Dryland Streams Research Articles

Developing plant functional groups to identify changes in functional composition and diversity in a dryland river experiencing artificially sustained flows

Land use and climate changes are driving significant shifts in the magnitude and persistence of dryland stream surface flows. The impact of these shifts on ecological functioning is largely unknown, particularly where streams have become wetter rather than drier. This study investigated relationships between hydrologic regime (including surface water persistence, differences in groundwater depth and altered flooding dynamics) with plant traits and riverine vegetation functional composition. Our study system was a previously ephemeral creek in semi-arid northwest Australia that has received groundwater discharge from nearby mining operations for >15 years; surface flows are now persistent for ∼27 km downstream of the discharge point. We aimed to (i) identify plant functional groups (FGs) associated with the creek and adjacent floodplain; and (ii) assess their distribution across hydrological gradients to predict shifts in ecological functioning in response to changing flow regimes. Seven FGs were identified using hierarchical clustering of 40 woody perennial plant species based on morphometric, phenological and physiologic traits. We then investigated how FG abundance (projective foliar cover), functional composition, and functional and taxonomic richness varied along a 14 km gradient from persistent to ephemeral flows, varying groundwater depths, and distances from the stream channel. Dominant FGs were (i) drought avoidant mesic trees that are fluvial stress tolerant, or (ii) drought tolerant xeric tall shrubs that are fluvial stress intolerant. The drought avoidant mesic tree FG was associated with shallow groundwater but exhibited lower cover in riparian areas closer to the discharge (persistent surface flows). However, there were more FGs and higher species richness closer to the discharge point, particularly on the floodplain. Our findings demonstrate that quantifying FG distribution and diversity is a significant step in both assessing the impacts of mine water discharge on riverine ecosystems and for planning for post-mining restoration.

Where the water goes: Partitioning surface flow and streambed infiltration in an ephemeral river laboratory experiment

Ephemeral and intermittent flow in dryland streams plays a vital role in supporting delicate ecosystems and recharging groundwater in underlying aquifers. Despite extensive research, accurately predicting surface runoff, streambed water infiltration, and groundwater recharge during ephemeral streamflow events remains challenging due to the complex physical processes involved. Such processes are logistically difficult to observe in the field and should not be assessed through numerical studies alone. To tackle this complexity, we utilized an 8 m long, 0.25 m wide, and 0.6 m deep laboratory flume filled with fine sand as a physical model for event flows along a dry stream. This flume allowed us to measure both streamflow and streambed infiltration volumetrically as the stream wetting front advanced along its length during 8 steady inflow rates. The flume was divided into separate compartments to measure water across the sediment surface (streamflow) and water exiting through the flume’s bottom (groundwater recharge). To validate our physical tests, we compared them with simulated flows in an integrated surface–subsurface hydrological numerical model. The model results exhibited good agreement with the observed stream and groundwater dynamics. Through a parametric numerical investigation, we identified that hydraulic conductivity significantly controlled the rate of surface infiltration and the advancement of the stream wetting front. Interestingly, we found a linear relationship between surface discharge and stream wetting front location, as well as groundwater recharge, under steady-state conditions, in line with the governing equations. However, our simulations of variable streambed heterogeneity and elevation unveiled the crucial influence of these factors in determining the timing of downstream flow progression. This insight is essential for establishing ecological connectivity along non-perennial rivers and has significant implications for delivering environmental flows in arid regions where ephemeral rivers are prevalent. Our study underscores the value of flume experiments in elucidating the transient physical processes during surface water and groundwater interactions in intermittent streams. By employing a combination of physical modelling and numerical simulations, we advance our understanding of these vital hydrological interactions, contributing to sustainable water management practices in semi-arid and arid environments.

