Alpine Headwater Stream Research Articles

Long‐term reconstruction of energy fluxes in an alpine river: Effects of flow regulation and restoration

AbstractFlow regulation of montane and alpine headwater streams can fundamentally alter food web structure and energy flows through changes in productivity, resource availability, and community assembly. Dam flow‐release schemes can be used to mitigate the environmental impacts of flow regulation via environmental flows, which can increase discharge variability and other ecologically important hydrological properties. In particular, managed floods can reintroduce disturbance to the system and stimulate the reactivation of physical habitat dynamics. However, how managed floods might restore ecosystem processes is virtually unknown. In this study, we examined patterns in potential energy fluxes before, during and after a long‐term experimental flood program on the river Spöl, a regulated alpine River in southeast Switzerland. We used benthic samples collected during long‐term monitoring and stable isotope analysis (δ13C and δ15N) of macroinvertebrates and their potential food sources to reconstruct secondary production, and potential energy fluxes, over a 20‐year study period. The experimental floods did not alter the relative importance of basal resources but resulted in a considerable decline in secondary production, which remained low after the discontinuation of the floods. Our data suggest that a lack of recolonization by mosses following the discontinuation of the experimental flood program on the river Spöl may have driven patterns in energy fluxes by limiting macroinvertebrates using mosses for habitat. The effects of environmental flows on energy flows in this system thus depend on flood disturbance and the environmental context following the discontinuation of floods.

Field observation and mathematical representation of the hydrogeological function of alpine landforms in the Canadian Rockies

AbstractGroundwater discharge sustains the baseflow of alpine headwater streams, which is critical for water supply and aquatic environments in mountainous regions. Periglacial landforms typical of alpine headwaters (e.g., talus, moraine, rock glacier, alpine meadows) are important aquifers in alpine watersheds. This study examines the hydrological function of an alpine aquifer complex in a small headwater basin in the Canadian Rockies. The aquifer complex consisting of talus, alpine meadow underlain by a bedrock depression, and recessional moraine provided essentially all baseflow of a 6.5 km2 watershed, even though the upper sub‐watershed containing the aquifer complex occupies only 14% of the watershed. Chemical and isotopic signatures indicated that the recessional moraine serves as a gatekeeper of the upper sub‐watershed, whereby it integrates groundwater components from multiple aquifers and controls the discharge from the outlet springs. Field observation of discharge and the water table in the moraine aquifer showed a nonlinear groundwater storage‐discharge relationship. Numerical groundwater flow models of the upper sub‐watershed showed that the transmissivity feedback resulting from a decrease in hydraulic conductivity with depth was essential for determining the nonlinear storage‐discharge relationship. A simple exponential function was proposed to represent the observed groundwater storage‐discharge relationship, which can be implemented within large‐scale hydrological models to simulate baseflow coming out of alpine headwater regions.

Long-lasting effects of experimental flow intermittency on alpine stream macroinvertebrates (Val Roseg, Switzerland)

Changing weather patterns and receding glaciers are predicted to increase flow intermittency in alpine streams. If aquatic macroinvertebrate communities largely comprise taxa adapted to perennial flows, an increase in flow intermittency substantially reduces biodiversity and affects functional processes. We conducted a before-after-control-impact field experiment to examine how macroinvertebrate communities in an alpine headwater stream responded to and recovered from a repeated experimental increase in flow intermittency. Flow in one channel was manipulated to simulate increased summer intermittency (June–September) over two consecutive years, whilst an adjacent channel served as a control. We monitored the density of benthic macroinvertebrates, periphyton and organic matter at approximately monthly intervals over three years during the snow-free period. Before manipulation, both channels had similar ecological properties. The flow manipulation reduced the overall macroinvertebrate density, and especially the proportional rheophile density, across both years. Recovery of the macroinvertebrate community following experimental flow intermittency took more than a year, and longer than our study period. This could be due to long aquatic life stages, dispersal limitation and biotic interactions. We conclude that climate-induced changes in alpine stream flow regimes can lead to a fundamental shift in macroinvertebrate assemblages through local extinctions, mostly of rheophilic species.

