Desert Stream Research Articles

Review: The distribution, flow, and quality of Grand Canyon Springs, Arizona (USA)

An understanding of the hydrogeology of Grand Canyon National Park (GRCA) in northern Arizona, USA, is critical for future resource protection. The ~750 springs in GRCA provide both perennial and seasonal flow to numerous desert streams, drinking water to wildlife and visitors in an otherwise arid environment, and habitat for rare, endemic and threatened species. Spring behavior and flow patterns represent local and regional patterns in aquifer recharge, reflect the geologic structure and stratigraphy, and are indicators of the overall biotic health of the canyon. These springs, however, are subject to pressures from water supply development, changes in recharge from forest fires and other land management activities, and potential contamination. Roaring Springs is the sole water supply for residents and visitors (>6 million/year), and all springs support valuable riparian habitats with very high species diversity. Most springs flow from the karstic Redwall-Muav aquifer and show seasonal patterns in flow and water chemistry indicative of variable aquifer porosities, including conduit flow. They have Ca/Mg-HCO3 dominated chemistry and trace elements consistent with nearby deep wells drilled into the Redwall-Muav aquifer. Tracer techniques and water-age dating indicate a wide range of residence times for many springs, supporting the concept of multiple porosities. A perched aquifer produces small springs which issue from the contacts between sandstone and shale units, with variable groundwater residence times. Stable isotope data suggest both an elevational and seasonal difference in recharge between North and South Rim springs. This review highlights the complex nature of the groundwater system.

Desert stream phototrophic mats: a promising source for biofuel production

ABSTRACTDesert streams occur in abundance in the mountain regions of the Arabian Peninsula, where massive areas are covered by phototrophic microbial mats. In this study, 11 different phototrophic microbial mats were screened for their carbohydrate and lipid contents and their ability to produce methane, and mats with potential were further used for biofuel production. A maximum bioethanol yield of 0.76 ± 0.1 g/L was obtained by enzymatic saccharification of one mat, which was dominated by the alga Spirogyra, followed by fermentation of the produced sugars using the native Clostridium strain AK-1. A lipid content of 9–18 wt% was measured from the microbial mats. An increase of 4% and 9% was observed, when the mat with the highest lipid content was incubated under 3% salinity and nitrogen deprivation conditions, respectively. Anaerobic digestion of one selected cyanobacterial mat for biomethane production yielded a maximum of 95 mL CH4/g VS (volatile solids) after 49 days. MiSeq analysis revealed that the microbial community of this mat was 99.5% dominated by archaea, mainly belonging to the genera Methanoculleus (56.7%), Methanobacterium (27%) and Methanosarcina (7.6%). Our results demonstrate the potential of using microbial mats from desert streams to produce bioethanol, biodiesel and biomethane.

Evidence for self-organization in determining spatial patterns of stream nutrients, despite primacy of the geomorphic template

Nutrients in freshwater ecosystems are highly variable in space and time. Nevertheless, the variety of processes contributing to nutrient patchiness, and the wide range of spatial and temporal scales at which these processes operate, obfuscate how this spatial heterogeneity is generated. Here, we describe the spatial structure of stream nutrient concentration, quantify the relative importance of the physical template and biological processes, and detect and evaluate the role of self-organization in driving such patterns. We examined nutrient spatial patterns in Sycamore Creek, an intermittent desert stream in Arizona that experienced an ecosystem regime shift [from a gravel/algae-dominated to a vascular plant-dominated (hereafter, "wetland") system] in 2000 when cattle grazing ceased. We conducted high-resolution nutrient surveys in surface water along a 10-km stream reach over four visits spanning 18 y (1995-2013) that represent different successional stages and prewetland stage vs. postwetland state. As expected, groundwater upwelling had a major influence on nutrient spatial patterns. However, self-organization realized by the mechanism of spatial feedbacks also was significant and intensified over ecosystem succession, as a resource (nitrogen) became increasingly limiting. By late succession, the effects of internal spatial feedbacks and groundwater upwelling were approximately equal in magnitude. Wetland establishment influenced nutrient spatial patterns only indirectly, by modifying the extent of surface water/groundwater exchange. This study illustrates that multiple mechanisms interact in a dynamic way to create spatial heterogeneity in riverine ecosystems, and provides a means to detect spatial self-organization against physical template heterogeneity as a dominant driver of spatial patterns.