Arroyo formation impacts on an early dryland agricultural community in Northeastern Utah, USA

AbstractIndigenous Fremont farmers in Cub Creek, a part of northeastern Utah's Dinosaur National Monument, occupied the northern ecological margin of maize cultivation in western North America from A.D. 300 to 1300. Agriculture in Cub Creek was a response to multidecadal precipitation variability, but when precipitation stabilized between A.D. 750 and 1050, agricultural conditions improved and populations expanded to form villages along the floodplains of local dryland streams. Did the very same conditions (i.e., decreased precipitation variability) that allowed the growth of agricultural societies make them simultaneously vulnerable to arroyo formation, a key geomorphic risk to floodplain agriculturalists? Preliminary results from Cub Creek show that rapid sedimentation punctuated by episodic arroyo formation characterized the last 2000 years. We use stratigraphic and chronological evidence formalized in a Bayesian age model to develop a set of working hypotheses that a 2.5 m‐deep discontinuous arroyo formed before either A.D. 1020 or A.D. 1275. The earlier age corresponds with occupation of the Cub Creek village, while the later age corresponds with the end of Fremont agriculture in Cub Creek, and demonstrates regional synchronicity with arroyo formation across the Colorado Plateau. A second arroyo formed before A.D. 1490, indicating rapid alluvial cycling in Cub Creek. We conclude that floodplain instability and arroyo formation combined with the return of the dominant multidecadal precipitation variability regime beginning at A.D. 1050 was a key constraint on the growth potential of local populations. These findings have potential implications for the development of early Indigenous dryland agricultural systems throughout the interior of western North America.

Chemotaxonomic responses of autotrophic periphyton communities to nutrient additions in pools of an intermittent stream

AbstractThe algal groups present in periphyton communities form an important base of autochthonous food webs in freshwater streams. Nitrogen (N) and phosphorus (P) are key macronutrients in aquatic systems. Excess nutrients benefit some algal groups over others.We paired a nutrient‐diffusing substrata limitation experiment with high performance liquid chromatography to (a) identify which nutrient(s) limit periphyton production, and (b) how the periphyton biomass and community structure changes between isolated pools of differing hydrological characteristics along an intermittent dryland stream.Unique peaks for 21 pigments were identified and matched with published values. We then produced a PERMANOVA model using pigment ratios and CHEMTAX analysis to explore changes in community structure resulting from nutrient addition.Periphyton communities in these pools were co‐limited by N and P. Nitrogen additions caused the periphyton to shift from diatom‐ to chlorophyte‐dominated community structure and benefited cyanophyta growth. Phosphorus additions reduced the relative proportion of diatoms and also resulted in an increase in pheophoribide‐a, a pigment indicative of cell lysis, demonstrating a detrimental impact of P additions.Outcomes of this study show that when adding nutrient to a system there may be subtle shifts in community composition which can be telescoped up the food web regardless of the system's nutrient status.

Phosphorus sorption characteristics and interactions with leaf litter-derived dissolved organic matter leachate in iron-rich sediments of a sub-tropical ephemeral stream

This study investigated the influence of dissolved organic matter (DOM) additions on phosphate sorption kinetics of iron-rich sediments (39–50% hematite and goethite) from an ephemeral stream in the arid Pilbara region of sub-tropical northwest Australia. While phosphate sorption in stream sediments is known to be strongly influenced by sediment mineralogy as well as interactions with DOM, the mechanisms and significance of DOM on P-release from sediments with high sorption capacities, are largely undescribed. We assessed phosphorus (P) sorption behaviours by adding a range of solutions of known inorganic P concentrations that were amended with variable loadings of DOM derived from leachates of leaf litter to sediments from stream pools during the non-flowing phase. We compared the sorption capacity of the sediments and concurrent changes in DOM composition measured using fluorescence spectroscopy. We show that the low-dose DOM addition (~ 4 mg L−1 DOC) had the effect of reducing sediment P adsorption capacity, while for the high-dose DOM addition (~ 45 mg L−1 DOC), it was increased. The high-dose DOM was similar to pore water DOC and likely saturated sediment surface adsorption sites and produced P–OM–Fe complexes. This resulted in increased removal of P from solution. Sediment P sorption characteristics were well fitted to both Freundlich and Langmuir isotherm models regardless of DOC concentration. Langmuir P sorption maxima ranged from 0.106 to 0.152 mg g−1. General P sorption characteristics of these iron-rich sediments did not differ among pools of contrasting hydrological connectivity. Our results show how humic-rich DOM can modulate the sediment P availability in dryland streams. Unravelling the complexities of P availability is of particular significance to further our understanding of biogeochemical processes in aquatic ecosystems where P often acts as a limiting nutrient.

