary succession. We asked if riparian and lotic assemblages at early successional stages (36 years after the eruption) showed predictable change along longitudinal gradients within catchments, and whether assemblages were similar among five replicate catchments.In July 2016, we collected environmental data and characterised riparian, algal, and benthic macroinvertebrate assemblages at 21 stream reaches distributed within and among five neighbouring catchments. We evaluated patterns of richness, abundance, biomass, multivariate taxonomic community structure, and functional traits both longitudinally and among catchments.We found minimal evidence that longitudinal gradients had developed within catchments at 36 years post‐eruption. Increases in diatom and macroinvertebrate richness with downstream distance were the only biological responses with longitudinal trends. Conversely, we documented substantial variation in community structure of riparian plants, soft‐bodied algae, diatoms, and macroinvertebrates at the among‐catchment scale. Among‐catchment differences consistently separated two eastern catchments from three western catchments, and these two groups also differed in stream water chemistry, water temperature, and geomorphology.Overall, we documented greater diversity in the young catchments than predicted by ecologists in the years immediately following the eruption, yet functional traits indicate that these catchments are still in relatively early stages of succession. Variation at the among‐catchment scale is likely to be driven in part by hydrological source variation, with the two eastern catchments showing environmental signatures associated with glacial ice‐melt and the three western catchments probably fed primarily by springs from groundwater aquifers. Contemporary flow disturbance regimes also varied among catchments and successional trajectories were probably reset repeatedly in streams experiencing more frequent disturbance.Similar to new stream channels formed following glacial retreat, our results support a tolerance model of succession in streams. However, contrasting abiotic templates among Mount St. Helens catchments appear to be driving different successional trajectories of riparian plant, algal, and macroinvertebrate assemblages among neighbouring small catchments sharing the same catastrophic disturbance history. [ABSTRACT FROM AUTHOR]

Many once‐perennial rivers have become intermittent. Channel drying can result in fish mortality if refuges are not available. Understanding where refuges occur and if fishes use these refuges can provide insight for species persistence and help stakeholders manage limited resources. Streamflow diversions in the Rio Grande of New Mexico can result in >60 km losses of aquatic habitat, affecting up to 30% of the range of imperiled Rio Grande silvery minnow (Hybognathus amarus). Potential refuges include areas with perennial flow below diversion dams, isolated pools, and irrigation return flows.We examined spatial and temporal patterns of both adult and young‐of‐year Rio Grande silvery minnow collected in isolated pools that formed during streamflow intermittency from 2009 to 2019. We hypothesised that: (1) Rio Grande silvery minnow would be more numerous in pools that persisted longer; (2) they would be more numerous in isolated pools located closer to upstream areas of perennial flow, due to upstream movement to escape drying; and (3) increased rate of aquatic habitat loss each day would result in more Rio Grande silvery minnow in isolated pools.During the 12 years of the study, we counted Rio Grande silvery minnow in 3,985 isolated pools that formed during streamflow intermittency. We related counts of Rio Grande silvery minnow in each pool to the maximum pool depth, rate of loss of aquatic habitat that occurred that day, and distance each pool was to an upstream barrier. In 2016, we examined persistence of 290 isolated pools until complete desiccation or reconnection with continuous flows occurred, and the factors that influenced pool persistence.Deeper pools persisted for longer, but depth had a small positive effect on counts of adult Rio Grande silvery minnow and no effect on counts of young‐of‐year in isolated pools. Adults were more numerous in upstream isolated pools, whereas young‐of‐year were more numerous in downstream isolated pools. Rate of channel drying had little effect on the numbers of adult Rio Grande silvery minnow in isolated pools, but more young‐of‐year were stranded when the rate of drying was faster. On average, pools persisted <4 days and 263 of 290 dried completely before continuous flows returned. Only 66 of 4,749 Rio Grande silvery minnow occurred in pools that did not dry completely.Rio Grande silvery minnow did not appear to escape channel intermittency; instead, they became stranded in shrinking isolated pools that did not persist long enough to act as refuges for fishes. Lack of refuge during channel intermittency would result in catastrophic mortality of fishes through complete desiccation of pools if there were no management actions, such as translocating fish. To increase persistence through streamflow intermittency, conservation actions should match the species response to intermittency by ensuring the availability of perennial‐water refuges at the appropriate spatial and temporal scale. [ABSTRACT FROM AUTHOR]

Pulsed flow disturbances strongly influence invertebrate drift in lotic ecosystems. However, drift-flow relationships are often context dependent and non-intuitive, suggesting that local abiotic and biotic conditions mediate the impacts of flow on the physical and behavioural drivers of drift entry. Two factors may be particularly important: physical channel architecture, which modulates how flow influences velocity and habitat area; and species traits, which determine behavioural responses to flow and susceptibility to passive entrainment., We examined how channel architecture and species traits (e.g. mobility, body size, and dislodgement susceptibility) mediated the effects of flow on bulk drift abundance and taxa-specific per capita drift rates (the rate of emigration from the benthos). In complementary experiments, we reduced and increased flows in stream mesocosms with contrasting cross-sectional channel profiles: concave channels, where habitat area contracted and expanded with altered flow but velocity remained relatively constant; and flat channels, which maintained constant habitat area but experienced greater changes in velocity., Total drift concentration increased following flow reductions and decreased following flow increases whereas drift flux (the total number drifting) showed the opposite pattern. Channel architecture did not influence drift during flow reductions, but during flow increases drift flux and concentration were amplified in flat channels that experienced larger increases in velocity and shear stress., Contrasting responses among individual taxa to flow manipulation were explained by variation in mobility (swimming and crawling ability) and body shape (susceptibility to drag). Per capita drift rates for the most mobile taxa increased c. 10% under flow reduction, indicating a behavioural response, whereas drift of other taxa declined. Per capita drift increased for all taxa following elevated flow but by the largest magnitude in taxa with body shapes that experience more drag, suggesting passive dislodgement., Our results imply that: (i) stream channel architecture can modify the impacts of flow increases on stream invertebrates; and (ii) invertebrate taxa vary in their vulnerability and behavioural responses to flow disturbance. Together these inferences clarify some of the previously unexplained context-dependent responses of drift to flow disturbances. [ABSTRACT FROM AUTHOR]