Molecular Genetics Informs Spatial Segregation of Two Desert Stream Gila Species

AbstractHeadwater Chub Gila nigra and Roundtail Chub G. robusta are two distinct but morphologically similar cyprinid species that historically were parapatric in Fossil Creek (Gila River basin) in central Arizona. The creek was chemically treated in 2005 to eradicate nonnative fishes, and chubs salvaged before the treatment were repatriated afterward. Annual samples from multiple sites along a transect throughout the treated reach were characterized for mtDNA and microsatellites from 2008 to 2014. Genetic variation was compared within and among Fossil Creek samples and with reference material for Headwater Chub from Fossil Springs at the headwaters of the creek and for Roundtail Chub from the Verde River and West Clear Creek. Measures of mtDNA and microsatellite diversity identified significant differences among Headwater Chub from Fossil Springs, Roundtail Chub from the Verde River, and samples above and below Irving Falls, a natural fish barrier within the treatment reach. The frequency of Headwater Chub mtDNA was low at the most downstream sampling sites, progressively increasing upstream. There was also temporal variation, with Headwater Chub mtDNA generally becoming rarer in later samples. F‐statistic analysis of microsatellite data and Bayesian clustering of microsatellite genotypes with STRUCTURE were generally consistent and identified three groups: Headwater Chub and two forms of Roundtail Chub. Using the species assignments provided by STRUCTURE and mitotypes, we found 153 “pure” Headwater Chub and only a single pure Roundtail Chub above Irving Falls and 27 pure Headwater Chub and 398 pure Roundtail Chub below the falls. Tests of cytonuclear disequilibrium indicated nonrandom mating (with the rare species more likely to mate than the more common one) and/or selection against hybrids. These data support the recognition of the Headwater Chub as a distinct full species and demonstrate that conservation actions within the genus Gila should typically be carried out at the population level.Received April 8, 2016; accepted August 16, 2016 Published online December 2, 2016

Caught between a Rock and a Hard Mineral Encrustation: Long-Lived Aquatic Insects Accumulate Calcium Carbonate Deposits in a Montane Desert Stream

Aquatic ecosystems overlying regions of limestone bedrock can feature active deposition of calcium carbonate in the form of travertine or tufa. Although most travertine deposits form a cement-like layer on stream substrates, mineral deposits can also form on benthic invertebrates. However, little is known about which taxa may be prone to calcium carbonate encrustation and which life history traits may make taxa more susceptible to becoming encrusted. Here we report the presence of calcium carbonate deposits on live insects collected from a montane stream in the Madrean Sky Islands (Huachuca Mountains) of Arizona between 2011 and 2013. Life history differences are examined between taxa with and without travertine deposits. Thirteen genera of aquatic insects were found with calcium carbonate deposits on the exoskeleton as well as 22 other genera, also encountered in the study stream, that have not previously been found with such deposits. Taxa with calcium carbonate encrustation had significantly longer-lived aquatic stages than those without encrustation. Furthermore, encrustation presence did not differ among aerial dispersal modes. These results suggest that the extent of calcium carbonate deposition on aquatic insects is primarily related to the length of time they are in the stream. Since mineral encrustation may reduce predation pressure and mobility, changes in patterns of travertine formation in these systems may have profound effects on ecological interactions. Los ecosistemas dulceacuicolas ubicados en regiones de roca caliza pueden contener depositos de carbonato de calcio en forma de travertino o tufa. Aunque muchos depositos de travertino se encuentran como una capa o manto de cemento sobre el fondo del arroyo, los depositos tambien pueden formarse sobre los invertebrados bentonicos. Sin embargo, poco se sabe acerca de las especies que son propensas a los depositos de travertino, o acerca de las caracteristicas de estas especies que influyen en tal proceso. En este manuscrito reportamos la presencia de depositos de travertino en insectos vivos y recolectados en un arroyo montanoso en el Archipielago Madrense de Arizona durante 2011–2013 y examinamos las diferencias en las historias de vida entre taxones con o sin depositos de travertino. Encontramos trece generos de insectos acuaticos con travertino sobre el exoesqueleto y vientedos generos sin travertino. Los taxones con travertino tienen una fase acuatica significativamente mas larga que los taxones sin travertino. Ademas, la presencia de travertino no difirio entre especies con distintas maneras de dispersion aerea. Estos resultados sugieren que las diferencias en los depositos de travertino en insectos acuaticos ocurren principalmente a causa de la duracion del periodo de vida acuatica del insecto. Aunque los depositos de travertino pueden reducir la presion por depredacion y movilidad, cambios en los patrones de formacion de travertino en estos sistemas pueden afectar profundamente las interacciones ecologicas.

