Drought refuges,spatial scale and recolonisation by invertebrates in non‐perennial streams

1. If resistance traits drive recolonisation after drought, then drought refuges should contribute strongly to assemblage composition within streams. If resilience traits drive recolonisation, macroinvertebrates emerging from refuges may disperse widely, colonising many streams. To determine whether the contribution of drought refuges to macroinvertebrate recolonisation in non‐perennial streams was mostly local (within stream) or broader scale (across streams), we measured the association between the composition of invertebrate assemblages in different types of in‐stream drought refuge and the assemblage composition of streams when flow resumed. 2. We sampled 16 streams of varying hydrological regime on the western side of the Victoria Range in the Grampians National Park, Victoria, Australia. Drought refuges (perennial pools, dry sediment, damp sediment, seeps, patches of leaf litter, beneath stones) were identified and sampled during autumn. Most taxa were found in perennial pools; few taxa were found aestivating beneath stones or having desiccation‐resistant stages in dry sediment. Perennial pools and perennially flowing reaches were the refuges that harboured the greatest diversity of macroinvertebrate taxa. 3. Streams were sampled again during spring. Assemblage composition of non‐perennial reaches in spring was unrelated to composition in nearby refuges in the previous autumn. In contrast, assemblage composition in perennial reaches during spring was strongly correlated with composition during autumn. Therefore, drought refuges did not directly influence assemblage composition locally within non‐perennial streams. Rather, both perennially flowing reaches and perennial pools acted as drought refuges across the broader landscape. Resilience traits are likely to drive recolonisation in these streams. 4. Monitoring of drought refuges in a particular stream will therefore not predict species composition when flow resumes. Drought refuges are likely to sustain biodiversity over larger spatial scales such as groups of streams or whole drainage networks. Consequently, stream networks will need to be managed as entities rather than as single waterways and the focus of drought refuge protection should be on perennial pools and reaches.     

Effects of floods on fish assemblages in an intermittent prairie stream

1. Floods are major disturbances to stream ecosystems that can kill or displace organisms and modify habitats. Many studies have reported changes in fish assemblages after a single flood, but few studies have evaluated the importance of timing and intensity of floods on long‐term fish assemblage dynamics. 2. We used a 10‐year dataset to evaluate the effects of floods on fishes in Kings Creek, an intermittent prairie stream in north‐eastern, Kansas, U.S.A. Samples were collected seasonally at two perennial headwater sites (1995–2005) and one perennial downstream flowing site (1997–2005) allowing us to evaluate the effects of floods at different locations within a watershed. In addition, four surveys during 2003 and 2004 sampled 3–5 km of stream between the long‐term study sites to evaluate the use of intermittent reaches of this stream. 3. Because of higher discharge and bed scouring at the downstream site, we predicted that the fish assemblage would have lowered species richness and abundance following floods. In contrast, we expected increased species richness and abundance at headwater sites because floods increase stream connectivity and create the potential for colonisation from downstream reaches. 4. Akaike Information Criteria (AIC) was used to select among candidate regression models that predicted species richness and abundance based on Julian date, time since floods, season and physical habitat at each site. At the downstream site, AIC weightings suggested Julian date was the best predictor of fish assemblage structure, but no model explained >16% of the variation in species richness or community structure. Variation explained by Julian date was primarily attributed to a long‐term pattern of declining abundance of common species. At the headwater sites, there was not a single candidate model selected to predict total species abundance and assemblage structure. AIC weightings suggested variation in assemblage structure was associated with either Julian date or local habitat characteristics. 5. Fishes rapidly colonised isolated or dry habitats following floods. This was evidenced by the occurrence of fishes in intermittent reaches and the positive association between maximum daily discharge and colonisation events at both headwater sites. 6. Our study suggests floods allow dispersal into intermittent habitats with little or no downstream displacement of fishes. Movement of fishes among habitats during flooding highlights the importance of maintaining connectivity of stream networks of low to medium order prairie streams.     