Extreme hydrological events (floods, droughts) in flowing waters can alter habitat availability and structure and produce significant shifts in biodiversity, species assemblages and ecosystem processes., Extreme floods and droughts occur over large spatial extents and can greatly alter links between streams and their catchments. The nature and strengths of these links differ between degraded and intact streams and their catchments, strongly influencing the responses to and recovery from extreme events. It is essential that these catchment-stream linkages be identified and considered when planning stream restoration., Despite the scarcity of comprehensive, long-term monitoring of most stream restoration projects, our understanding of the influence of extreme events on running waters suggests both positive and negative impacts on restoration efforts. Extreme hydrological events can impair stream restoration efforts giving rise to delays or complete failures in achieving restoration objectives. Alternatively, extreme floods may facilitate progress towards restoration goals by engendering key ecological processes., Understanding the impacts and the temporal-spatial scales over which extreme events alter the ecological structure and function of streams (in both natural and damaged systems), as well as their underlying mechanisms, is a challenge for stream restorers that will only progress with the accumulation of long-term data. Restorers should recognise and plan for hydrological extreme events and their interactions with other disturbances (e.g. fire, urban development). Increasing connectivity of restoration efforts along with their spatial extent can lessen the negative effects of extreme hydrological events on stream restoration., Successful ecological stream restoration requires long time spans and thus with a changing climate the prediction of more frequent and intense extreme events only strengthens the need to incorporate their likelihood into restoration planning and implementation. [ABSTRACT FROM AUTHOR]

Climate change is expected to intensify drought in many regions, but ecological impacts on stream communities are poorly understood. Many arid-land streams are characterised by predictable seasonal cycles of wetting and drying, to which species are adapted, but unpredictable supraseasonal droughts may constitute extreme events that challenge resident biota., In this article, we synthesise research conducted in arid-land streams of the Madrean Sky Islands (MSI) in Arizona, U.S.A, to evaluate the resistance and resilience of invertebrate communities to drying disturbances caused by normal seasonal drying and severe supraseasonal drought. We also highlight how spatial context (e.g. distance to perennial refuges) influences recovery patterns., Invertebrate community structure changes predictably as habitat contraction progresses from loss of lateral connectivity to complete drying of MSI streams. When drying events are predictable (e.g. seasonal drying), post-drought community recovery is often rapid, since most MSI taxa possess life history traits conferring high resistance and/or resilience to stream drying., Extreme supraseasonal droughts, in contrast, cause unprecedented transitions from perennial to intermittent flow in some MSI streams. While species richness may recover quickly following this flow regime shift, marked turnover in community structure can occur and may delay or preclude recovery to pre-drought conditions. In such cases, short-lived (<1 year) strong dispersers replace relatively long-lived (≥1 year) weak dispersers. As habitat isolation increases, the potential for community recovery from extreme drought decreases., Many MSI aquatic species are threatened by extreme drought. Extinctions of endemic aquatic species due to habitat drying have already been observed in nearby deserts. Further studies are urgently needed to identify drought-sensitive species and understand how the loss of such species may affect stream ecosystem functioning. [ABSTRACT FROM AUTHOR]

Global climate change can increase the mean and variability of temperature and may increase the occurrence of extremes such as heatwaves. Both gradual and abrupt warming may perturb freshwater communities, but understanding of ecological resilience to such events is limited., We report the response of mollusc communities in the River Saone and its two main tributaries to the European heatwave of 2003 and determine the extent of community recovery over 8 years following this rare extreme event, during a period of gradual warming., The 2003 heatwave had a major impact on the density and species richness of mollusc communities across the study area. After the heatwave, abiotic conditions favoured recolonisation by molluscs, yet full recovery of community states (defined as the combination of structure × density × species richness) was not observed at any site., Given the profound changes observed in the mollusc community structure in the Saone River and in its two main tributaries and the observed slow pace of recovery relative to that observed elsewhere for pulse disturbances (typically < 3 years), we suggest that global change, including warming, may preclude community recovery from pulse disturbances, such as extreme events, and instead yield communities comprising new combinations of species. [ABSTRACT FROM AUTHOR]

Flood-related disturbances are predicted to be seriously altered by climate change effects, and this will have strong implications for stream communities. Predicting how and why community structure responds to changes in disturbance regimes will require measures of disturbance that are closely linked to community variability. A range of disturbance measures have been tested for their ability to explain patterns in stream periphyton and invertebrate assemblages, but assessments of fish have largely focussed on flow measures as predictors. Consequently, the mechanisms driving fish assemblage responses to disturbance are poorly known., Stream disturbance was quantified using seven measures, three based on disturbance of the streambed (assessed using painted tracer particles), three associated with variation in stream discharge and one from a subjective measure of stream channel stability. Twenty streams were sampled on five occasions to evaluate which disturbance measure explained the most variation in fish assemblages and also to quantify the influence of disturbance on spatial and temporal assemblage variability. To determine whether disturbance affected fish directly or via its effect on benthic food supply (i.e. stream invertebrates), a one-off survey of 52 streams was conducted., Path analysis on data from the 20 streams indicated that fish biomass responded most strongly to flow-mediated bed movement, not flow variability. Consequently, bed-movement measures were better predictors of spatial and temporal variability in fish biomass and assemblage structure than purely hydrological measures. Furthermore, variations in fish biomass and assemblage structure were higher in more disturbed streams. A second path analysis conducted on data from the 52 stream survey compared the effects of physical habitat and food-related components of bed disturbance on fish biomass. It indicated that both had a similar influence on disturbance-related reductions in fish biomass., These results indicate that variation in fish assemblages can best be understood by measuring or predicting bed movement, because bed disturbance is likely to encompass more effectively major factors that influence stream fish, such as habitat availability and food supply. A shift in disturbance regime is likely to change the biomass of fish a habitat can support and fish assemblage structure. [ABSTRACT FROM AUTHOR]