Wood decay in desert riverine environments

Floodplain forests and the woody debris they produce are major components of riverine ecosystems in many arid and semiarid regions (drylands). We monitored breakdown and nitrogen dynamics in wood and bark from a native riparian tree, Fremont cottonwood (Populus deltoides subsp. wislizeni), along four North American desert streams. We placed locally-obtained, fresh, coarse material [disks or cylinders (∼500–2000cm3)] along two cold-desert and two warm-desert rivers in the Colorado River Basin. Material was placed in both floodplain and aquatic environments, and left in situ for up to 12years. We tested the hypothesis that breakdown would be fastest in relatively warm and moist aerobic environments by comparing the time required for 50% loss of initial ash-free dry matter (T50) calculated using exponential decay models incorporating a lag term. In cold-desert sites (Green and Yampa rivers, Colorado), disks of wood with bark attached exposed for up to 12years in locations rarely inundated lost mass at a slower rate (T50=34yr) than in locations inundated during most spring floods (T50=12yr). At the latter locations, bark alone loss mass at a rate initially similar to whole disks (T50=13yr), but which subsequently slowed. In warm-desert sites monitored for 3years, cylinders of wood with bark removed lost mass very slowly (T50=60yr) at a location never inundated (Bill Williams River, Arizona), whereas decay rate varied among aquatic locations (T50=20yr in Bill Williams River; T50=3yr in Las Vegas Wash, an effluent-dominated stream warmed by treated wastewater inflows). Invertebrates had a minor role in wood breakdown except at in-stream locations in Las Vegas Wash. The presence and form of change in nitrogen content during exposure varied among riverine environments. Our results suggest woody debris breakdown in desert riverine ecosystems is primarily a microbial process with rates determined by landscape position, local weather, and especially the regional climate through its effect on the flow regime. The increased warmth and aridity expected to accompany climate change in the North American southwest will likely retard the already slow wood decay process on naturally functioning desert river floodplains. Our results have implications for designing environmental flows to manage floodplain forest wood budgets, carbon storage, and nutrient cycling along regulated dryland rivers.

Temporal variability in hydrology modifies the influence of geomorphology on wetland distribution along a desert stream: a commentary on Dong et al. (2016)

Temporal variability in hydrology modifies the influence of geomorphology on wetland distribution along a desert stream: a commentary on Dong <i>et al</i>. (2016)

The effects of riparian restoration following saltcedar (Tamarix spp.) biocontrol on habitat and herpetofauna along a desert stream

Amphibians and reptiles (herpetofauna) have been linked to specific microhabitat characteristics, microclimates, and water resources in riparian forests. Our objective was to relate variation in herpetofauna abundance to changes in habitat caused by a beetle used for Tamarix biocontrol (Diorhabda carinulata; Coleoptera: Chrysomelidae) and riparian restoration. During 2013 and 2014, we measured vegetation and monitored herpetofauna via trapping and visual encounter surveys (VES) at locations affected by biocontrol along the Virgin River in the Mojave Desert of the southwestern United States. Twenty‐one sites were divided into four riparian stand types based on density and percent cover of dominant trees (Tamarix, Prosopis, Populus, and Salix) and presence or absence of restoration. Restoration activities consisted of mechanically removing non‐native trees, transplanting native trees, and restoring hydrologic flows. Restored sites had three times more total lizard and eight times more yellow‐backed spiny lizard (Sceloporus uniformis) captures than other stand types. Woodhouse's toad (Anaxyrus woodhousii) captures were greatest in unrestored and restored Tam‐Pop/Sal sites. Results from VES indicated that herpetofauna abundance was greatest in the restored Tam‐Pop/Sal site compared with the adjacent unrestored Tam‐Pop/Sal site. Tam sites were characterized by having high Tamarix cover, percent canopy cover, and shade. Restored Tam‐Pop/Sal sites were most similar in habitat to Tam‐Pop/Sal sites. Two species of herpetofauna (spiny lizard and toad) were found to prefer habitat components characteristic of restored Tam‐Pop/Sal sites. Restored sites likely supported higher abundances of these species because restoration activities reduced canopy cover, increased native tree density, and restored surface water.