Discharge‐Modulated Soil Organic Carbon Export From Temperate Mountainous Headwater Streams

AbstractErosion and riverine transport of organic carbon is an important component of the global carbon cycle, but the significance of this process for Earth's surface carbon budgets depends on the sources of carbon being mobilized. In this study, we aim to constrain how runoff‐driven erosion modulates the contribution of different carbon source endmembers, that is, bedrock, soil and vegetation, in three forested headwater catchments in the Swiss Prealps. The sources of organic carbon are determined using an inverse model based on bulk carbon isotope signatures and the abundances and distributions of long‐chain n‐alkane plant wax biomarkers in suspended sediments collected over a range of discharges. Despite landcover differences and contrasting bulk particulate organic carbon (POC) signatures, the increase of soil‐sourced organic carbon with runoff is similar in all three studied catchments. This apparent existence of common processes implies that export fluxes of soil organic carbon may be extrapolated to similar catchments. Overall, our analysis shows that runoff‐driven soil erosion in these alpine headwater streams is responsible for the export of at least 0.3 to 0.8 gC m−2 yr−1 as POC. Our study also shows that despite a significant variability in isotopic and molecular POC signatures at low runoff, all three catchments show a convergence of these signatures at higher discharges. Suspended sediment samples collected at above‐average runoff are hence most representative of overall endmember contributions.

Spatial–temporal variations in riverine carbon strongly influenced by local hydrological events in an alpine catchment

Abstract. Headwater streams drain >70 % of global land areas but are poorly monitored compared with large rivers. The small size and low water buffering capacity of headwater streams may result in a high sensitivity to local hydrological alterations and different carbon transport patterns from large rivers. Furthermore, alpine headwater streams on the “Asian water tower”, i.e., Qinghai–Tibetan Plateau, are heavily affected by thawing of frozen soils in spring as well as monsoonal precipitation in summer, which may present contrasting spatial–temporal variations in carbon transport compared to tropical and temperate streams and strongly influence the export of carbon locked in seasonally frozen soils. To illustrate the unique hydro-biogeochemistry of riverine carbon in Qinghai–Tibetan headwater streams, here we carry out a benchmark investigation on the riverine carbon transport in the Shaliu River (a small alpine river integrating headwater streams) based on annual flux monitoring, sampling at a high spatial resolution in two different seasons and hydrological event monitoring. We show that riverine carbon fluxes in the Shaliu River were dominated by dissolved inorganic carbon, peaking in the summer due to high discharge brought by the monsoon. Combining seasonal sampling along the river and monitoring of soil–river carbon transfer during spring thaw, we also show that both dissolved and particulate forms of riverine carbon increased downstream in the pre-monsoon season due to increasing contribution of organic matter derived from thawed soils along the river. By comparison, riverine carbon fluctuated in the summer, likely associated with sporadic inputs of organic matter supplied by local precipitation events during the monsoon season. Furthermore, using lignin phenol analysis for both riverine organic matter and soils in the basin, we show that the higher acid-to-aldehyde (Ad/Al) ratios of riverine lignin in the monsoon season reflect a larger contribution of topsoil likely via increased surface runoff compared with the pre-monsoon season when soil leachate lignin Ad/Al ratios were closer to those in the subsoil than topsoil solutions. Overall, these findings highlight the unique patterns and strong links of carbon transport in alpine headwater catchments with local hydrological events. Given the projected climate warming on the Qinghai–Tibetan Plateau, thawing of frozen soils and alterations of precipitation regimes may significantly influence the alpine headwater carbon transport, with critical effects on the biogeochemical cycles of the downstream rivers. The alpine headwater catchments may also be utilized as sentinels for climate-induced changes in the hydrological pathways and/or biogeochemistry of the small basin.

Coarse sediment transfer and geomorphic changes in an alpine headwater stream

Headwater sediment dynamics is very important for fluvial processes in downstream reaches, but have been poorly studied until now, and existing sediment processes are not well understood. In fact, quantifying bedload transport and assessing river changes is challenging, in mountain environments, and traditional bedload formulas tend to overestimate sediment fluxes. This work aims to analyze the coarse sediment transfer processes acting in the high-altitude headwater reach of Rio Cordon (eastern Italian Alps). Pebbles provided by Passive Integrated Transponders (PITs) were deployed in the study site and their position was periodically monitored over 20 months, in order to quantify sediment mobility (i.e., travel distances and virtual velocities) and, the bedload volumes. The geomorphic changes due to flood events were analyzed through high resolution DEMs (0.05 m cell), derived with Structure from Motion (SfM) in three different periods (October 2017, September 2018 and June 2019), allowing to compute the DEMs of Difference (DoDs). The study period was characterized by two distinct flood events, a short and intense summer rainstorm in August 2018 (recurrence interval RI > 2 years) and a severe cyclonic rainfall event (i.e. Vaia storm, RI > 50 years) in October 2018. The combination of tracer and DoDs analysis shows that ordinary flood maintained the reach in an apparent equilibrium state, with balanced volumes of erosion and deposition. Differently, the high-magnitude flood was characterized by bedload fluxes an order of magnitude higher than ordinary event and an evident net erosion. However, in both events the volumes mobilized in the headwater did not contribute significantly to the sediment fluxes delivered at the Rio Cordon basin outlet. This highlighted the reduced efficiency in sediment propagation mainly due to a marked dis-connectivity associated with the presence of a hanging valley in the middle part of the basin. This study discusses the complex dynamism of headwater streams, stressing the importance of understanding sediment fluxes and connectivity conditions that can strongly affect human activities and infrastructures even in lowland reaches.