Effect of fire on benthic algal assemblage structure and recolonization in intermittent streams

Abstract: Dry biofilm on rocks and other substrata forms an important drought refuge for benthic algae in intermittent streams following the cessation of flow. This dry biofilm is potentially susceptible to disturbance from bushfires, including direct burning and/or scorching and damage from radiant heat, particularly when streams are dry. Therefore, damage to dry biofilms by fire has the potential to influence algal recolonization and assemblage structure in intermittent streams following commencement of flow. The influence of fire on benthic algal assemblages and recolonization was examined in intermittent streams of the Grampians National Park, Victoria, Australia, using a field survey and manipulative field experiment. The field survey compared assemblages in two intermittent streams within a recently burnt area (within 5 months of the fire) with two intermittent streams within an unburnt area. The two burnt streams were still flowing during the fire so most biofilms were not likely to be directly exposed to flames. Considerable site‐to‐site and stream‐to‐stream variation was detected during the field survey, which may have obscured potential differences attributable to indirect effects of the fire. The manipulative field experiment occurred in two intermittent streams and consisted of five treatments chosen to replicate various characteristics of bushfires that may influence dry biofilms: dry biofilm exposed directly to fire; dry biofilm exposed to radiant heat; dry biofilm exposed to ash; and two procedural controls. After exposure to the different treatments, rocks were replaced in the streams and algae were sampled 7 days after flow commenced. Differences occurred across treatments, but treatment differences were inconsistent across the two streams. For example, direct exposure to fire reduced the abundance of recolonizing algae and altered assemblage structure in both streams, while radiant heat had an effect on assemblage structure in one stream only. The manipulative field experiment is likely to have represented the intensity of a small bushfire only. Nonetheless, significant differences across treatments were detected, so these experimental results suggest that fire can damage dry biofilms, and hence, influence algal recolonization and assemblage structure in intermittent streams.     

Stream–floodplain connectivity and fish assemblage diversity in the Champlain Valley, Vermont, U.S.A.

To evaluate the influence of main channel–floodplain connectivity on fish assemblage diversity in floodplains associated with streams and small rivers, fish assemblages and habitat characteristics were surveyed at 24 stream reaches in the Champlain Valley of Vermont, U.S.A. Fish assemblages differed markedly between the main channel and the floodplain. Fish assemblage diversity was greatest at reaches that exhibited high floodplain connectivity. Whereas certain species inhabited only main channels or floodplains, others utilized both main channel and floodplain habitats. Both floodplain fish α-diversity and γ-diversity of the entire stream corridor were positively correlated with connectivity between the main channel and its floodplain. Consistent with these results, species turnover (as measured by β-diversity) was negatively correlated with floodplain connectivity. Floodplains with waterbodies characterized by a wide range of water depths and turbidity levels exhibited high fish diversity. The results suggest that by separating rivers from their floodplains, incision and subsequent channel widening will have detrimental effects on multiple aspects of fish assemblage diversity across the stream–floodplain ecosystem.     

Loss of functionally important and regionally endemic species from streams forced into intermittency by global warming

Climate change is altering hydrological cycles globally, and in Mediterranean (med-) climate regions it is causing the drying of river flow regimes, including the loss of perennial flows. Water regime exerts a strong influence over stream assemblages, which have developed over geological timeframes with the extant flow regime. Consequently, sudden drying in formerly perennial streams is expected to have large, negative impacts on stream fauna. We compared contemporary (2016/17) macroinvertebrate assemblages of formerly perennial streams that became intermittently flowing (since the early 2000s) to assemblages recorded in the same streams by a study conducted pre-drying (1981/82) in the med-climate region of southwestern Australia (the Wungong Brook catchment, SWA), using a multiple before-after, control-impact design. Assemblage composition in the stream reaches that remained perennial changed very little between the studies. In contrast, recent intermittency had a profound effect on species composition in streams impacted by drying, including the extirpation of nearly all Gondwanan relictual insect species. New species arriving at intermittent streams tended to be widespread, resilient species including desert-adapted taxa. Intermittent streams also had distinct species assemblages, due in part to differences in their hydroperiods, allowing the establishment of distinct winter and summer assemblages in streams with longer-lived pools. The remaining perennial stream is the only refuge for ancient Gondwanan relict species and the only place in the Wungong Brook catchment where many of these species still persist. The fauna of SWA upland streams is becoming homogenised with that of the wider Western Australian landscape, as drought-tolerant, widespread species replace local endemics. Flow regime drying caused large, in situ alterations to stream assemblage composition and demonstrates the threat posed to relictual stream faunas in regions where climates are drying.     