Temporal variability in hydrology modifies the influence of geomorphology on wetland distribution along a desert stream

Summary Both geomorphic setting and dynamic environmental variables influence riverine wetland vegetation distributions. Most studies of species distributions in riverine systems emphasize either hydrological variability or geomorphic controls, but rarely consider the interaction between the two. It is unknown whether and to what extent the relationship between the geomorphic template and species distribution is modified by fluctuating environmental conditions. This study examines how spatial patterns of riverine wetlands in a desert stream change in response to environmental shifts brought about by interannual variability in the hydrologic regime. We surveyed wetland spatial distribution and measured its abundance every June over 5 years (2009–2013) by recording patch size and presence/absence of five wetland plant species along the 12‐km main stem of Sycamore Creek, Arizona, U.S.A. The study period encompassed a very large flood in January 2010, a wet year (2010), two average years (2009 and 2013) and two extremely dry years (2011 and 2012). We used a Bayesian statistical approach to analyse the relationship between geomorphic variables and wetland distribution under different hydrological conditions. The geomorphic variables provided much greater explanatory power in dry years than in average to wet years. Hydrological conditions modified the interactions between geomorphic template and species distribution. Annual hydrological conditions affected the direction (i.e. positive or negative effect) and magnitude (i.e. the size and significance level of an effect) of these interactions, both of which gave rise to spatial patterns of wetlands. Ecosystem temporal variability, such as inter‐annual and multi‐year hydrological variability and longer‐term ecosystem state changes, triggered complex species responses. Synthesis. The effect of geomorphic setting on stream wetland plant distribution in this desert system is conditioned on the temporal variability in hydrology among years. Temporal transferability of the relationship between geomorphology and species distributions is therefore questionable.

Effects of rapid urbanization on streamflow, erosion, and sedimentation in a desert stream in the American Southwest

Abstract Rapid urbanization has resulted in a series of sequential effects on a desert stream in the American Southwest. Lower Las Vegas Wash was a dry wash characterized by infrequent flood deposition when Las Vegas, Nevada was established in 1905. Wastewater effluent was discharged into the wash in low volumes for over 3 decades. Wastewater volumes increased commensurably with accelerated population growth during the late 20th century and created a sequence of feedback effects on the floodplain. Initially slow saturation of the valley fill created a desert oasis of dense floodplain vegetation and wetlands. Annual streamflow began in 1958 and erosion began a decade later with shallow incision in discontinuous channel segments. Increasing baseflow gradually enlarged channels; headcutting was active during the 1970s to 1984. The incised channels concentrated storm runoff, which accelerated local channel erosion, and in 1984 the headcuts were integrated during a series of monsoon floods. Wetlands were drained and most floodplain vegetation destroyed. Channel erosion continued unabated until engineering interventions began in the 21st century. No natural channel recovery occurred after initial urbanization effects because streamflow never stabilized in the late 20th century. A 6.6 M m3 sediment slug, eroded from the wash in ∼25 years, was deposited in Las Vegas Bay in Lake Mead. Falling reservoir levels during the 21st century are responsible for sediment redistribution and infilling of the bay. Close monitoring of impacts is recommended when urban wastewater and storm runoff are discharged on a desert wash. Channel interventions, when necessary, are advised in order to prevent costly engineering schemes of channel stabilization, flood control, and floodplain restoration.

Hydrology shapes taxonomic and functional structure of desert stream invertebrate communities

Hydrology is a fundamental factor influencing ecosystem dynamics, life-history strategies, and diversity patterns in running-water habitats. However, it remains unclear how hydrology may structure the taxonomic and functional composition of communities, especially in systems with high spatiotemporal variability in flow. We examined invertebrate diversity from 7 desert streams in the Huachuca Mountains of southeastern Arizona, USA, that span a flow permanence continuum from highly intermittent to perennial. We examined the relative roles of flow permanence, habitat size, season, and microhabitat in determining taxonomic and functional structure (according to 7 species traits compiled for 234 taxa) of these communities. We predicted that both functional and taxonomic diversity would be positively related to flow permanence and negatively related to the duration and number of stream drying events. As predicted, increased flow permanence was associated with increased functional richness, functional evenness, and taxonomic richness. Conversely, drying events reduced functional diversity across all measured indices. We found a saturating relationship between functional richness and taxonomic richness, indicating functional redundancy in species-rich communities, which may promote resilience of ecosystem function to environmental variation. Our study adds further evidence that hydrology is a key determinant of aquatic invertebrate diversity, and that stream hydroperiod strongly influences both functional and taxonomic diversity in arid-land streams.