Linking sediment flux to river migration in arid landscapes through mass balance

ABSTRACT Lateral migration is a key process shaping sinuous rivers and controlling sediment exchange with floodplains. The rate at which channels migrate is affected by bend curvature, bank erodibility, and sediment supply. The relationship between migration rate and sediment supply is poorly understood in dryland regions, where direct measurements are scarce. Here we propose a simple mass-balance model to estimate the sediment flux of ephemeral streams in North America's Great Basin and establish a comparison with timelapse photogrammetric data of lateral migration. The model takes into consideration variables such as long-term hillslope erosion, transient sediment storage in intra-catchment lowlands, and sediment bypass to depocenters. Our results point to first-order similarities in how sediment supply drives channel migration across diverse hydro-climatic regimes. However, we find that, for a given sediment supply and channel width, and despite their ephemeral discharge, dryland streams with minimal bank vegetation migrate about three times faster than humid-climate, vegetated ones. This difference in migration pace likely results from the compound effect of bank erodibility and bend geometry. Our model sheds new light on the driving mechanisms of channel mobility in dryland streams and may find application in assessing the sediment budgets of ungauged streams, reservoir trapping, and morphodynamic adjustments in stressed watersheds.

Natural infrastructure in dryland streams (NIDS) can establish regenerative wetland sinks that reverse desertification and strengthen climate resilience

In this article we describe the natural hydrogeomorphological and biogeochemical cycles of dryland fluvial ecosystems that make them unique, yet vulnerable to land use activities and climate change. We introduce Natural Infrastructure in Dryland Streams (NIDS), which are structures naturally or anthropogenically created from earth, wood, debris, or rock that can restore implicit function of these systems. This manuscript further discusses the capability of and functional similarities between beaver dams and anthropogenic NIDS, documented by decades of scientific study. In addition, we present the novel, evidence-based finding that NIDS can create wetlands in water-scarce riparian zones, with soil organic carbon stock as much as 200 to 1400 Mg C/ha in the top meter of soil. We identify the key restorative action of NIDS, which is to slow the drainage of water from the landscape such that more of it can infiltrate and be used to facilitate natural physical, chemical, and biological processes in fluvial environments. Specifically, we assert that the rapid drainage of water from such environments can be reversed through the restoration of natural infrastructure that once existed. We then explore how NIDS can be used to restore the natural biogeochemical feedback loops in these systems. We provide examples of how NIDS have been used to restore such feedback loops, the lessons learned from installation of NIDS in the dryland streams of the southwestern United States, how such efforts might be scaled up, and what the implications are for mitigating climate change effects. Our synthesis portrays how restoration using NIDS can support adaptation to and protection from climate-related disturbances and stressors such as drought, water shortages, flooding, heatwaves, dust storms, wildfire, biodiversity losses, and food insecurity.

Diel cycles of δ13CDIC and ecosystem metabolism in ephemeral dryland streams

Streams in hot, arid environments often exist as a series of isolated pools along main channels. During these periods, shallow alluvial through flow may strongly influence key ecological processes within pools. We measured diel changes in δ13C values of dissolved inorganic carbon (DIC) and dissolved oxygen (DO) in two pools of ephemeral, dryland streams. We quantified alluvial water connectivity through stable isotope analysis (δ18O and δ2H) of pool and alluvial water. We also estimated gross primary productivity (GPP) and ecosystem respiration (ER) rates across a wider set of pools in both streams. δ13CDIC values displayed regular diel cycles, where both pools displayed small but similar daily amplitude (0.7–0.9‰) despite contrasting amplitudes of change in DO (0.8 mg L−1 vs. 2.8 mg L−1) and contrasting alluvial water connectivity (connected vs. disconnected). Water temperature was the strongest predictor of both δ13CDIC values and rates of change in δ13CDIC across both pools. Across both streams, all pools were net heterotrophic. GPP (0.35–1.73 g O2 m−2 d−1) and ER (0.49–2.64 g O2 m−2 d−1) rates were linked to aquatic vegetation cover. The disconnect between diurnal amplitudes of δ13C values and DO concentrations thus suggests that ecological drivers of gas exchange became increasingly localised as pools contracted.