1. Unpredictable, extreme climatic events (e.g. droughts) can potentially destabilize aquatic communities. From 1998 to 2002, southwestern Georgia, U.S.A., experienced the third worst drought of the last 100 years, leading to loss of surface flow in many small streams. We sampled macroinvertebrates, flow and water chemistry in small headwater streams from 2001 to 2007 in two adjacent coastal plain streams of contrasting headwater type (wetland and groundwater-fed seep) following resumption of flow. 2. Decreasing water temperature, conductivity and nutrient concentrations within the first 2 years of the study indicated flushing of the streambed associated with increased discharge. Invertebrate community composition became less variable over time and during wetter years, reflecting water chemistry, hydrological and climate conditions. 3. A core set of species appeared immediately following breaking of the drought in both streams, reflecting a shared species pool. These species exhibited resilience traits, including short life cycles and resistance to desiccation, which allowed for rapid recovery from disturbance. Such species, which were small-bodied, sclerotized and abundant in the drift, were then replaced as flows increased by those that were larger, soft-bodied and rare in drift, suggesting a more stable and less ephemeral habitat. 4. Hydrologic regime and long-term precipitation indices were strongly correlated with invertebrate community and trait structure. Long-term data allowed for better interpretation of the effects of infrequent disturbances on aquatic ecosystems. Additionally, long-term precipitation indices (i.e. 48-month standardized precipitation index) can indicate the likelihood of a return to drought, allowing for the collection of pre-disturbance data. [ABSTRACT FROM AUTHOR]

1. We propose a model of plant strategies in temperate fluvial hydrosystems that considers the hydraulic and geomorphic features that control plant recruitment, establishment and growth in river floodplains. 2. The model describes first how the disturbance gradient and the grain-size of the river bed load affect the relative proportion of erosion and deposition processes, and how the frequency of flood disturbance affects the intensity of such processes. 3. Secondly, the model predicts plant strategies according to direct and indirect effects of floods (disturbances through erosion versus deposition processes, and associated nutrient excess or limitation). 4. The relevance of the model as a prediction tool is discussed. Some proposals are made to validate the model, and traits are proposed that should be considered in future research for improving the predicting value of the model. [ABSTRACT FROM AUTHOR]

1. Agriculture causes high sediment, nutrient and light input to streams, which may affect rates of ecosystem processes, such as organic matter decay. In the southern Appalachians, socioeconomic trends over the past 50 years have caused widespread abandonment of farmland with subsequent reforestation. Physical and chemical properties of streams in these reforested areas may be returning to pre-agriculture levels thereby creating the potential for recovery of ecosystem processes. 2. We examined wood breakdown and microbial activity on wood substrata in streams with different historical and current agricultural activity in their catchments. We analysed historical (1950) and recent (1998) forested land cover from large areas of the southern Appalachians and categorized streams based on percent forested land cover in these two time periods. Categories included a gradient of current agriculture from forested to heavily agricultural and reforestation from agriculture due to land abandonment. We compared microbial respiration on wood veneer substrata and breakdown of wood veneers among these land-use categories. We also compared temperature, sediment accumulation and nitrogen and phosphorus concentrations. 3. Streams with current agriculture had higher concentrations of dissolved inorganic nitrogen than forested streams. Despite reforestation from agriculture, nitrogen concentrations were also elevated in streams with agricultural histories relative to forested streams. Temperature was also higher in agricultural streams but appeared to recover from historical agriculture through reforestation and stream shading. 4. Wood breakdown rates ranged from 0.0015 to 0.0076 day−1 and were similar to other studies using wood veneers to determine breakdown rate. Microbial respiration increased with incubation time in streams up to approximately 150 days, after which it remained constant. Neither wood breakdown nor microbial respiration was significantly different among land-use categories, despite the observed physical and chemical differences in streams based on land-use. Wood breakdown rates could be predicted by microbial respiration indicating microbial control of wood breakdown in these streams. Both breakdown and microbial respiration were negatively correlated with the amount of inorganic sediment accumulated on wood veneers. 5. Higher nutrients and temperature led us to expect faster breakdown and higher microbial respiration in agricultural streams, but sediment in these streams may be limiting microbial activity and breakdown of organic material resulting in little net effect of agriculture on wood breakdown. Wood may not be desirable as a tool for functional assessment of stream integrity due to its unpredictable response to agriculture. [ABSTRACT FROM AUTHOR]

1. This study analysed changes occurring in Mediterranean stream fish assemblages over a sequence of dry years followed by a generally wet period (1991–98). Variations in assemblage attributes were quantified at the basin and stream reach scales, and related to variables reflecting the occurrence of unusually dry or wet conditions. 2. Assemblage variability increased along with the resolution of analysis, with little changes in species richness, composition and rank abundances, but significant variation in individual species abundances. Fluctuations in relative abundances were significantly affected by variables reflecting the severity of summer droughts and the occurrence of rainy springs. These patterns were evident at the basin scale, while variability at individual stream reaches tended to be higher and less related to rainfall patterns. 3. At least three response guilds to rainfall variation could be identified: two of the four abundant and widespread species (chub and loach) declined following dry years, whereas the two other core species (nase and eel) declined after rainy spring; one scarce native species (stickleback) increased in dry years. 4. Except at the two most upstream reaches, the assemblages tended to recover quickly to previous configuration after the changes occurring during the sequence of dry years. 5. Temporal variability of local assemblages was concordant among reaches but did not follow any consistent spatial pattern. Instead, spatial patterns in assemblage attributes changed over time in response to environmental variability, with a tendency for a disruption of upstream–downstream gradients following dry years. 6. Results supported the view that present-day droughts cause relatively small and transient changes to Mediterranean stream fish assemblages. However, longer and more severe droughts expected under altered future climates, may result in declines or local extinctions of the most sensitive species and their potential replacement by more resistant species. Changing drought regimes thus need to be duly considered in the development of conservation strategies for Mediterranean stream fish. [ABSTRACT FROM AUTHOR]