Microstructure and cyanobacterial composition of microbial mats from the High Arctic

Arctic lakes, ponds and streams contain benthic microbial mats that are dominated by cyanobacteria, and these communities often account for a large proportion of the total ecosystem biomass and productivity. The vertical structure and composition of mats from two different aquatic habitats in the Canadian High Arctic, Ward Hunt Lake and a polar desert stream were analyzed in detail by microscopy techniques. Two distinct layers were identified in each mat: a surface layer with a high density of cells and associated extracellular polymeric substances (EPS), and a less cohesive bottom layer with an accumulation of mineral particles. The matrix formed by the cyanobacterial filaments and EPS produced the complex microstructure of all three mats, and likely favoured different microenvironments where specialized microbial interactions and biomineralization processes could take place. Structural and compositional differences were found among the mats. The lake mat had a surface layer of Dichothrix, and contained abundant particles of calcium carbonate, while Tychonema-like and Tolypothrix-like appeared mainly in the stream mats, along with a higher diversity of Chroococcales. A black microbial mat from one of the stream sites had markedly lower diversity than the other mat types. The observed differences in cyanobacterial composition and physical structure may be related to habitat stability and the availability of liquid water.

Long-term changes in seasonal fish assemblage dynamics in an adventitious desert stream

Abstract Tornillo Creek, a tributary of the Rio Grande in Texas, United States, has historically been an important nursery and spawning habitat for several native fish species. We examined variation in the seasonal fish assemblages in Tornillo Creek within and between two time periods (1967–1970 vs. 2009–2011), and contemporary fish assemblage–environment associations, in order to understand what environmental factors were associated with seasonal fish abundance and occupancy patterns. Our results indicated that fish assemblages were very different between the two time periods. Contemporary seasonal fish assemblage patterns in Tornillo Creek were less variable than historical assemblages and were linked to several environmental factors including water temperature, stream depth, and current velocity. We suggest that the maintenance of stream flow and connectivity are important for the immigration and emigration of certain riverine fishes in Tornillo Creek and that decreases in stream flow could accelerate ...

Metapopulation analysis indicates native and non‐native fishes respond differently to effects of wildfire on desert streams

AbstractAsh flows and flooding associated with wildfires represent important but understudied sources of disturbance for fish populations. Knowledge concerning these disturbances is especially limited for larger streams where warm water species dominate. Fire‐related disturbances have been hypothesised to differentially affect native and non‐native fishes, although this hypothesis has only been tested for salmonids. The objective of our research was to contrast effects of uncharacteristically large wildfires followed by flooding on metapopulations of native and non‐native fishes in theGilaRiver of southwestNewMexico. Probabilities of occupancy, colonisation and local extinction of fishes were calculated across sites before and during disturbance and were also measured across a broader spatial scale during disturbance to identify potential refuge locations. Occupancy was higher for native fishes than non‐natives, but multiple wildfire and flood events increased extinction probabilities of native species. Responses of non‐native species to wildfires were mixed; extinction of non‐native salmonids increased during disturbance, while extinction of several warm water species remained unchanged or decreased. Several undisturbed sites were poor refugia for natives as they were impacted by non‐native piscivores, dewatering, and fragmentation. However, despite exposure to multiple disturbances, sites located in large tributary and valley reaches were consistently occupied by native species, suggesting these habitats provided refugia. We suggest that management actions (forest thinning; prescribed burning) that restore a more natural disturbance regime of small and less severe fires coupled with habitat remediation activities (non‐native removal; decreased water withdrawal; improved connectivity) might diminish extinction risk for native fishes exposed to wildfire disturbance.