Water Quality and Macroinvertebrate Community in Dryland Streams: The Case of the Tehuacán-Cuicatlán Biosphere Reserve (México) Facing Climate Change

The Tehuacán-Cuicatlán Biosphere Reserve (TCBR), the southernmost semi-arid zone of North America, includes two dryland streams, the Río Salado (RS) and Río Grande (RG); it is surrounded by high vegetation diversity, a cacti diversification center, and the densest columnar cacti forest worldwide. However, no scientific knowledge is currently available on these dryland streams. We evaluated water quality, its relationship with the local geological characteristics, land uses, and the composition of aquatic macroinvertebrates (AM), analyzing their bioindicator potential. These results were discussed in relation to climate change predictions. The RS showed higher mineralization, salinity, hardness, water and air temperature, and low water quality index (WQI), relative to the RG. A discriminant analysis showed spatial (mineralization, salinity, and hardness in the RS) and temporal patterns (higher nitrogen compounds and temperature in the rainy season). The RS showed a lower AM diversity (40 taxa) compared to the RG (73 taxa); Ephemeroptera-Plecoptera-Trichoptera reached higher values in the RG. A co-inertia analysis identified five groups of sites with different AM assemblages and water quality characteristics. Climate change predictions for the TCBR suggest increased aridity, higher temperature, and lower rainfall, leading to reduced river flow and increased salinity and mineralization. These could alter habitat features and connectivity, with loss of AM diversity, highlighting the vulnerability of these unique ecosystems to climate change.

Revisiting the modified Hvorslev formula to account for the dynamic process of streambed clogging: Field validation

Estimating streambed hydraulic conductivity is particularly important in arid and semi-arid regions, where surface water infiltration from ephemeral dryland streams helps maintain riparian ecosystems. In situ permeameter tests are widely used to estimate the hydraulic conductivity of streambed sediments. Previous studies have demonstrated that one of the major factors controlling spatiotemporal variations in streambed hydraulic conductivity in such areas is streambed clogging by suspended sediments. Recently, a modified Hvorslev (1951) formula was theoretically proposed to account for the effect of streambed clogging on streambed hydraulic properties due to suspended particle sedimentation during falling-head tests. To validate the modified formula, fieldwork was conducted at five sites in the lower Heihe River, a typical intermittent river of north-western China, in September 2017. A simple PVC pipe with an inner diameter of 4 cm was used as a permeameter for the field tests. Both muddy river water and clean water were used to fill the permeameter to conduct comparative tests. To differentiate the interpretation of the field testing data obtained with clean water, the data obtained with muddy river water were analysed with an equation considering an iteratively increasing total hydraulic resistance of riverbed sediments due to the effects of riverbed clogging. The estimated vertical hydraulic conductivity for the top 0.3 m of riverbed sediments varied from 1 to 41 m/day. The proposed model was shown to accurately estimate the hydraulic conductivity of riverbed sediments using both clean and muddy water during slug tests.

Relação peso-comprimento de duas espécies de peixes em um rio intermitente no nordeste do Brasil

Given the context of high hydrological uncertainties and stress in dryland streams, the evaluation of length-weight relationships and condition factors of fish becomes of chief concern for the understanding of these systems. Length-weight relationships were estimated for two fish species, Astyanax fasciatus (Cuvier 1819) and Astyanax bimaculatus (Linnaeus 1758), from temporary pools in a dry river bed. Length-weight relationship information was compared between dry and rainy seasons. Sampling was performed using gillnets, manual trawls and cast nets at different reaches of the river. Estimates for the values of b of the length-weight relationship were 2.63 for A. fasciatus, and 2.78 for A. bimaculatus, indicating a negative allometric growth. The condition factor ranged between 0.0180 and 0.0187 for both species, without significant difference between seasons, and meaning a greater investment in growth rather than weight gain. Results indicated that fishes tended to be larger during the rainy season, likely representing the result of a higher frequency of immature and smaller individuals during the previous dry season as a consequence of reproductive activity of adults. Keywords: Astyanax fasciatus, Astyanax bimaculatus, temporary pools, condition factor.