Pathways for algal recolonization in seasonally‐flowing streams

1. In semi‐arid climates, seasonally‐flowing streams provide most of the water required for human use, but knowledge of how water extraction affects ecological processes is limited. Predicted alterations in stream flows associated with the impacts of climate change further emphasize the need to understand these processes. Benthic algae are an important base for stream food webs, but we have little knowledge of how algae survive dry periods or respond to altered flow regimes. 2. We sampled 19 streams within the Grampians National Park, south‐eastern Australia and included four components: a survey of different drought refuges (e.g. permanent pools, dry biofilm on stones and dry leaf packs) and associated algal taxa; a survey of algal regrowth on stones after flows recommenced to determine which refuges contributed to regrowth; reciprocal transplant experiments to determine the relative importance of algal drift and regrowth from dry biofilm in recolonization; direct measurement of algal drift to determine taxonomic composition in relation to benthic assemblage composition. 3. Algae showed little specificity for drought refuges but did depend on them; no species were found that were not present in at least one of the perennial pool, dry biofilm or leaf pack refuges. Perennial pools were most closely correlated with the composition of algal assemblages once flows resumed, but the loss or gain of perennial pools that might arise from stream regulation is unlikely to affect the composition of algal regrowth. However, regulated streams were associated with strong increases in algal density in dry biofilm, including increased densities of Cyanobacteria. 4. A model for algal recolonization in seasonally‐flowing streams identified three pathways for algal recolonization (drift‐dependent, dry biofilm‐dependent and contributions from both), depending on whether streams are diatom‐dominated or dominated by filamentous algae. The model predicted the effects of changes to stream flow regimes on benthic algal recolonization and provides a basis for hypotheses testable in streams elsewhere.     

Can bryophytes be used to characterize hydrologic permanence in forested headwater streams?

Recent court cases have questioned whether all headwater streams, particularly those that are not perennial, fall within the protective boundaries of the Clean Water Act. Rapid field-based indicators of hydrologic permanence are critically needed for jurisdictional determinations. The study objective was to determine whether characteristics of bryophyte assemblages in forested headwater streams can be useful indicators of hydrologic permanence. We sampled bryophytes from the channel thalweg at 113 sites across 10 forests in the U.S. which varied in hydrologic permanence. Hydrologic permanence was based on hydrologic status during spring (wet season) and late summer (dry season) visits. Perennial sites had flow (surface or visibly interstitial) during both spring and summer visits. Intermittent sites flowed during spring but were either dry or had surface water limited to isolated pools in summer. Ephemeral sites did not have flow during either visit. The species composition significantly differed between ephemeral and perennial stream reaches. The species composition of intermittent streams overlapped with both ephemeral and perennial streams. Three species and six families were identified as indicators of particular permanence classes. Liverworts occurred more frequently at perennial than at intermittent or ephemeral sites. Bryophytes with cushion and turf growth forms were common to ephemeral sites, whereas mat and weft forms were more common to perennial sites. Acrocarpous mosses were more frequent at ephemeral than at perennial sites. The frequencies of bryophyte-moisture associations (derived from the literature) across the three permanence categories were significantly different. Species associated with dry habitats were more frequently encountered at ephemeral than perennial sites, whereas species associated with wet habitats were more frequent at perennial than ephemeral sites. Species richness varied among forests and permanence classes. Ephemeral sites tended to have higher species richness than perennial sites; however, this pattern did not hold for all forests. Bryophytes should be included among the tools for jurisdictional determinations, much like vascular plants are used to help delineate wetlands. Growth forms and higher level taxonomy of bryophytes (i.e., phyla) can be particularly useful for making jurisdictional determinations because of their ability to discriminate ephemeral from intermittent and perennial reaches and they are measures that can be rapidly trained and routinely used by agencies making such determinations.     

