Stretch Of Stream Research Articles

Refining stream ecosystem evaluation: A novel approach using the family ecological index (FEIS)

The use of ecological indices based on benthic macroinvertebrate population for streams ecosystem quality assessment receives great attention. Streams in general have high variability characteristics; thus, using a single index used for the whole stream stretch renders less accurate readings. Therefore, this study was conducted to produce an ecological index using benthic macroinvertebrates as a bioindicator for a clean stream ecosystem. In this study, 42 recreational streams throughout Peninsular Malaysia were sampled and 663 units of water and benthic macroinvertebrates samples were analysed. Three phyla, 19 orders, 117 families and 55,410 macroinvertebrate individuals were recorded. Benthic macroinvertebrate data were analysed using existing indices such as Shannon Diversity Index (H’), biotic indices (BMWPTHAI and FBI) and WQI to analyse water quality. Canonical Correspondence Analysis (CCA) was used to evaluate the strength of water quality and physical habitat in determining benthic macroinvertebrate community structure. Next, Principal Component Analysis (PCA) was used to produce new weights. Based on the similarities and dissimilarities of each family in their ecological functions, the components were truncated into only three significant groups with respective weights and a new ecological index, namely Family Ecological Index for Streams (FEIS). From the FEIS values calculation, three scales which are 2.11 to 3.00 (first class: excellent ecosystem), 1.36 to 2.10 (second class: healthy ecosystem), and 0.00 to 1.35 (third class: moderately healthy ecosystem) were obtained. It is found that FEIS is capable of exhibiting variations in stream quality even within clean streams and able to detect early deterioration in streams which is highly useful for management stakeholders to manage recreational streams in Malaysia. The new data-driven approach in the development of FEIS in this study can be applied to other rivers and the difference in the weights determination as well as formulation obtained can be observed.

Procedure of Numerical Modelling and Estimation of Sieve Curve Changes as a Tool to Define Riverbed’s Erodibility

The numerical 1-D HEC-RAS modelling tool was supported by the estimation of the sieve curve changes procedure to measure the scale of predicted discharges along a stretch of stream in southern Poland on the Olkusz Upland. The procedure was calibrated in southern Poland on the mountain streams during high-stage events, using a radiotracer application in bedload transport. Particular terrain hypsometry, created by the dissolution of limestone, forced the deep erosion of the river valley bottom; it is here that the current shape of the riverbed of the Prądnik stream is placed. While numerical modelling is widely used in hydraulics, standards have been set for the estimation of flood risk zones; these estimations suggest that the densities of the measured cross-sections are less then optimal, and that the erosive processes are more frequent. This was proved by identifying a number of erosive sections. A new procedure proposed combining the prediction of grain size distribution with hydraulic modelling. Calculations using the estimation of sieve curves, based on the processes of creation and destruction in the armouring layer, have proven to be a challenge for the existing standards of hydraulic modelling. We believe that it is easy to expand the usefulness of the 1D model by utilising its results for this procedure. For the purpose of this type of analysis, dense cross-section measurements are involved, careful modelling is required and a wide range of additional in-field data has to be gathered. For the interpretation of the results, the relation between channel-forming discharge, bankfull discharge, present and critical shear stresses, as well as the mean diameter of the grain size and other estimated sieve curve parameters, were evaluated. Channel-forming discharge is smaller than the bankfull discharge in more than one third of the segment where the erosion process is more frequent and the stability of the riverbed is compromised. Channel-forming discharge was at least twice as high in the stable sections, compared to the erosive section. The presented method will help to find unstable riverbed sections, in order to mitigate the dimension of river training techniques and protect the natural state of the river. While we are in the period of development in this region of Europe, limiting the scope of interference in rivers and streams by applying this method may create an opportunity for the concept of river training close to nature.

Assessment of physicochemical parameters of Vishav stream: an important tributary of river Jhelum, Kashmir Himalaya, India.