Bacterial diversity and fatty acid composition of hypersaline cyanobacterial mats from an inland desert wadi

Hypersaline cyanobacterial mats are infrequently reported in desert streams and information on such mats is very scarce. We investigated bacterial diversity and fatty acid composition in hypersaline cyanobacterial mats from Wadi Muqshin, located inland near the Empty Quarter desert in Oman. Most of the detected cyanobacteria belonged to known halotolerant, thermotolerant and UV resistant types that were typically reported in other hypersaline mats. A total of 84,834 ribosomal sequences were obtained, with 62–79% of the sequences affiliated to Cyanobacteria, Proteobacteria, Bacteroidetes, Clostridia and Chloroflexi. Cluster analysis showed that Mat 5 with the highest salinity was profoundly different from the other mats and shared species were ≤72% between all mats. While most Deltaproteobacteria in the wadi mats belonged to sulfate-reducing bacteria, a number of sequences related to purple sulfur, purple non-sulfur as well as green no-sulfur bacteria were also detected. Different saturated, branched and mono- and di-unsaturated fatty acids were detected in all mats, with the saturated 16:0 and 18:0 and the monounsaturated 16:1 and 18:1 fatty acids accounting for relative amounts of 70–77% of total fatty acids. We conclude that microbial diversity and fatty acids composition in the desert wadi hypersaline cyanobacterial mats resemble their counterparts from other hypersaline environments.

Dispersal ability and habitat requirements determine landscape-level genetic patterns in desert aquatic insects.

Species occupying the same geographic range can exhibit remarkably different population structures across the landscape, ranging from highly diversified to panmictic. Given limitations on collecting population-level data for large numbers of species, ecologists seek to identify proximate organismal traits-such as dispersal ability, habitat preference and life history-that are strong predictors of realized population structure. We examined how dispersal ability and habitat structure affect the regional balance of gene flow and genetic drift within three aquatic insects that represent the range of dispersal abilities and habitat requirements observed in desert stream insect communities. For each species, we tested for linear relationships between genetic distances and geographic distances using Euclidean and landscape-based metrics of resistance. We found that the moderate-disperser Mesocapnia arizonensis (Plecoptera: Capniidae) has a strong isolation-by-distance pattern, suggesting migration-drift equilibrium. By contrast, population structure in the flightless Abedus herberti (Hemiptera: Belostomatidae) is influenced by genetic drift, while gene flow is the dominant force in the strong-flying Boreonectes aequinoctialis (Coleoptera: Dytiscidae). The best-fitting landscape model for M.arizonensis was based on Euclidean distance. Analyses also identified a strong spatial scale-dependence, where landscape genetic methods only performed well for species that were intermediate in dispersal ability. Our results highlight the fact that when either gene flow or genetic drift dominates in shaping population structure, no detectable relationship between genetic and geographic distances is expected at certain spatial scales. This study provides insight into how gene flow and drift interact at the regional scale for these insects as well as the organisms that share similar habitats and dispersal abilities.

High and dry: intermittent watersheds provide a test case for genetic response of desert fishes to climate change

In desert streams, fishes and other organisms that depend on surface water are predicted to inhabit smaller and more isolated wetted reaches, while the frequency and severity of disturbance is expected to increase under most climate change models. Together, these factors should reduce population genetic diversity and persistence probabilities. In this study, our goal was to understand genetic responses of stream fish populations to disturbance in an intermittent stream network. This network is occupied by Rio Grande sucker (Pantosteus plebeius) that is native to highland desert streams in North America. Sample localities in upland perennial reaches were connected by moderate to high levels of gene flow even when separated by up to a 30-km intermittent reach. However, drier and lower-elevation reaches were significant barriers to gene flow. Effects of genetic drift (lower allelic diversity and higher levels of inbreeding) were more pronounced in the watershed with fewest wetted reaches. Temporal analysis of genetic diversity indicated that streams with several spatially distinct wetted reaches were more genetically resistant to wildfire-induced demographic bottlenecks than a stream with only one wetted reach. Maintenance of multiple wetted reaches within streams and facilitated gene flow among watersheds could slow losses of genetic diversity in upland desert stream fishes, and will be important strategies for conserving stream biodiversity in the face of habitat fragmentation and disturbance related to climate change.