Seasonal variation in the composition of fish assemblages in small Amazonian forest streams: evidence for predictable changes

1. The effects of seasonal inundation on the biology of fishes on floodplains of large Amazonian rivers are well studied. However, the small seasonal changes in headwater streams are generally considered to have little effect on fish assemblages. 2. In this study, we analysed seasonal changes in the species composition and abundance of fish in small Amazonian forest streams. We sampled fish with hand and seine nets in headwater streams in a 10 000 ha terra‐firme forest reserve near Manaus, Brazil. Each stream was surveyed at the end of the 2005 dry season, at the beginning of the 2006 rainy season and at the beginning of the 2006 dry season, by means of a standardized sampling effort. 3. The numbers of individuals and species caught were higher in the dry season, but rarefaction analyses indicated that greater species numbers could have been due simply to the larger number of individuals caught. 4. Between the dry and rainy season, the direction of changes in species composition in multivariate space varied among sites, especially for quantitative (abundance) data. However, the observed variation among sites was the less than expected if the directions of change were random. 5. Fish assemblages in the second dry season were more similar to those in the previous dry season than expected if changes in species composition among seasons were random. This indicates that a general seasonal pattern in fish assemblages can be detected, despite the existence of some erratic site‐specific changes. 6. Most of the species that showed large seasonal variations in density occupy temporary ponds during the rainy season, when much of the valley is inundated and pond networks form adjacent to streams. Short‐duration lateral migrations to these ponds may play an important role in the seasonal fish‐assemblage dynamics in Amazonian headwater streams. 7. Our results contrast with previous studies on small Amazonian streams, which have found little seasonal change in fish assemblages, and highlight the importance of the abundance of common species as an indicator of general fish assemblage structure in biological monitoring programmes.     

Taxonomic distinctness and aquatic Coleoptera: comparing a perennial and intermittent stream with differing geomorphologies in Hidalgo, Mexico

Taxonomic distinctness combines species richness and phylogenetic diversity to detect changes in taxonomic structure. Most studies have reported on changes due to marine impacts, with little emphasis on habitat influences or freshwater systems. The composition and organizational structure of aquatic Coleoptera assemblages are susceptible to local influence, yet their colonization capacities suggest greater assemblage similarity over larger spatial scales. Surveys were conducted in two contrasting local streams (approximately 20 km apart), one intermittent with multiple stair-step cascades and intervening pools during the rainy season (and only pools during the dry season), and the other perennial with low gradient, in Hidalgo, Mexico. Published information from these initial surveys and their associated collections of adults were examined and the data converted to monthly presence/absence data for analysis using taxonomic distinctness. Despite reported physicochemical differences between the streams, only the average monthly proportion of swimmer species was significantly higher in the intermittent stream where elmids and psephenids were largely missing due to the general absence of protective substrate and the presence of pools; higher taxonomic structure was not affected. Well-developed colonization capabilities and a widely distributed species pool likely reduced detectable differences between the stream assemblages. Although the average monthly value of Sorenson’s Similarity Index for aquatic Coleoptera between both streams was low, differentiation was not achieved using taxonomic distinctness and presence/absence data.     