The deteriorating conditions in stream ecosystems are detrimental for society as far as its health, and development is concerned if the underlying factors continue to operate without regular monitoring. In order to maintain the health of a stream ecosystem, assessment of spatiotemporal changes in its physicochemical attributes and identification of all factors that could alter its hydrological regime is an essential component for managing it. The current 2-year study (October-2017 to September-2019) assessed the physicochemical regime of lower stretches of the Vishav stream, a major left-bank tributary of river Jhelum on a spatiotemporal basis. The physicochemical data was analyzed through linear regression, ANOVA (followed by Duncan's test), multivariate statistical analysis, viz., principal component analysis (PCA) and cluster analysis (CA). Linear regression pointed out the nature and magnitude of the relationship between different physicochemical variables (p < 0.05). PCA showed that WT, pH, EC, NO3-N, TDS, TH, and DO are the major factors reflecting the water quality of the Vishav stream. The range in water quality parameters of the Vishav stream was found conducive for the inhabitant fishes. Two well-defined clusters were obtained, wherein Cluster-I comprising of Site-III (a downstream pollution prone site) and Cluster-II comprising of Site-II and Site-I (mid- and upstream site respectively) are less prone to human interferences. The present study could serve as baseline information to manage and conserve this precious element of the aquatic ecosphere in terms of better water quality for humans and its inhabitant faunal elements especially fish which play a significant role in the economy of that region.

Hydromorphological rehabilitation improves channel morphology, instream biotopes, and macroinvertebrate communities, and thus enhances the conservation of an urban river

Abstract Many river rehabilitation projects have been criticized for failing to meet their goals or for being insufficiently monitored. There is, therefore, an urgent need to develop robust approaches for assessing treatment efficacy and thus guide the increasing investment in rehabilitation. Instream biotopes (formerly called ‘functional habitats’ or ‘mesohabitats’ by different authors) and their macroinvertebrate assemblages were used to assess the effectiveness of entire‐channel hydromorphological rehabilitation of a 1.8‐km stretch of a lowland stream through the town of Market Harborough, Leicestershire, UK. The project successfully enhanced the physical diversity, measured in the rehabilitated reach as the coefficients of variability for channel water depth and width, wet surface area, number of instream biotopes, and biotope diversity. The project also enhanced the biodiversity conservation value, measured as macroinvertebrate total density, total biomass, richness, diversity, Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness, EPT diversity, EPT count%, and EPT biomass%, all of which significantly increased following rehabilitation. Chironomidae count% and biomass% significantly decreased following rehabilitation. Rehabilitation was also successful in significantly increasing macroinvertebrate shredder, scraper, and filter‐feeder group density. Changes in the macroinvertebrate community metrics in the rehabilitated reach were related significantly to changes in the percentages of cover of instream biotopes and increases in biotope diversity in the rehabilitated reach. Macroinvertebrate structural and functional metrics can provide quantitative data for assessing reach‐level rehabilitation outcomes, if samples are collected in a pre‐defined sampling protocol stratified at the instream biotope level. The practical implications of this work are that the design of rehabilitation projects, if based upon the recreation of biotope heterogeneity, will succeed in improving biological value and restoring the area to near naturalness if a suitable upstream source of macroinvertebrates for natural recolonization is available. The study shows that the concept of biotopes has an important role to play in river conservation management.

Tracking Downstream Variability in Large Grain‐Size Distributions in the South‐Central Andes