Effects of longitudinal and lateral stream channel complexity on native and non‐native fishes in an invaded desert stream

Summary Understanding how habitat heterogeneity influences the structure of communities has been a longstanding goal of ecologists. Identifying how stream channel complexity affects fishes will be particularly important in systems simplified by anthropogenic activities and encroachment of non‐native riparian vegetation. Here, we assessed how large‐scale longitudinal distribution of lateral stream channel complexity from anthropogenically channelised, naturally braided and canyon‐bound reaches of the San Juan River in New Mexico and Utah, U.S.A., correlated with species richness, evenness and the relative abundance of small‐bodied native and non‐native fishes. We also contrasted fish assemblages at a smaller scale, in the most laterally complex reach of the river, by comparing fish assemblages between primary channel and secondary channel habitats as well as in newly restored secondary channels. Rarefied fish species richness was generally highest in the braided reach of the river and the longitudinal distribution of total fish densities varied temporally. Contrary to our predictions, native fish densities were highest in the most upstream and anthropogenically channelised reach of the river. Non‐native fish densities tended to be highest in the downstream braided reach, and all fishes were sparse in the least complex canyon‐bound reach. Total fish densities were higher in secondary channels compared with primary channels, and non‐native fishes were responsible for this difference. Fish assemblages in recently restored secondary channels were generally similar to those in naturally occurring ones. Our results suggest that lateral channel complexity can facilitate greater species richness at larger scales and is likely to provide habitats for fishes that are rare or not found in larger primary channels. These findings also suggest reduced channel complexity may lower the ability of riverine systems to support diverse fish assemblages.

Efficacy of Mechanically Removing Nonnative Predators from a Desert Stream

AbstractNative fish faunas throughout the American Southwest have declined dramatically in the past century, mainly a consequence of habitat alteration and alien species introductions. We initiated this 6‐year study to evaluate the efficacy of mechanical removal of nonnative predaceous rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta, yellow bullhead Ameiurus natalis and smallmouth bass Micropterus dolomieu from an open 4.6‐km reach of West Fork Gila River in southwest New Mexico, USA. Removal efforts involved intensive sampling with a 10‐ to 12‐person crew using backpack electrofishers and seines to capture fish over a 4‐ to 5‐day period each year. Additionally, two reference sites were sampled with similar methods to compare temporal changes in species mass in the absence of a removal effort. Results were mixed. Mass of yellow bullhead, rainbow trout and brown trout declined in the removal reach from 2007 through 2012, but there was no change in smallmouth bass. Concurrently, mass of Rainbow trout, yellow bullhead and smallmouth bass did not change at reference sites, but brown trout mass declined, indicating factors other than removal were driving abundance of brown trout. Occurrence of several large flathead catfish Pylodictis olivaris in the removal reach in 2012 changed what would have been a decline in overall nonnative mass to no change over the course of the study. Spikedace Meda fulgida was the only native species positively responding to predator removal. Results of this study suggest that with moderate effort and resources applied systematically, mechanical removal can benefit some native fish species, but movement of problem species from surrounding areas into removal reaches necessitates continued control efforts. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Long‐term mark‐and‐recapture study of a freshwater mussel reveals patterns of habitat use and an association between survival and river discharge

Summary Climate change and human population growth threaten the supply of fresh water for human use and freshwater biodiversity. Long‐term studies are necessary to identify the effects of such temporal trends on biological and ecological phenomena; however, the collection of long‐term data can be costly and time‐consuming. We investigated the effect of hydrological variation over time on population dynamics in a perennial river of the northern Chihuahuan Desert, using an imperilled freshwater mussel (Popenaias popeii) as a model. We conducted a 15‐year mark‐and‐recapture study, and distance sampling, to estimate demographic parameters while accounting for habitat heterogeneity and changes in river discharge. Recapture probability varied between microhabitats, and survival was positively correlated with river discharge. Survival and the finite rate of population growth were relatively stable over time. Over 60% of individuals were found at relatively high density in riffle habitats, which compose c. 16% of the total study area. Mean monthly temperature in the region increased over the past 100 years, and mean monthly discharge of the Black River declined over the past 65 years. With no significant trends in total monthly precipitation, declines in discharge suggest that reduction of stream flow is likely due to lowering of the water table and decreased groundwater recharge. Significant changes in climate and hydrological regimes, and increases in anthropogenic threats (increased water demand, degraded water quality) in the region, may induce significant declines in population size of this imperilled mussel. We demonstrated the importance of considering habitat heterogeneity and hydrological cycles over time to examine population dynamics. Survival of benthic invertebrates in desert streams is sensitive to hydrological cycles, which are expected to be altered via climate change and extensive water use. Species recovery plans need to incorporate knowledge of spatial distributions when designing strategies for habitat assessment and making conservation decisions.