Riffle-pool geomorphology disrupts longitudinal patterns of stream benthos

An Ozark Plateau stream was studied to determine the influence of distinct pool and riffle geomorphology on the longitudinal zonation of macroinvertebrate species assemblages and functional group classification. All study sites were dominated by alluvial pool and riffle channel form and the first two orders became intermittent during summer months. Nine benthos samples were collected seasonally from riffles and pools at each of five sites using a vacuum benthos sampler. Diel temperature pulse and coarse particulate organic matter (CPOM) were measured at each site also. Water temperature was most variable in second order, and CPOM on riffles was not more abundant in upstream reaches. Annual average density and biomass of invertebrates were highest in third and fourth orders, respectively. Diversity was depressed in the intermittent headwaters sites. Macroinvertebrate functional groups did not exhibit strong longitudinal trends as predicted by the river continuum model, with species assemblages apparently more strongly affected by the segment-level physical template, although shredders were more abundant in the headwaters during fall and winter. This study indicates that a reach-level perspective based on channel form is a necessary complement to holistic stream ecosystem models, especially in alluvial gravel streams.     

Ecogeography of the herpetofauna of a northern California watershed: linking species patterns to landscape processes

Ecosystems are rapidly being altered and destabilized on a global scale, threatening native biota and compromising vital services provided to human society. We need to better understand the processes that can undermine ecosystem integrity (resistance‐resilience) in order to devise strategies to ameliorate this trend. We used a herpetofaunal assemblage to first assess spatial patterns of biodiversity and then to discover the underlying landscape processes likely responsible for these patterns. Reptiles and amphibians are a phylogenetically diverse set of species with documented sensitivity to environmental perturbations. We examined ecogeographic patterns of these taxa in aquatic and riparian environments across the landscape mosaic of the Mattole River watershed of northern California, USA. We analyzed species distributions relative to three primary vegetation types (grassland, second‐growth forest, late‐seral forest) and two hydrologic regimes (perennial vs intermittent). We sought evidence for the processes behind these patterns by modeling animal distributions relative to multi‐scale compositional, structural, and physical attributes of the vegetation or hydrologic type. Total herpetofaunal diversity was higher along perennial streams, with reptile diversity higher in mixed grassland. Amphibian and reptile richness, and reptile evenness, varied significantly among the three vegetations. Evidence indicated that distinct assemblages were associated with each end of a seral continuum. Four amphibians were more abundant in late‐seral forest, while two amphibians and two reptiles were more abundant in second‐growth forest, or mixed grassland, or both. Two amphibians were more abundant along intermittent streams. Models for predicting reptile richness, or abundances of the two amphibian assemblages, indicated water temperature was the best predictor variable. Based on these results and the physiological limits of several sensitive species, we determined the primary processes influencing faunal assemblage patterns on this landscape have been vegetation changes resulting from the harvesting of late‐seral forests and the clearing of forest for pasture. Comparing past with present landscape mosaics indicated that these changes have transformed the dominant amphibian and reptile species assemblage from a mostly cold‐water and cool forest‐associated assemblage to one now dominated by warm‐water and mixed grassland/woodland species.     

Spatial and temporal variation of fish assemblage in two intermittent streams in north‐western Zimbabwe

This study examined the spatial and temporal variation in fish assemblage composition within two intermittent streams in north‐western Zimbabwe. Eight sites were sampled during three different periods of flow. Twenty‐five species in eight families, dominated by cichlids and cyprinids, were collected. Redundancy analysis (RDA) was used to explore the relationship between species and their environmental correlates. According to variation partition, 68.1% of assemblage variation was explained by both habitat and temporal variables. Most species were associated with large pools, whereas small species were associated with habitats that had gravel and boulders. The temporal aspect suggested a pattern associated with high catch per unit effort (CPUE) for the mormyrids, Clarias gariepinus and Labeo altivelis, during the early rain period. By comparison, the late rain period was associated with high CPUE for Labeo cylindricus, Labeobarbus marequensis, Chiloglanis neumanni and Oreochromis mortimeri, whereas the rest of the species had high CPUE during both the late rain and dry periods. The assemblage composition was more diverse during the late rain period. Refuge availability during the dry period appeared to be essential for the assemblage of the intermittent streams. Protection of these temporal habitats is therefore important in conserving fish assemblages within intermittent streams of the Lake Kariba area.     