AbstractMixed sand‐ and gravel‐bed rivers record erosion, transport, and fining signals in their bedload size distributions. Thus, grain‐size data are imperative for studying these processes. However, collecting hundreds to thousands of pebble measurements in steep and dynamic high‐mountain river settings remains challenging. Using the recently published digital grain‐sizing algorithm PebbleCounts, we were able to survey seven large ( 1,000 ) channel cross‐sections and measure thousands to tens‐of‐thousands of grains per survey along a 100‐km stretch of the trunk stream of the Toro Basin in Northwest Argentina. The study region traverses a steep topographic and environmental gradient on the eastern margin of the Central Andean Plateau. Careful counting and validation allows us to identify measurement errors and constrain percentile uncertainties using large sample sizes. In the coarse 2.5 cm fraction of bedload, only the uppermost size percentiles (95th) vary significantly downstream, whereas the 50th and 84th percentiles show less variability. We note a relation between increases in these upper percentiles and along‐channel junctions with large, steep tributaries. This signal is strongly influenced by lithology and geologic structures, and mixed with local hillslope input. In steep catchments like the Toro Basin, we suggest nonlinear relationships between geomorphic metrics and grain size, whereby the steepest parts of the landscape exert primary control on the upper grain‐size percentiles. Thus, average or median metrics that do not apply weights or thresholds to steeper topography may be less predictive of grain‐size distributions in such settings.

Spatial-temporal variation in land use in a coastal watershed under pressure of population growth

ABSTRACT Population progress is one of the main factors affecting the environment, modifying the natural dynamics of ecosystems. The Camboriú River Basin is located on the southern coast of Brazil and is strongly affected by population growth. This paper aimed to verify the space-time variation in the land use of the Camboriú River Basin by identifying changes in occupation and land use between 1986 and 2017. The rapid environmental assessment protocol was applied using the following parameters: bottom substrate, habitat complexity, backwaters quality, stability of banks, vegetative protection, vegetative cover, vegetative quality, and riparian vegetation presence. The changes in occupation and land use show rapid urbanization with a rise of 9% in 1986 to 24% in 2017. In riparian forest, higher altitude regions of the watershed show better results when compared to lower course regions, which are, thus, reflected in the environmental integrity of the stretches of stream studied. As for the behavior of urban occupation in space, it was verified that it did not occur continuously in comparison with the population, data during the studied period.

Environmental factors structuring the assemblage of aquatic insects in an epigean and hypogean stretch of a Neotropical karst stream

The aim of the present study was to identify which variables structure the assemblage of aquatic insects in the subterranean environment, because knowledge about how subterranean communities are structured in rivers with hypogean sections is still emerging. In addition to identifying whether the hypogean assemblage is structured in the same way as the epigean assemblage, we sought to identify whether the environmental restrictions caused by the subterranean habitat resulted in reduced species richness and abundance compared with the epigean habitat. We sampled a 345-m stretch of a stream in Brazil and collected aquatic insects from both the epigean and hypogean environments. In the stream stretch investigated, there was a positive relationship between richness and the percentage of immersion, fine substrate and organic matter, and a negative relationship between richness and conductivity. We also verified differences between the epigean and hypogean environments regarding composition. The variables that best explained the overall assemblage structure included the epigean–hypogean environment and the percentage of coarse and fine substrate. Given the differences between these environments, we emphasise the need for management actions in karst regions that consider the discontinuity of these regions’ river communities to conserve all components of diversity.

Estimation of Manning’s Roughness Coefficient Through Calibration Using HEC-RAS Model: A Case Study of Rohri Canal, Pakistan

In understanding the hydraulic characteristics of river system flow, the hydraulic simulation models are essential tools. The suitable value of Manning’s roughness coefficient “n” is chosen through method of calibration; i.e., the value which reproduces observed data to an acceptable accuracy. In the present study, the unsteady flow model HEC-RAS is applied to Rohri Canal (upstream Rohri) to estimate value of manning’s coefficient through the procedure. Through a series of systematic. Studies to identify the n values in a hypothetical open channel and a natural stream stretch, several identification procedures based on unconstrained and constrained minimizations are analyzed. However, the decision on what value to adopt is a complex task, especially when dealing with natural water courses due to the various factors that affect this coefficient ‘n’. The data was collected in the period of January (2010) to December (2011), and divided equally into two sets. The first set is for calibration purpose, estimation of (n) and the second set for the verification process of testing the model with actual data to establish its predictability accuracy. Graphical and statistical approaches were used for model calibration and verification. Results show that the manning’s roughness coefficient “n” for Rohri Canal which shows good agreement between observed and computed hydrograph is 0.042.