1. Nutrient enrichment and resulting eutrophication is a widespread anthropogenic influence on freshwater ecosystems, but recovery from nutrient enrichment is poorly understood, especially in stream environments. We examined multi-year patterns in community recovery from experimental low-concentration nutrient enrichment (N + P or P only) in three reaches of two Arctic tundra streams (Kuparuk River and Oksrukuyik Creek) on the North Slope of Alaska (U.S.A.). 2. Rates of recovery varied among community components and depended on duration of enrichment (2–13 consecutive growing seasons). Biomass of epilithic algae returned to reference levels rapidly (within 2 years), regardless of nutrients added or enrichment duration. Aquatic bryophyte cover, which increased greatly in the Kuparuk River only after long-term enrichment (8 years), took 8 years of recovery to approach reference levels, after storms had scoured most remnant moss in the recovering reach. 3. Multi-year persistence of bryophytes in the Kuparuk River appeared to prevent recovery of insect populations that had either been positively (e.g. the mayfly Ephemerella, most chironomid midge taxa) or negatively (e.g. the tube-building chironomid Orthocladius rivulorum) affected by this shift in dominant primary producer. These lags in recovery (of >3 years) were probably driven by the persistent effect of bryophytes on physical benthic habitat. 4. Summer growth rates of Arctic grayling (both adults and young-of-year) in Oksrukuyik Creek (fertilised for 6 years with no bryophyte colonisation), which were consistently increased by nutrient addition, returned to reference rates within 1–2 years. 5. Rates of recovery of these virtually pristine Arctic stream ecosystems from low-level nutrient enrichment appeared to be controlled largely by duration of enrichment, mediated through physical habitat shifts caused by eventual bryophyte colonisation, and subsequent physical disturbance that removed bryophytes. Nutrient enrichment of oligotrophic Arctic stream ecosystems caused by climate change or local anthropogenic activity may have dramatic and persistent consequences if it results in the colonisation of long-lived primary producers that alter physical habitat. [ABSTRACT FROM AUTHOR]

1. We investigated the effects of local disturbance history and habitat parameters (abiotic and biotic) on the microdistribution of benthic invertebrates during several floods in two streams, the Schmiedlaine in Germany (four events) and the Kye Burn in New Zealand (two events). 2. Bed movement patterns were quantified using metal-link scour chains. Before and after each flood, quantitative invertebrate samples were taken from replicate bed patches that had experienced sediment scour, fill or remained stable. 3. Patterns of invertebrate density in the different bed stability types (i.e. scour, fill, stable) varied between floods, sampling dates and streams, but invertebrate density was highest in stable patches in >50% of all the patch type effects detected and lowest in fill patches in 75% of all detected effects. Stable bed patches acted as a refugium for Liponeura spp. and Leuctra spp. in the Schmiedlaine and for Hydracarina and Deleatidium spp. in the Kye Burn. 4. Averaged across both streams, only near-bed current velocity was correlated with invertebrate distribution on the streambed more often than disturbance history. In the Kye Burn, disturbance history and water depth were the most influential habitat parameters. 5. Our results suggest that a thorough understanding of the microdistribution of benthic invertebrates requires knowledge of disturbance history, as well as more readily measured habitat parameters such as current velocity or water depth. [ABSTRACT FROM AUTHOR]

1. We investigated the effects of a flood on the fauna and physical habitat of the hyporheic zone of the Kye Burn, a fourth order gravel-bed stream in New Zealand.2. Freeze core hyporheic samples (to 50 cm depth) and benthic samples (to 10 cm) were taken, along with measurements of vertical hydrological gradient, before, 2 days after and 1 month after the flood (estimated return period: 1.5 years, estimatedQmax = 10.4 m3 s−1).3. The composition of the hyporheos differed over the three sampling occasions with fewer taxa collected immediately postflood than preflood. The equitability of the community was higher on both postflood occasions, consistent with the reduced densities of two abundant taxa (Leptophlebiidae and Copepoda).4. Total invertebrate abundance was lower on the postflood occasions than preflood in both benthic (0–10 cm) and hyporheic (10–50 cm) sediments. Several taxa, including asellotan isopods and amphipods, recovered within 1 month of the event. Hyporheic densities of larvalHydoraand nematodes did not differ among the three sampling occasions, but the water mitePseudotryssaturuswas more abundant 1 month after the flood than preflood. There was no evidence of vertical movements (to 50 cm) by any taxa in response to the flood.5. The proportion of fine sediments (<1 mm) in the subsurface sediments (10–50 cm) increased over the three sampling occasions and median particle size declined, but sediment porosity did not change. More particulate organic matter was found in the sediments after the flood.6. Our study provides little evidence that the hyporheic zone (to 50 cm) acted as a significant refuge during the flood event, although movements to or recolonisation from sediments deeper than 50 cm could explain the recovery of many crustacean and mite taxa within 1 month. [ABSTRACT FROM AUTHOR]