A Statistical Method to Predict Flow Permanence in Dryland Streams from Time Series of Stream Temperature

Intermittent and ephemeral streams represent more than half of the length of the global river network. Dryland freshwater ecosystems are especially vulnerable to changes in human-related water uses as well as shifts in terrestrial climates. Yet, the description and quantification of patterns of flow permanence in these systems is challenging mostly due to difficulties in instrumentation. Here, we took advantage of existing stream temperature datasets in dryland streams in the northwest Great Basin desert, USA, to extract critical information on climate-sensitive patterns of flow permanence. We used a signal detection technique, Hidden Markov Models (HMMs), to extract information from daily time series of stream temperature to diagnose patterns of stream drying. Specifically, we applied HMMs to time series of daily standard deviation (SD) of stream temperature (i.e., dry stream channels typically display highly variable daily temperature records compared to wet stream channels) between April and August (2015–2016). We used information from paired stream and air temperature data loggers as well as co-located stream temperature data loggers with electrical resistors as confirmatory sources of the timing of stream drying. We expanded our approach to an entire stream network to illustrate the utility of the method to detect patterns of flow permanence over a broader spatial extent. We successfully identified and separated signals characteristic of wet and dry stream conditions and their shifts over time. Most of our study sites within the entire stream network exhibited a single state over the entire season (80%), but a portion of them showed one or more shifts among states (17%). We provide recommendations to use this approach based on a series of simple steps. Our findings illustrate a successful method that can be used to rigorously quantify flow permanence regimes in streams using existing records of stream temperature.

Adjustment of dryland stream channels over four decades of urbanization

Abstract Human settlements are increasingly expanding into drylands and mountain areas that are less accommodating to urban communities, creating hazards related to the conveyance of water and sediment. Comparatively few studies of urbanization effects on streams have focused on drylands, especially steep terrains. Development of the town of Fountain Hills in the foothills of the McDowell Mountains (Arizona, USA) since the 1970s provides an example of dryland streams adjusting to urbanization over several decades. Urbanization has introduced numerous roads with dip crossings that fragment channels, causing management challenges. Field surveys in 1987, 2006, and 2016 showed that wash channels in urban areas are up to ten times larger than comparable natural channels. Roads have exerted a spatial pattern of geomorphic influence, where channels downstream of dip crossings are narrower and deeper than those upstream, reflecting erosion with input of urban drainage. Repeat measurements showed that the pattern of downstream incision and upstream accretion has become less evident in the recent decade. The muted response by 2016 results from evolving management that has improved the conveyance of water and sediment, including stormwater controls and erosion protection. Additionally, comparison of channel changes at dip crossings with those at culverted crossings revealed greater impact at dip crossings. Less morphological change at culverts suggests more effective throughput of water and sediment. Results demonstrate successful adaptive management including decisions to employ culverts in recent developments. The evolving landscape of Fountain Hills provides a possible template for understanding and predicting changes in other dryland areas undergoing urbanization.

Estimating groundwater-ephemeral stream exchange in hyper-arid environments: Field experiments and numerical simulations

Surface water infiltration from ephemeral dryland streams is particularly important in hyporheic exchange and biogeochemical processes in arid and semi-arid regions. However, streamflow transmission losses can vary significantly, partly due to spatiotemporal variations in streambed permeability. To extend our understanding of changes in streambed hydraulic properties, field investigations of streambed hydraulic conductivity were conducted in an ephemeral dryland stream in north-western China during high and low streamflow periods. Additionally, streamflow transmission losses were numerically estimated using combined stream and groundwater hydraulic head data and stream and streambed temperature data. An analysis of slug test data at two different river flow stages (one test was performed at a low river stage with clean water and the other at a high river stage with muddy water) suggested that sedimentation from fine-grained particles, i.e., physical clogging processes, likely led to a reduction in streambed hydraulic properties. To account for the effects of streambed clogging on changes in hydraulic properties, an iteratively increasing total hydraulic resistance during the slug test was considered to correct the estimation of streambed hydraulic conductivity. The stream and streambed temperature can also greatly influence the hydraulic properties of the streambed. One-dimensional coupled water and heat flux modelling with HYDRUS-1D was used to quantify the effects of seasonal changes in stream and streambed temperature on streamflow losses. During the period from 6 August 2014 to 4 June 2015, the total infiltration estimated using temperature-dependent hydraulic conductivity accounted for approximately 88% of that using temperature-independent hydraulic conductivity. Streambed clogging processes associated with fine particle settling/wash up cycles during flow events, and seasonal changes in streamflow temperature are two considerable factors that affect water infiltration in ephemeral dryland streams. Our results show that time series measurements of stream and sediment temperature and surface and groundwater head can be used to effectively determine the seasonal dynamics of streambed water exchange. Such combined heat and head monitoring at field sites is useful for calibrating regional surface-groundwater models. The results of this study may provide insights into hyporheic exchange in ephemeral dryland streams.