Genotoxic effect of heavy metals on Astyanax lacustris in an urban stream

Uncontrolled urbanization growth contributes to the pollution of aquatic environments. Heavy metals released by domestic and industrial effluents can negatively affect aquatic organisms. This study aimed to evaluate the effect of environmental pollutants, such as metals, on fish DNA damage, in stretches of an urban stream. Specimens of the Neotropical fish, Astyanax lacustris, were exposed in situ for 96 h along the Antas stream, a Brazilian aquatic system deteriorated by anthropogenic factors. Water and sediment samples were collected simultaneously for physicochemical and heavy metal analyses. The comet assay was performed as a biomarker of genotoxicity. Fish located downstream had a higher frequency of DNA damage than in the reference site. We found concentrations of Cr and Ni above acceptable levels in sediment samples. Generally, Ba, Mn, Mg, Zn, Cr, and Ni were the elements most associated with genotoxic damage. Water and sediment of the Antas stream showed genotoxic potential in A. lacustris according to the urbanization gradient, demonstrating the importance to prevent the release of environmental pollutants, especially heavy metals in urban areas.

Ichthyofauna from “serranias costeiras” of the Ribeira de Iguape River basin, Southeast Brazil

Abstract: The Ribeira de Iguape River basin has about 100 fish species. This study aimed to characterize the fish community from “serranias costeiras” of the Ribeira de Iguape River basin. Samplings were conducted with electrofishing during the dry season in the years 2018-2019. The sampling effort consisted of 30 streams stretches. As a result, 50 species were captured, distributed in 37 genera, 11 families, and six orders. The species richness estimate (SChao1) was 57 species, and the coverage estimate for the entire data set was C = 0.998. Harttia kronei and Chasmocranus lopezae are endemic species and can be used as bioindicators of streams in this river basin. We captured approximately nine species by stream stretch. Beta diversity was found to be more critical for gamma diversity than alpha diversity. This finding highlights the streams environmental heterogeneity importance for maintaining regional fish diversity. We captured eight individuals of the threatened species Spintherobolus papilliferus and this indicates an expansion in the geographic distribution of this species.

Habitat simplification changes temporal patterns of invertebrate beta diversity in a high-Andean stream

ABSTRACT Several studies have revealed that beta-diversity patterns are tightly coupled to differences in habitat heterogeneity and the spatial distance among headwaters. However, addressing the relative role of these factors over time is yet to be done, particularly in those factors acting at the local scale. Here, we investigated the temporal responses of stream invertebrate communities to habitat simplification and flow variability in two stretches of a high-Andean stream: an undisturbed and a channelized stretch. Both stretches were sampled every month in two hydrologically different years: a “regular year” and a year under the influence of the La Niña phenomenon. Using four types of beta diversity (Jaccard, Percentage difference, Raup-Crick, and Gower), we tested two hypotheses: (H1) the temporal flow dynamics affect beta diversity, increasing its temporal variability. (H2) The effect of flow temporal dynamics would change because of the habitat simplification, being the temporal variability of beta diversity higher in the simplified (channelized) stretch. Contrarily to our expectations, we observed a significant mismatch between beta diversity metrics: while some indicated an increase, others indicated a decrease in the temporal variability. Nevertheless, taken together and interpreted in an ecological context, our results indicate that habitat simplification changes the temporal patterns of beta diversity. Specifically, our findings suggest that habitat simplification induces the functional homogenization of the community and diminishes the extent of change in community composition between sampling occasions. At the channelized stretch, we found that the community comprised species with adaptations to flow-related constraints primarily whose occurrence varied little among sampling occasions. These findings are relevant to Andean streams’ bioassessment, as certain programs rely on taking a ‘snapshot” to assess the ecosystem’s health or the effect of an anthropogenic stressor. Our study suggests that this snapshot may be biased if both the habitat heterogeneity and the history of high-flow events are not explicitly considered.