1. The ability of many introduced fish species to thrive in degraded aquatic habitats and their potential to impact on aquatic ecosystem structure and function suggest that introduced fish may represent both a symptom and a cause of decline in river health and the integrity of native aquatic communities.2. The varying sensitivities of many commonly introduced fish species to degraded stream conditions, the mechanism and reason for their introduction and the differential susceptibility of local stream habitats to invasion because of the environmental and biological characteristics of the receiving water body, are all confounding factors that may obscure the interpretation of patterns of introduced fish species distribution and abundance and therefore their reliability as indicators of river health.3. In the present study, we address the question of whether alien fish (i.e. those species introduced from other countries) are a reliable indicator of the health of streams and rivers in south-eastern Queensland, Australia. We examine the relationships of alien fish species distributions and indices of abundance and biomass with the natural environmental features, the biotic characteristics of the local native fish assemblages and indicators of anthropogenic disturbance at a large number of sites subject to varying sources and intensities of human impact.4. Alien fish species were found to be widespread and often abundant in south-eastern Queensland rivers and streams, and the five species collected were considered to be relatively tolerant to river degradation, making them good candidate indicators of river health. Variation in alien species indices was unrelated to the size of the study sites, the sampling effort expended or natural environmental gradients. The biological resistance of the native fish fauna was not concluded to be an important factor mediating invasion success by alien species. Variation in alien fish indices was, however, strongly related to indicators of disturbance intensity describing local in-stream habitat and riparian degradation, water quality and surrounding land use, particularly the amount of urban development in the catchment.5. Potential confounding factors that may influence the likelihood of introduction and successful establishment of an alien species and the implications of these factors for river bioassessment are discussed. We conclude that the potentially strong impact that many alien fish species can have on the biological integrity of natural aquatic ecosystems, together with their potential to be used as an initial basis to find out other forms of human disturbance impacts, suggest that some alien species (particularly species from the family Poeciliidae) can represent a reliable‘first cut’ indicator of river health. [ABSTRACT FROM AUTHOR]

1. Perturbation pathways affecting interactions between feeding habitat, food supplies and diet of a lotic avian predator, blue duck ( Hymenolaimus malacorhynchos Gemlin), were tracked in a New Zealand river following substantial inputs of sediment from a volcanic eruption. Sediment impacts were separated temporally into two distinct phases: (i) deposition of fine ash from volcanic fall-out, and (ii) pulsed releases of volcanic sand and gravel retained by an upstream dam. 2. Levels of interstitial suspendable inorganic sediments increased by several orders of magnitude following ash inputs to the river, but returned to low levels within 3 months. Flushing of volcanic sand and gravel retained by the dam resulted in sediment deposition upstream of a large island where coarse material lodged firmly between larger benthic substratum elements in blue duck feeding habitat. 3. Changes in algal biomass appeared to largely reflect seasonal and hydrological influences, but the percentage inorganic content of periphyton increased significantly during both posteruption phases. 4. Diversity and abundance of blue duck food supplies on boulders and on the riverbed declined following the eruption, whereas significant impacts on biomass were only detected in benthic habitats which were more susceptible to sediment deposition. However, percent composition of the benthic and boulder invertebrate food resources appeared largely unaffected by the eruption. 5. There was no apparent association between the eruption and the composition or electivity ( E*) of blue duck diet, but there was a significant increase in the percent inorganic content of faeces, suggesting a reduction in the quality of food rations. 6. Changes in food quantity and quality associated with deposition of volcanic sediments were linked to an observed decline in blue duck population density and fecundity following the eruption. An interacting continuum of adaptive responses to perturbation intensity and impact duration is proposed for blue duck, and potentially other long-lived, riverine birds, that enable them to cope with disturbances in highly variable and heterogenous lotic environments. [ABSTRACT FROM AUTHOR]

Summary 1. It is axiomatic that unusually long dry periods (droughts) adversely affect aquatic biota. Recovery after drought is rapid by macroinvertebrates that possess strategies to survive drying or are highly mobile but other taxa take longer to recolonise depending on the timing, intensity, and duration of the dry phase. 2. Although drought acts as a sustained ‘ramp’ disturbance, impacts may be disproportionately severe when certain critical thresholds are exceeded. For example, ecological changes may be gradual while a riffle dries but cessation of flow causes abrupt loss of a specific habitat, alteration of physicochemical conditions in pools downstream, and fragmentation of the river ecosystem. Many ecological responses to drought within these habitats apparently depend on the timing and rapidity of hydrological transitions across these thresholds, exhibiting a ‘stepped’ response alternating between gradual change while a threshold is approached followed by a swift transition when a habitat disappears or is fragmented. 3. In two Australian intermittent streams, drought conditions eliminated or decimated several groups of macroinvertebrates, including atyid shrimps, stoneflies and free-living caddisflies. These taxa persisted during the early stages of the drought but did not recruit successfully the following year, despite a return to higher-than-baseflow conditions. This ‘lag effect’ in response to drought emphasises the value of long-term survey data. Although changes in faunal composition were inconsistent among sites, marked shifts in taxa richness, abundance and trophic organisation after the riffle habitat dried provide evidence for a stepped response. 4. Responses by macroinvertebrate assemblages to droughts of differing severity in English chalk streams were variable. The prolonged 1988–92 drought had a greater impact than shorter droughts in the early 1970s but recovery over the next 3 years was... [ABSTRACT FROM AUTHOR]

Summary 1. Drought is a natural disturbance of aquatic ecosystems and can be a major factor in structuring aquatic communities. For individuals, populations and communities to persist in disturbed environments, they must have refuge from disturbance or disturbance must be minimal. Refugia convey spatial and temporal resistance or resilience in the face of disturbance, but the role of refugia in aquatic systems remains poorly understood. 2. We review available literature on aquatic refugia for fishes in order to synthesise current knowledge and provide suggestions for needed research. Our objectives were to clarify definitions of disturbance and refugia in the context of drought in aquatic systems, review how refuge habitats influence fish community structure, and consider the potential impact of refugia on fish population and community dynamics during drought. 3. Droughts cause a decrease in surface area/volume and an increase in extremes of physical and chemical water quality parameters. These conditions are linked with biotic interactions that structure the community of fishes residing in low-flow or dry season refugia by increasing mortality rates, decreasing birth rates and/or increasing migration rates. Many aquatic organisms seek refuge from disturbance and/or have adaptations (e.g. physiological tolerance) that provide refuge. 4. Drought in aquatic systems leads to shifts in refugia spacing and connectance at multiple spatial and temporal scales. Refuge size, disturbance intensity, and mobility of organisms is predicted to play a large role in population persistence. We expect that refuge habitats will experience net immigration during drying and net emigration after rewetting, with the opposite occurring in surrounding habitat patches. Population dynamics of fishes using refugia during drought are best modelled by modified source-sink dynamics, but dynamics are likely to change with spatial scale. [ABSTRACT FROM AUTHOR]