Diatom communities along stream longitudinal gradients

1. Summer diatom communities on artificial substrates were sampled weekly for a month in three first- to sixth-order tributaries of the Kentucky River to determine how community structure varied with stream size. 2. Diatom cell abundances were generally higher in the headwaters. Species diversity increased in a downstream direction in two of the streams, and in an upstream direction in the third. However, diversity in general seemed more closely related to current regimes than to stream size per se, with highest species diversity at intermediate current velocities. 3. Variation in diatom accumulation rates was greater in downstream communities than in the headwater assemblages of two streams, suggesting that downstream communities may experience greater fluctuations in abundance, at least under low-flow conditions. 4. Patterns of species distributions suggested a relationship between morphological growth forms (guilds) and stream size, as well as the influence of current. Achnanthes spp., Eunotia spp., erect, and stalked taxa were more commonly associated with headwater assemblages. Filamentous and centric diatoms occurred with greater frequency downstream.     

Spatial-temporal distribution of preimaginal blackflies in Neotropical streams

The current study investigates the spatial-temporal distribution of preimaginal blackflies in three a priori established, adjacent areas of Brazil and Venezuela. First we tested the hypothesis that the distribution of individual simuliid species, over two geographic scales, were predictable on the basis of several habitat parameters. Secondly, we determined if local species assemblages remain constant over two spatial axes, i.e., were species assemblages predictable within and between defined regions. Finally we compared the species assemblage between Venezuelan streams sampled in both the wet (October) and dry (February) seasons. As stream conditions changed across a north–south gradient, so did the occurrence of the most common species. This change in species composition from upland to lowland areas is consistent with the patterns of faunal change seen in North American simuliid assemblages. Discriminant Function Analysis showed a strong association between regional demarcations and stream site conditions. These results were paralleled by the strong correspondence between species assemblages and these regions. Thus each region presented both distinct stream environments and species assemblages. Monte Carlo analysis suggests that some if not all species are not only present in both the wet and dry season but that they are continuing to use the same stream sites. Three broad contingent rules for blackfly species assemblages in new world streams are put forward; (i) the ubiquitous nature of simuliids; (ii) species often distribute themselves along an axis of stream size and; (iii) there is a strong and consistent correspondence between regional characteristics of streams and the species assemblage.     

The Goldilocks effect: intermittent streams sustain more plant species than those with perennial or ephemeral flow

1. Drylands worldwide are typified by extreme variability in hydrologic processes, which structures riparian communities at various temporal and spatial scales. One key question is how underlying differences in hydrology over the length of interrupted perennial rivers influence spatial and temporal patterns in species richness and species composition. 2. We examined effects of differences in dry season hydrology on species richness, composition and cover of herbaceous plant communities in the streamside zone (the zone influenced directly by low flows in the channel). Data were collected at ephemeral, intermittent and perennial flow reaches on three rivers of the desert Southwest (Arizona, U.S.A.): Lower Cienega Creek, Hassayampa River and Lower San Pedro River. 3. Patterns of species richness varied with temporal scale of analysis, that is between single‐year and multi‐year time frames. At the annual timescale, quadrat species richness (m^?2) and herbaceous cover were higher at sites with perennial flow than at either intermittent or ephemeral sites. In contrast to this single‐year pattern, the highest long‐term richness occurred at intermittent sites. 4. Quadrat species richness, total species richness at a site (per 18 1‐m² plots) and cover were more variable year to year at non‐perennial sites than at perennial flow sites. On two of the three rivers, ephemeral sites had the highest inter‐annual compositional variance, while the perennial sites had the lowest. 5. Compositional differences between the hydrologic site types were dominated by species turnover, not nestedness. The perennial sites had more wetland and perennial species than the other two site types. The intermittent sites had more annual species than did the other two types. 6. High long‐term species richness and distinct species composition of intermittent sites are probably sustained by pronounced temporal variability in environmental conditions (i.e. frequent and persistent flow events, and dry periods). Plants at these sites take advantage of greater moisture than those at ephemeral sites and also experience less competition from resident species than those at perennial sites. 7. Conservation of desert riparian diversity depends upon the protection of consistently wet conditions at perennial flow sites, as well as the maintenance of the processes that cause fluctuations in environmental conditions at non‐perennial sites.     