Community structure of benthic macroinvertebrates in different types of habitat in a stream stretch of the Atlantic Rainforest

The benthic macroinvertebrates are organisms of extremely importance to the energy flow of lotic ecosystems, acting as a link between basal resourcers and predators. This study aimed to analyze patterns of abundance, richness and diversity of a benthic macroinvertebrates community in different types of substrate and different velocities of water. The study was carried on in a stretch of 40 meters of a headwater stream located at the Tijuca National Park, the Tijuca River. The sampling was perfomed in September 2016. We sampled four types of samples, which we denominated as “habitats”: litter in riffle, litter in pool, sand in riffle and sand in pool. A total total of 2.208 individuals were collected, with most of them being immature insects. We confirmed our assumptions by finding difference between the composition of the four types of habitats. It was found higher abundance, richness and diversity in litter habitats, confirming that the characteristics of this type of substrate are more favorable for the majority of the benthic macrofauna.

The Effect of Stream Discharge on Hyporheic Exchange

Streambed morphology, streamflow dynamics, and the heterogeneity of streambed sediments critically controls the interaction between surface water and groundwater. The present study investigated the impact of different flow regimes on hyporheic exchange in a boreal stream in northern Sweden using experimental and numerical approaches. Low-, base-, and high-flow discharges were simulated by regulating the streamflow upstream in the study area, and temperature was used as the natural tracer to monitor the impact of the different flow discharges on hyporheic exchange fluxes in stretches of stream featuring gaining and losing conditions. A numerical model was developed using geomorphological and hydrological properties of the stream and was then used to perform a detailed analysis of the subsurface water flow. Additionally, the impact of heterogeneity in sediment permeability on hyporheic exchange fluxes was investigated. Both the experimental and modelling results show that temporally increasing flow resulted in a larger (deeper) extent of the hyporheic zone as well as longer hyporheic flow residence times. However, the result of the numerical analysis is strongly controlled by heterogeneity in sediment permeability. In particular, for homogeneous sediments, the fragmentation of upwelling length substantially varies with streamflow dynamics due to the contribution of deeper fluxes.

Assessment of system characteristics of Gangotri glacier headwater stream

The study examine the major ion chemistry and p(CO2) variations of Ganga headwater stream for a stretch of 18 km from Gangotri glacier snout at Gomukh to Gangotri for characterising the open and closed system conditions and its temporal variations. The study has been carried out at three locations along the stream continuum, at the glacier snout (0 km), Bhojwasa (4 km) and Gangotri (18 km) covering three consecutive melt seasons from the year 2014 to 2016 and reveals the persistence of closed system conditions along the stream stretch. The year 2014 and 2016 melt seasons experience high p(CO2) closed system conditions associated with high suspended sediment flux, whereas the year 2015 experienced low p(CO2) closed system condition associated with low sediment flux suggesting in-stream sulphide oxidation during high sediment flux years and results into low values of the C-ratio. On the other hand, the melt season with low sediment flux such as the year 2015 showed dominance of HCO3− over SO42− and higher C-ratio. The study shows that the headwater reach of River Bhagirathi from Gomukh to Gangotri has prevalence of high p(CO2) closed system characteristics associated with high sediment flux and dominance of SO42− during the seasonal peak flow. This is suggested as a unique characteristic of the meltwaters of the upper Bhagirathi basin.

Long-term effects of a rural landscape on the structure and trophic organization of a fish assemblage

Abstract Aim We investigated the long-term effects of a rural landscape on the structure and trophic organization of a fish assemblage. Methods We compared environmental data and data from fish assemblage structure and trophic organization sampled in a stream located in a rural area, within a time gap of 20 years. Results We observed only punctual changes in the environmental variables and fish structure, which may not be related to the rural landscape. In fish diet, insectivory remained predominant in all sample periods. However, when we analyzed the groups of hexapods consumed by the fish species, we found a substitution of Ephemeroptera and Trichoptera by Diptera over time. Conclusions Although the fish assemblage structure was not affected, the insectivore diet analysis showed that the stream stretch could be in an ongoing process of environmental quality loss, once the simplification in the macroinvertebrates assemblage structure over time is indicative of environmental degradation.