SUMMARY 1. The effects of physical disturbance in terms of wave exposure, shore slope and substrate mobility on the presence, species richness, cover and depth limits of the low-growing, shallow water macrophyte community (called the low mixed community) were examined at 41 shore sites in Lake Wanaka, South Island, New Zealand. 2. Wave exposure at a site was described by the previous year's maximum depth of sediment motion and maximum vertical extension of waves, determined from a computer wave model. Shore slope at each site was recorded from -1 to 0 m depth, and sediment stability was assessed as the cover of small gravel. 3. The low mixed community was only present on sites where the previous year's maximum depth of sediment motion was <8 m, maximum wave run-up was <0.3 m, shore slope <0.12 m m-1 , and small gravel cover <78%. 4. Species richness, cover and depth limits of the low mixed community decreased with increasing disturbance on the sites. Sixty-two percentage of the variation in species richness could be explained by physical disturbance variables when all sites were included (N=41). When only sites with a low mixed community were included (N=22), only 18% of the variation was explained. Species richness within sites supporting a low mixed community is thus poorly explained by physical disturbance, whereas presence or absence is better explained. There was no evidence to support the intermediate disturbance hypothesis in this study. 5. Disturbance as a result of waves explained 86% of the variation in cover among sites with a low mixed community and accounted for 68 and 58% of the variation in upper and lower depth limit of the low mixed community, respectively. 6. The models obtained in this study can be used as predictive models for the low mixed community in New Zealand lakes in relation to natural physical disturbance on the shore. By integrating these results with previous studies on the effect of water level fluctuation, we describe... [ABSTRACT FROM AUTHOR]

SUMMARY 1. Microform bed clusters are bedform microunits in streams. They consist of an obstacle clast (a large surface stone) against which other stones are stacked in a specific manner. Stream ecologists have suggested that these clusters are better flood refugia and more valuable habitats for invertebrates than single surface stones because of their higher stability during high-flow events and their greater diversity of microhabitats available for colonisation. 2. To test these predictions in the Schmiedlaine, a flood-prone prealpine stream, we sampled invertebrates on clustered and single stones on a total of seven occasions including before and after a moderate spate, two major floods and a minor spate. We also monitored surface particle stability during the second major flood and the minor spate. 3. Before the second flood (return period 1–2 years), we determined the exact positions of 60 clusters (60 obstacle clasts with 136 associated smaller particles) and 50 single stones in a 250-m reach. We sampled the fauna on stones in the clusters, on well-embedded single stones, and on loose single stones. Only six obstacle clasts and one single stone remained after the flood, implying movement of almost the entire surface layer. Therefore, we sampled the invertebrates on stones in newly formed clusters, on well-embedded and single loose stones 3 and 11 days after the flood. The minor spate, in contrast to the second flood, moved only one of 495 monitored surface particles. 4. Three days after the flood, invertebrate density on clustered stones and both single stone categories were similar and equalled 34% of the mean pre-flood density. Eight days later, density had almost doubled. The relatively high survival and rapid recovery suggest that invertebrates found refugia during the flood. However, the very low stability of clusters and single stones implied that surface particles were unimportant as refugia. 5. Total invertebrate density and taxon richness... [ABSTRACT FROM AUTHOR]

1. The palatability of aquatic macrophytes to the snail Lymnaea stagnalis was investigated in the laboratory. Eight species of macrophyte were selected from habitats that differed in either flood disturbance regime or nutrient status. 2. In a non-choice test, single macrophyte species were offered to individual snails. The average amount of plant dry mass consumed per Lymnaea dry mass ranged from 3.6 ± 1.4 (±SE) to 63.6 ± 13.9 mg g–1 day–1 across plant species. In a choice test, all eight plant species were presented simultaneously to sets of five snails. The average total consumption was 66.1 ± 3.8 mg g–1 day–1 and the maximum average consumption for a single plant was 26.2 ± 3.6 mg g–1 day–1 . 3. In both tests, the amount consumed by snails differed significantly between the plant species. The species growing in undisturbed habitats were the least consumed. Habitat nutrient status was unrelated to plant palatability. 4. These results suggest that macrophyte species growing in habitats that are rarely disturbed by floods allocate a greater proportion of their resources to resisting herbivory. [ABSTRACT FROM AUTHOR]

1. The aim of ecological risk assessments is to evaluate the likelihood that ecosystems are adversely affected by human-induced disturbance that brings the ecosystem into a new dynamic equilibrium with a simpler structure and lower potential energy. The risk probability depends on the threshold capacity of the system (resistance) and on the capacity of the system to return to a state of equilibrium (resilience). 2. There are two complementary approaches to assessing ecological risks of riverine landscape dynamics. The reductionist approach aims at identifying risk to the ecosystem on the basis of accumulated data on simple stressor–effect relationships. The holistic approach aims at taking the whole ecosystem performance into account, which implies meso-scale analysis. 3. Landscape patterns and their dynamics represent the physical framework of processes determining the ecosystem's equilibrium. Assessing risks of landscape dynamics to riverine ecosystems implies addressing complex interactions of system components (e.g. population dynamics and biogeochemical cycles) occurring at multiple scales of space and time. 4. One of the most important steps in ecological risk assessment is to establish clear assessment endpoints (e.g. vital ecosystem and landscape attributes). Their formulation must recognise that riverine ecosystems are dynamic, structurally complex and composed of both deterministic and stochastic components. 5. Remote sensing (geo)statistics and geographical information systems are primary tools for quantifying spatial and temporal components of riverine ecosystem and landscape attributes. 6. The difficulty to experiment at the riverine landscape level means that ecological risk management is heavily dependent on models. Current models are targeted towards simulating ecological risk at levels ranging from single species to habitats, food webs and meta-populations to ecosystems and entire riverine landscapes, with some including socio-economic... [ABSTRACT FROM AUTHOR]