Characterising the dynamics of surface water-groundwater interactions in intermittent and ephemeral streams using streambed thermal signatures

Ephemeral and intermittent flow in dryland stream channels infiltrates into sediments, replenishes groundwater resources and underpins riparian ecosystems. However, the spatiotemporal complexity of the transitory flow processes that occur beneath such stream channels are poorly observed and understood. We develop a new approach to characterise the dynamics of surface water-groundwater interactions in dryland streams using pairs of temperature records measured at different depths within the streambed. The approach exploits the fact that the downward propagation of the diel temperature fluctuation from the surface depends on the sediment thermal diffusivity. This is controlled by time-varying fractions of air and water contained in streambed sediments causing a contrast in thermal properties. We demonstrate the usefulness of this method with multi-level temperature and pressure records of a flow event acquired using 12 streambed arrays deployed along a ∼ 12 km dryland channel section. Thermal signatures clearly indicate the presence of water and characterise the vertical flow component as well as the occurrence of horizontal hyporheic flow. We jointly interpret thermal signatures as well as surface and groundwater levels to distinguish four different hydrological regimes: [A] dry channel, [B] surface run-off, [C] pool-riffle sequence, and [D] isolated pools. The occurrence and duration of the regimes depends on the rate at which the infiltrated water redistributes in the subsurface which, in turn, is controlled by the hydraulic properties of the variably saturated sediment. Our results have significant implications for understanding how transitory flows recharge alluvial sediments, influence water quality and underpin dryland ecosystems.

Importance of neutral processes varies in time and space: Evidence from dryland stream ecosystems.

Many ecosystems experience strong temporal variability in environmental conditions; yet, a clear picture of how niche and neutral processes operate to determine community assembly in temporally variable systems remains elusive. In this study, we constructed neutral metacommunity models to assess the relative importance of neutral processes in a spatially and temporally variable ecosystem. We analyzed macroinvertebrate community data spanning multiple seasons and years from 20 sites in a Sonoran Desert river network in Arizona. The model goodness-of-fit was used to infer the importance of neutral processes. Averaging over eight stream flow conditions across three years, we found that neutral processes were more important in perennial streams than in non-perennial streams (intermittent and ephemeral streams). Averaging across perennial and non-perennial streams, we found that neutral processes were more important during very high flow and in low flow periods; whereas, at very low flows, the relative importance of neutral processes varied greatly. These findings were robust to the choice of model parameter values. Our study suggested that the net effect of disturbance on the relative importance of niche and neutral processes in community assembly varies non-monotonically with the severity of disturbance. In contrast to the prevailing view that disturbance promotes niche processes, we found that neutral processes could become more important when the severity of disturbance is beyond a certain threshold such that all organisms are adversely affected regardless of their biological traits and strategies.

Hydrology drives seasonal variation in dryland stream macroinvertebrate communities

Many lotic ecosystems are spatially and temporally heterogeneous but none more so than dryland streams flowing through arid and semi-arid landscapes. Understanding seasonal variation in richness and trait composition is critical to our fundamental understanding of these dynamic stream networks. Here, we analyzed aquatic macroinvertebrate communities within perennial and intermittent reaches in replicate dryland watersheds in southwestern USA across 10 seasons and 4 years. We quantified how hydrology, season, and microhabitat type affected taxa richness and trait composition. Taxa richness was higher in perennial than intermittent reaches, in pools than riffles, and in fall and summer than in spring. The interaction between hydrology and season was important; the difference between fall high and spring low richness was greater in intermittent than perennial streams. Hydrology was the main predictor of trait composition; intermittent streams supported a larger fraction of small taxa, taxa with the ability to undergo diapause, and uni- or multivoltine taxa (i.e., taxa with shorter life cycles). Trait composition did not vary across seasons among perennial reaches whereas the fraction of aerial dispersers appeared to be greater in spring than other seasons among intermittent reaches. Our results were largely consistent with predictions of the habitat templet; r-selected traits were more frequently represented in intermittent communities. The temporal characterization of macroinvertebrate community structure in dryland streams provides a powerful glimpse of how stream communities may respond to a drying climate.

River network architecture, genetic effective size and distributional patterns predict differences in genetic structure across species in a dryland stream fish community.