Composition, abundance and biomass of a fish assemblage in a southern Brazilian coastal stream during polyhaline/euhaline condition

Abstract: The coastal streams of southernmost Brazil, Rio Grande do Sul state, are marked by a period of regular marine intrusion resultant from intense oceanic winds. In the present study we aimed to investigate the species composition, abundance and relative biomass of the ichthyofauna in the lower stretch of a coastal stream during summer, a period of regular marine intrusion. Estreito is a coastal hydrological complex composed by lakes, swamps and a perennial stream, located at the central-south portion of the coastal plain of Rio Grande do Sul state. During the summer of 2018, the ichthyofauna of lower Estreito stream was sampled by beach hauls applied in 17 random points distributed in a stretch of ca. 2km. Measurements at the sampled stretch revealed salinities between 19.3 to 31.3 ppt, characterizing the studied system as polyhaline/euhaline during summer. The sample of 4,533 specimens revealed the occurrence of 20 species, being the great majority marine-dwelling. The most abundant species were the anablepid Jenynsia lineata (70.3%), the cichlid Geophagus brasiliensis (19.3%) and the the mugilid Mugil curema (7.5%). The highest relative biomass was recorded for J. lineata, followed by M. curema and G. brasiliensis. The dominance of marine-dwelling species in the assemblage composition and the high abundance of limnic-estuarine J. lineata corroborate previous studies conducted in other washouts of Rio Grande do Sul.

Effect of the riparian vegetation removal on the trophic network of Neotropical stream fish assemblage

The study of the diet of fish is an important tool to assess different levels of environmental degradation, since the availability of food in the environment is a key factor for the fish occurrence. The removal of riparian vegetation usually degrades environmental quality, as this vegetation has an important role in providing energy to the ecosystem. This study investigates the effects of the removal of riparian vegetation on the fish assemblage trophic network. The study was carried out in two stretches of a southeastern Brazilian stream, one in a forest fragment and another in a pasture, during the wet and dry seasons of 2014. We analyzed the items consumed by each fish species using the frequency of occurrence and area of each item, which were combined to calculate the alimentary index, which was used to determine the food niche overlap of the fish and the specialization index of the trophic network. Aquatic Hexapoda, vegetal debris and organic matter dominated the trophic network of the two stretches. We detected higher values of food niche overlap in the forested stretch and more complex trophic networks in the pasture stretch. We found few seasonal variations in the items consumed and calculated indices in both stretches studied. The presence of grass on the banks in the pasture stretch and the importation of food resources from the upstream area may have provided a higher diversity of resources and consequently showed a more complex trophic network when compared to the forested stretch.

Modeling Remobilization of Neonicotinoid Residues from Tree Foliage in Streams-A Relevant Exposure Pathway in Risk Assessment?

Systemic neonicotinoid insecticides are increasingly used as a crop protection measure to suppress insect pests on trees. However, senescent foliage falling from treated trees represents a rarely studied pathway through which neonicotinoids may enter nontarget environments, e.g., surface waters. To estimate risk posed by this pathway, neonicotinoid residues were analyzed in foliage from black alder trees treated with one of three neonicotinoid insecticides (imidacloprid, thiacloprid, or acetamiprid) at five concentrations, each ranging from 0.0375-9.6 g active ingredient/cm trunk diameter at breast height (n = 3). Foliar residues measured at the time of leaf fall were used as input parameters for a model predicting imidacloprid water concentrations over a 100-m-long stream stretch as a consequence of remobilization from introduced foliage (input: 600 g foliage/m2 containing 80 μg imidacloprid/g). The water concentration (up to ∼250 ng/L) predicted by the model exceeded the recently proposed Maximum Permissible Concentration of 8.3 ng/L for ∼6.5 days. Moreover, dietary uptake was identified as an additional exposure route for aquatic organisms. The alternative pathway (i.e., introduction via leaf fall) and exposure route (i.e., dietary uptake) associated with the systemic nature of neonicotinoids should be accounted for during their registration process in order to safeguard ecosystem integrity.