1. Environmental variables, benthic algal biomass and macroinvertebrate fauna were examined from September 1999 to January 2000 (austral summer) along two glacier-fed rivers in South Island, New Zealand. 2. The rivers were characterized by high flow variability, high turbidity and physically disturbed beds. Water temperature ranged from <1 °C near the glacier margin to 10 °C further downstream. 3. Epilithic algal biomass was very low (<0.1 mg m–2) in months characterized by heavy rainfall, but ranged from 1.1 to 14.4 mg m–2 following an extended period with negligible precipitation. 4. Abundance and diversity of invertebrates in both rivers was low. Dominant taxa were Chironomidae (Orthocladiinae, Podonominae, Diamesinae), although mayfly species (Deleatidium: Leptophlebiidae) also occurred at most sites. A species of Eukiefferiella (Orthocladiinae) was collected at all sites and was the most abundant invertebrate close to the glacier margins. No meiofauna were found in either river. 5. Faunal diversity increased at the lowermost stations where species of Plecoptera, Trichoptera, Coleoptera and non-chironomid Diptera also occurred. 6. The faunas of the two New Zealand rivers conformed to the conceptual model of Milner & Petts (1994) in that taxon richness increased downstream with water temperature. However, invertebrate abundance increased downstream in only one of the two rivers. Also in contrast to the model predictions, Leptophlebiidae and Orthocladiinae, rather than Diamesinae, dominated the fauna at the coldest sites. [ABSTRACT FROM AUTHOR]

1. The extensive British canal system potentially provides a favourable habitat for aquatic plants and, because of its uniformity, it is ideal for studying relationships between disturbance intensity (largely from boat traffic) and communities of aquatic macrophytes. 2. The standing crop and species composition of aquatic plants were measured in summer at 396 sites distributed randomly over the canal network. We also quantified the number of rare species and ‘attribute groups’ (groups of species sharing similar suites of biological traits). These data were analysed in relation to standing crop, assumed to indicate a disturbance gradient. 3. Consistent with unimodal models and the Intermediate Disturbance Hypothesis, maximum hydrophyte species richness occurred at an intermediate biomass (50–200 g DWm-2). This corresponded to a low frequency of low magnitude disturbance (light boat traffic) on navigable sections, or occasional high magnitude disturbance (the post-interventionist phase) on sections currently unnavigable and subject to active vegetation management. The frequency of rare species was also the greatest under these conditions, reflecting the availability of regeneration niches. 4. Sorting of species into attribute groups revealed that the overall relationship between species richness and standing crop comprised of closely overlapping unimodal responses of nine attribute groups, superimposed on a core vegetation of Potamogeton pectinatus, together with greater representation of filamentous algae, lemnids and elodeids with increasing standing crop (i.e. assumed low disturbance). High species richness was associated with the overlap of functionally different groups of species, rather than with disturbance-mediated coexistence of functionally similar plants. 5. The analysis of a matrix of sites and the representation of plant traits weighted by the biomass of species that displayed them, in relation to different aspects of disturbance, highlighted a shift from traits associated with resilience (turions, unanchored floating or submerged leaves, low body flexibility, budding, small body size), or competitiveness (entire leaves, low reproductive output, high biomass density, large body size) at high standing crop, through to attributes more associated with resistance to disturbance (rhizomes, tubers, streamlining of foliage, low biomass density) at low standing crop. 6. Comparison with a stochastic null model of change in species number along a constrained gradient, after correction for variation in sampling effort, indicated that sites towards the tails of the gradient (excluding those with extremely low biomass) supported more species than might be expected from chance alone, while the most species-rich sites in mid-gradient generally supported many fewer species than expected. 7. We suggest that a disturbance regime that maintains intermediate standing crops would be appropriate for the conservation of species-rich aquatic vegetation. Precise definition of this regime, under a range of circumstances, requires the study of temporal change at representative sites. [ABSTRACT FROM AUTHOR]

1. A broadened concept of biodiversity, encompassing spatio-temporal heterogeneity, functional processes and species diversity, could provide a unifying theme for river ecology. 2. The theoretical foundations of stream ecology often do not reflect fully the crucial roles of spatial complexity and fluvial dynamics in natural river ecosystems, which has hindered conceptual advances and the effectiveness of efforts at conservation and restoration. 3. Inclusion of surface waters (lotic and lentic), subsurface waters (hyporheic and phreatic), riparian systems (in both constrained and floodplain reaches), and the ecotones between them (e.g. springs) as interacting components contributing to total biodiversity, is crucial for developing a holistic framework of rivers as ecosystems. 4. Measures of species diversity, including alpha, beta and gamma diversity, are a result of disturbance history, resource partitioning, habitat fragmentation and successional phenomena across the riverine landscape. A hierarchical approach to diversity in natural and altered river-floodplain ecosystems will enhance understanding of ecological phenomena operating at different scales along multidimensional environmental gradients. 5. Re-establishing functional diversity (e.g. hydrologic and successional processes) across the active corridor could serve as the focus of river conservation initiatives. Once functional processes have been reconstituted, habitat heterogeneity will increase, followed by corresponding increases in species diversity of aquatic and riparian biota. [ABSTRACT FROM AUTHOR]