Assessing effects of flow alteration on macroinvertebrate assemblages in Australian dryland rivers

1. Possible impacts of water‐resource development on assemblages of freshwater macroinvertebrates were investigated in the upper Darling River and some of its tributaries in north‐western New South Wales (Australia), an arid and semi‐arid region of low relief where alteration of river flows has intensified through expansion of irrigated agriculture. 2. Study sites were grouped into four hydrological regimes resulting from impoundment, flow regulation, water abstraction and natural variation, namely (i) intermittent flow with relatively little hydrological alteration from water‐resource development, (ii) intermittent flow with substantial alteration, (iii) near‐perennial flow with substantial alteration but unimpounded and (iv) near‐perennial flow with substantial alteration plus impoundment by weirs that stabilise water levels. 3. Macroinvertebrates were sampled with three methods (a quantitative cylinder sampler, handnet sampling and baited traps) in three periods with differing hydrology (recessional low flow in June 2003, high flow in March 2004 and increasing flow after drought in December 2004). 4. Taxonomic richness, assemblage composition and catch per unit effort of the crayfish Cherax destructor differed significantly among the site groups, but total macroinvertebrate density and the AUSRIVAS O/E (Australian River Assessment System observed‐over‐expected) index did not. The principal spatial differences were between the intermittent and near‐perennial rivers, and apparent effects of water‐resource development and impoundment were more subtle. Temporal differences in richness, abundance and composition were substantial and appeared to be related mainly to variations in discharge and temperature. 5. Current macroinvertebrate‐based methods for assessing the ‘condition’ or ‘health’ of Australian dryland rivers are inadequate. Such assessments might be improved with (i) reference data that take adequate account of antecedent hydrological conditions, (ii) consideration of long‐term taxonomic richness as well as richness on individual sampling occasions, (iii) evaluation of invertebrate population sizes, (iv) analysis of assemblage data by trait composition and (v) adoption of the genus as the default level of taxonomic resolution.     

Importance of low-flow and high-flow characteristics to restoration of riparian vegetation along rivers in arid south-western United States

1. Riparian vegetation in dry regions is influenced by low‐flow and high‐flow components of the surface and groundwater flow regimes. The duration of no‐flow periods in the surface stream controls vegetation structure along the low‐flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape heterogeneity and an abundance of pioneer wetland plant species in the floodplain. Vegetation changes on hydrologically altered river reaches are varied, given the great extent of flow regime changes ranging from stream and aquifer dewatering on reaches affected by stream diversion and groundwater pumping to altered timing, frequency, and magnitude of flood flows on reaches downstream of flow‐regulating dams. 3. As stream flows become more intermittent, diversity and cover of herbaceous species along the low‐flow channel decline. As groundwater deepens, diversity of riparian plant species (particularly perennial species) and landscape patches are reduced and species composition in the floodplain shifts from wetland pioneer trees (Populus, Salix) to more drought‐tolerant shrub species including Tamarix (introduced) and Bebbia. 4. On impounded rivers, changes in flood timing can simplify landscape patch structure and shift species composition from mixed forests composed of Populus and Salix, which have narrow regeneration windows, to the more reproductively opportunistic Tamarix. If flows are not diverted, suppression of flooding can result in increased density of riparian vegetation, leading in some cases to very high abundance of Tamarix patches. Coarsening of sediments in river reaches below dams, associated with sediment retention in reservoirs, contributes to reduced cover and richness of herbaceous vegetation by reducing water and nutrient‐holding capacity of soils. 5. These changes have implications for river restoration. They suggest that patch diversity, riparian plant species diversity, and abundance of flood‐dependent wetland tree species such as Populus and Salix can be increased by restoring fluvial dynamics on flood‐suppressed rivers and by increasing water availability in rivers subject to water diversion or withdrawal. On impounded rivers, restoration of plant species diversity also may hinge on restoration of sediment transport. 6. Determining the causes of vegetation change is critical for determining riparian restoration strategies. Of the many riparian restoration efforts underway in south‐western United States, some focus on re‐establishing hydrogeomorphic processes by restoring appropriate flows of surface water, groundwater and sediment, while many others focus on manipulating vegetation structure by planting trees (e.g. Populus) or removing trees (e.g. Tamarix). The latter approaches, in and of themselves, may not yield desired restoration outcomes if the tree species are indicators, rather than prime causes, of underlying changes in the physical environment.     