An influence of mesohabitat structures (pool, riffle, and run) and land-use pattern on the index of biological integrity in the Geum River watershed

Previous studies on the biological integrity on habitat and landuse patterns demonstrated ecological stream health in the view of regional or macrohabitat scale, thus ignored the mesoscale habitat patterns of pool, riffle, and runs in the stream health analysis. The objective of this study was to analyze influences on the mesohabitat structures of pool, riffle, and run reaches on the fish guilds and biological integrity in Geum-River Watershed. The mesohabitat structures of pool, riffle, and run reaches influenced the ecological stream health along with some close relations on the fish trophic and tolerance guilds. The mesoscale components altered chemical water quality such as nutrients (TN, TP) and BOD and these, then, determined the primary productions, based on the sestonic chlorophyll-a. The riffle-reach had good chemical conditions, but the pool-reach had nutrient enrichments. The riffle-reach had a predominance of insectivores, while the pool-reach has a predominance of omnivores. Also, the riffle-reach had high proportions of sensitive fish and insectivore fish, and the pool-reach had high proportions of tolerant species in the community composition. The intermediate fish species in tolerance and omnivorous fish species in the food linkage dominated the community in the watershed, and the sensitive and insectivorous fishes decreased rapidly with a degradation of the water quality. All the habitat patterns were largely determined by the land-use patterns in the watershed. Trophic guilds and tolerance guilds of fish were determined by land-use pattern and these determined the stream health, based on the Index of Biological Integrity. This study remarks the necessity to include additional variables to consider information provided by mesohabitats and land-use distributions within the selected stream stretch. Overall, our data suggest that land-use pattern and mesohabitat distribution are important factors to be considered for the trophic and tolerance fish compositions and chemical gradients as well as ecological stream health in the watershed.

Habitat use by two extremophile, highly endemic, and critically endangered fish species (Gambusia eurystoma and Poecilia sulphuraria; Poeciliidae)

Abstract Effective conservation measures for endangered species require basic knowledge of habitat use and critical environmental variables influencing the species' occurrence. However, setting priorities may be difficult when multiple endangered species inhabit the same area and differ in habitat use. This study characterized the physical and chemical environment at 30 sites along a 2.5 km stretch of stream associated with the Baños del Azufre hydrogen sulphide spring complex, home to the only population of the widemouth gambusia (Gambusia eurystoma) in existence. Also present in the spring is Poecilia sulphuraria, a narrowly endemic and endangered species known from only a few populations. This study provides the most detailed report to date for physical and chemical drivers of species density in this small, extreme environment. Gambusia eurystoma were generally rare throughout the stream, and stream flow and substrate size were the best predictors of G. eurystoma densities. Conversely, P. sulphuraria were considerably more abundant, but no physical or chemical variables predicted their density among sites. Size distributions of adult P. sulphuraria were significantly influenced by stream flow and water chemistry, indicating a cost to living in greater proximity to toxic springs with high hydrogen sulphide concentrations. Overall, the results provide the first extensive report of environmental variation and factors associated with G. eurystoma densities, and indicate that conservation measures prioritizing environmental conditions for G. eurystoma will also benefit P. sulphuraria. Fish in sulphide springs provide prime examples of narrowly endemic species in desperate need of conservation, and their habitats face conservation challenges similar to those found in other springs with sympatric, endemic species in the world's arid regions. Copyright © 2016 John Wiley &amp; Sons, Ltd.