1. Gross primary production (GPP) and ecosystem respiration (ER) were analysed for 18 months in two reaches of the River Thur, a prealpine river in Switzerland. The upper reach at 655 m above sea level (a.s.l.) is bedrock constrained, has a high slope (0.60%) and a catchment area of 126 km2. The lower reach at 370 m a.s.l. has a more extensive hyporheic zone, a lower slope (0.17%) and a catchment of 1696 km2. 2. In both reaches, temporal patterns of stream metabolism reflected the occurrence of bed-moving spates. Average reductions of GPP and ER by spates were 53 and 24% in the upper reach, and 37 and 14% in the lower reach, respectively. The greater resistance of ER than GPP in both reaches shifted the ecosystem metabolism towards heterotrophy (decrease of the ratio of GPP to ER (P/R)) following spates. 3. Recovery of GPP was significantly faster in the lower reach and exhibited distinct seasonal variation (positive correlation with incident light). The differences in stability (both resistance and resilience) between reaches reflected differences in geomorphic settings and disturbance regime. 4. Stepwise regression analysis was used to explore the potential influence of season, disturbance and prevailing environmental conditions on stream metabolism in each reach. Time since spate plus temperature explained 73 and 86% of variation in ER and GPP, respectively, in the upper reach and 55% of variation in ER in the lower reach. Season plus prevailing environmental conditions explained 67% of variation in GPP in the lower reach. 5. To test how the perception of stability may change with increasing scale of observation, the disturbance regimes of 12 sites were compared with the disturbance regime of the entire Thur catchment. The analysis suggests that stream metabolism at the catchment scale is far more resistant to high flow events than at the reach scale. [ABSTRACT FROM AUTHOR]

1. In unshaded, nutrient-rich streams, prolific growth of stream macrophytes often results in flows that over-top the banks and in high primary production and respiration that may result in extreme diel variations in dissolved oxygen. Consequently, water protection authorities commonly remove macrophytes periodically. 2. We investigated the effect of plant removal on stream metabolism and oxygen balance in two Swiss streams with a high macrophyte biomass. We monitored the concentration of dissolved oxygen before and after macrophytes were removed by cutting and dredging, and calculated rates of gross primary production and ecosystem respiration by means of diel oxygen curves. 3. The removal of plants, which had reached a dry biomass of 320–420 g m-2 immediately before plant removal, had a different impact on stream metabolism in the two streams. In the first (plants removed in May), neither primary production nor ecosystem respiration were significantly affected. In the second (plants removed in late July), gross primary production and ecosystem respiration were reduced by about 70%. In this latter stream gross primary production increased in the first 2 weeks after plant removal but never recovered to pre-disturbance levels. 4. The removal of plants coincided with only a moderate increase in nocturnal oxygen concentration (+1 mg L-1). This, and the rapid partial recovery of stream metabolism in the second stream, suggests that an increase in the oxygen concentration after plant cutting is transient in unshaded, nutrient-rich streams. [ABSTRACT FROM AUTHOR]

1. The monsoon causes major flood events in some Himalayan streams, but their seasonal predictability might reduce the resulting disturbance. We assessed seasonal change in the benthos of 16 streams in central Nepal over a gradient of declining rainfall and increasing altitude from 600 to 3800 m. All sites were surveyed on four occasions, two in winter (November) and two pre-monsoon (June), with additional sampling during the monsoon (August) at four low altitude sites. Invertebrate abundance, taxon richness and persistence were assessed at all sites, and density and meso-habitat distribution at the four low altitude sites only. 2. Strong seasonal variation among invertebrates was confined primarily to streams at low altitude (600–800 m) where monsoon rainfall was greatest and catchments were dominated by terraced agriculture. At these sites, a significant reduction in benthic density (on average by 77%) and taxon richness (by 20%) occurred between the winter and pre-monsoon periods, so that invertebrate numbers were already low before the monsoon. A further significant decline occurred in all meso-habitats during the monsoon, but the change in density was small in absolute terms. 3. Persistence in rank abundance was equally low at all sites, but turnover in composition was significantly lower at sites in semi-natural forest than in catchments managed for terracing or alpine pasture. 4. These data provide no evidence that monsoonal floods represent major disturbance, instead supporting the view that the ecological response might reflect an adjustment to predictable flow pattern. However, catchment land use in the Himalaya appears to be a significant source of ecosystem instability, and confounds the simple interpretation of monsoon effects. [ABSTRACT FROM AUTHOR]

1. The hyporheic zone has long been regarded as a potential refugium for lotic invertebrates during disturbance. However, there have been few attempts to quantify the stability of this habitat during high flow events. In a New Zealand stream with an unstable bed, the present authors monitored spatial patterns of scour and fill in a riffle in a wide flood plain and at two sites in a constrained reach: a pool-riffle with bedrock outcrops and a plane-bed (a bedform characterized by long stretches of planar stream bed). 2. At each 20-m site, 100 scour chains were installed in a systematic grid with about 1 m between chains. Scour was measured by comparing the length of chain exposed before and after a high flow event, whereas filling depth was equivalent to the thickness of the sediment deposited on top of the chains during the event. For each chain, the present authors noted dominant particle size and degree of packing of the surrounding bed, water depth and presence or absence of large stones upstream. Chains were re-located after four smaller spates, one intermediate event and one large flood. 3. Most events caused a complex mosaic of bed patches which experienced scour, fill or remained undisturbed. These patterns, which were mostly site- and event-specific, were often significantly influenced by the longitudinal or lateral position of the chains in the spatial grids. 4. The cumulative effect of the six high flow events differed substantially between sites. The first site experienced predominantly scour, the second both scour and fill, and the third almost exclusively fill. These differences were partly explained by channel geomorphology. The bedrock outcrops at the constrained pool-riffle site forced the flow at high discharge, causing deep scour in these areas, whereas a backwater effect at the third site reduced near-bottom shear stress during larger events and led to sediment deposition. 5. Except for a single event at the second site, scour affected mainly the uppermost 10–15 cm of the stream bed. Therefore, almost the entire hyporheic zone below this depth would have been available as refugium for invertebrates, in addition to the often consider-able number of bed patches which remained undisturbed during the six high flow events. 6. Fill without earlier scour during the same high flow event was common at all sites. Most previous studies have assumed that lotic invertebrates are mainly affected by scour during high flow events, but the consequences of sediment deposition may be just as far reaching. [ABSTRACT FROM AUTHOR]