Droughts and anti-droughts: the low flow hydrology of Australian rivers

1. Droughts are not easily defined other than by culturally driven judgements about the extent and nature of impact. Natural ecosystems are adapted to the magnitude and frequency of dry periods and these are instrumental in controlling the long term functioning of these systems. 2. In unregulated rivers, low flows are derived from water in long‐term storage in the catchment, commonly as shallow groundwater. Four types of low flow sequences are evident for representative rivers from each of the seven flow regime zones in Australia and an arid zone stream: perennial streams with low annual flow variability that have seasonal low flows but do not cease to flow; perennial streams with high annual variability that cease to flow in extreme years; ephemeral streams that regularly cease to flow in the dry season; and arid zone streams with long and erratic periods of no flow. 3. Although Australian rivers record runs of consecutive years of low flows longer than would be expected theoretically, the departures from the expected are not statistically significant. Trends and quasi‐cycles in sequences of low‐flow years are observed over decadal time scales. 4. Examples of the effects of river regulation on low flows in southern Australia indicate that, while in detail the impacts of regulation vary, in general regulation mitigates the severity of low flows. 5. It is our contention that the indigenous biota of Australian rivers are adapted to the naturally occurring low flow conditions and that, while there is considerable scientific interest in the effects of climate change on stream ecology, such studies have little practical relevance for the management of indigenous biota in unregulated rivers. 6. The changes brought about by the regulation of rivers are much more rapid and dramatic than those which might occur as a result of climate change and it is possible to develop management procedures to mitigate them. In regulated rivers, the real problem may be ‘anti‐droughts’– the removal of significant natural low‐flow events from the flow pattern.     

Downstream fish assemblage response to river impoundment varies with degree of hydrologic alteration

River impoundments can fundamentally restructure downstream fish assemblages by altering flow regimes. However, the degree of alteration and associated ecological change may depend on pre-existing hydrologic regimes. We used long-term datasets to compare downstream hydrological and fish assemblage responses to impoundment in two catchments classified as having intermittent and perennial-flashy natural hydrologic regimes. We observed significant shifts in fish assemblage structure at both sites after stream impoundment. The historically intermittent stream shifted to a stable perennial flow regime. Changes in fish assemblage structure covaried with changes in five different components of the flow regime; most species that increased in abundance require fluvial habitats and likely benefited from increased flows during historically low flow seasons. Shifts in fish assemblage structure were also observed in the perennial stream, despite minimal flow alteration after impoundment; however, most species shifts were associated with lentic environments, and were more likely related to proximity of reservoirs in the drainage system rather than changes in stream flow. Findings from this study confirm that downstream fish assemblage response to river impoundment can be associated with high levels of hydrologic alteration, but other factors including expansion of lentic species into lotic environments also influence shifts in assemblage structure.