Dendritic ecological network (DEN) architecture can be a strong predictor of spatial genetic patterns in theoretical and simulation studies. Yet, interspecific differences in dispersal capabilities and distribution within the network may equally affect species' genetic structuring. We characterized patterns of genetic variation from up to ten microsatellite loci for nine numerically dominant members of the upper Gila River fish community, New Mexico, USA. Using comparative landscape genetics, we evaluated the role of network architecture for structuring populations within species (pairwise FST ) while explicitly accounting for intraspecific demographic influences on effective population size (Ne ). Five species exhibited patterns of connectivity and/or genetic diversity gradients that were predicted by network structure. These species were generally considered to be small-bodied or habitat specialists. Spatial variation of Ne was a strong predictor of pairwise FST for two species, suggesting patterns of connectivity may also be influenced by genetic drift independent of network properties. Finally, two study species exhibited genetic patterns that were unexplained by network properties and appeared to be related to nonequilibrium processes. Properties of DENs shape community-wide genetic structure but effects are modified by intrinsic traits and nonequilibrium processes. Further theoretical development of the DEN framework should account for such cases.

Are Chironomidae (Diptera) good indicators of water scarcity? Dryland streams as a case study

Water scarcity is becoming one of the greatest challenges that human societies will face during this century. Monitoring water availability is expensive and technically challenging. In this regard, biological communities (e.g. aquatic insects) offer a cost-effective alternative, since they integrate temporal and spatial hydrological variability.Here we explore the potential of Chironomidae (Diptera), which have been usually neglected due to their complex taxonomy, as indicators of both local habitat condition and water scarcity. The study took place in 28 sites across seven dryland streams distributed within a 400km2 section of the Upper San Pedro River basin, southeastern Arizona. The selected streams covered a wide range of hydrological variability, which was continuously measured through the deployment of 15 electrical resistance (ER) sensors. Chironomidae taxa with no drought-resistance strategies were rarely found in streams that experienced frequent drying events (i.e. intermittent and ephemeral streams), suggesting that droughts have the potential to reduce species richness due to local extinctions of drought-intolerant taxa. Intermittent and ephemeral streams registered low canopy cover and a significantly higher abundance of scrapers (which mainly feed on algae) and shredders (feeding on poorly decomposed coarse organic material). This suggests that structural changes associated to drought (e.g. reduced canopy cover and decomposition rates) might lead to changes in the functional composition of the Chironomidae assemblages. We conclude that Chironomidae species can be used as indicators of hydrological variability and the impacts of drought on streams in the absence of flow gauges.

Alluvial ground water influences dissolved organic matter biogeochemistry of pools within intermittent dryland streams

Summary The dissolved organic matter (DOM) biogeochemistry of intermittent rivers is affected by periods of disconnection and drying after cessation of surface flow, with a pronounced concentration of solutes as pools evaporate and contract. However, inputs of alluvial ground water to stream pools may offset the effects of evaporation and influence the quantity and composition of DOM. We used δ18O and δ2H values of surface water and alluvial ground water (AW) together with DOM fluorescence excitation–emission spectroscopy to investigate changes in DOM biogeochemistry of pools from four intermittent streams in semi‐arid north‐west Australia. Our objective was to determine whether the fluorescence characteristics of DOM in pools are driven by their connectivity to alluvial ground water. Pool water δ18O values ranged from −8 to 13.1 ‰ and showed progressive enrichment as pools became disconnected from AW and subsequently evaporated. Parallel factor analysis of DOM fluorescence revealed that DOM in pools was generally dominated by humic‐like components, with lesser contributions from tryptophan‐like and tyrosine‐like components. DOC concentrations were higher in more evaporated pools. Humic‐like and tryptophan‐like DOM were also highest in pools isolated from AW, suggesting that inputs of AW had a dilution effect while also moderating evaporative concentration of solutes. In contrast, tyrosine‐like DOM was only weakly correlated with the hydrological status of pools. The proportion that each component contributed to total DOM fluorescence remained mostly consistent across both AW‐connected and evaporating pools. However, some pools with AW throughflow had higher contributions from tyrosine‐like fluorescence and lower contributions from humic‐like fluorescence. Disconnection from AW and subsequent evaporative concentration of solutes is an important control on DOM fluorescence characteristics in these intermittent streams. However, production and removal of DOM were only rarely influenced by AW connectivity, likely reflecting the high hydrological variability of intermittent rivers, even following cessation of surface flow.