Exploring the Sources of Unexpected High Methane Concentrations and Fluxes From Alpine Headwater Streams

AbstractThe dynamics of methane generation and evasion from well‐oxygenated, oligotrophic streams have been traditionally neglected. We estimated evasion of methane and assessed its sources and production pathways using a stable isotope approach in 16 oxygen‐rich and C‐poor (dissolved organic carbon: 55.32 ± 57.56 μmol/L) Alpine headwater streams. Methane was often supersaturated relative to the atmosphere (0.093 ± 0.179 μmol/L). Fluxes (0.87 ± 1.34 mmol·m−2·day−1) were unexpectedly high and comparable to those from high‐latitude lakes and reservoirs. Our findings suggest that methane in the streambed was largely produced from carbon dioxide reduction, whereas acetoclastic pathways and major deliveries from adjacent soils, assessed from a mass balance, may have contributed to stream water methane. This study sheds new light on high‐alpine streams as a hitherto unaccounted source of methane to the atmosphere.

Macroinvertebrate diversity and rarity in non-glacial Alpine streams

Alpine landscapes are being transformed through the rapid recession of glaciers, resulting in the development of numerous non-glacial headwater streams inhabited by a diverse assemblage of macroinvertebrates. We examined spatial patterns in biodiversity and rarity of macroinvertebrates in 41 non-glacial streams from five glacierized catchments in the Swiss Alps undergoing rapid glacial recession over the last decades. Water physico-chemistry and food resources (periphyton, benthic organic matter) varied widely among streams within each catchment, while no significant differences occurred among catchments. Variability in community composition was similar among streams within each catchment but differed among catchments, reflecting differences in catchment-scale species pools due to biogeographical context and season. Overall, 101 taxa from ca 33,000 individuals collected were identified in the streams with 7–33 taxa found in individual streams. Some 64% of the taxa comprised less than 5% of the total abundances at the streams (rare in abundance) with 78% of the taxa being represented by less than 5% of the most common taxon (Baetis sp.), whereas 47% of the taxa were found in less than 10% of the streams (rare in distribution). No taxon was found at all sites (maximum presence at 85% of the sites), while 15% of the taxa were found at 50% of the sites or more. However, analyzing the rank-abundance distribution showed that rarity was less prevalent than previously shown in other ecosystems. The results indicated that community assembly of alpine headwater streams is a complex interaction between environmental properties (habitat filtering), habitat stability coupled with dispersal (source sink dynamics), and time since deglaciation (island biogeography). Integrating these processes is essential towards understanding ongoing colonization events in headwater streams of alpine catchments as glaciers continue to recede.

Lentification in Alpine rivers: patterns of diatom assemblages and functional traits

Hydrological alterations in Alpine rivers have been largely increasing due to the combined effects of global climate change and local impacts. Water flow intermittency starts with a process called lentification, which leads to the transformation of the aquatic ecosystem with severe consequences on biotic communities. These consequences have a greater impact in Alpine headwater streams, which are considered important and fragile hotspots of biodiversity and a source of threatened species; the new regime creates conditions to which native biota are poorly adapted. The main aim of our work was to evaluate taxonomical, functional and structural changes of diatom communities in Alpine rivers during the lentification process that precedes a supra-seasonal drought. Contrary to what was expected, communities exposed to river shrinking showed a level of homogeneity comparable to those collected in permanent sections. We therefore hypothesized that lentification created an intermediate disturbance that favoured heterogeneity of assemblages. Conversely, ecological guilds, growth forms and eco-morphological groups varied along temporal and spatial gradients driven by the lentification process. Small, motile and medium-sized low profile species were more abundant during summer, as well as adnate and stalked ones. Lentification limited the presence of high profile guilds and mucous-forming colonies. Droughts in Alpine streams are fairly recent processes, and the dynamics of aquatic communities facing water scarcity are largely unknown. The results we obtained provide important insights towards a more refined functional classification of diatoms for evaluating hydrological alterations.

Chemical regulation of alpine headwater streams during a storm event (Bogong High Plains, Victoria, Australia)

The headwater streams originating in the Australian Alps are the highest water yielding landscape in South-Eastern Australia and are projected to be impacted by climate change through longer dry periods and more episodic rainfall. In this work we studied the major ion and DOC responses of three alpine peatlands (and the broader catchment containing these systems) to a high intensity (summer) storm event. Despite the high volume of rainfall, major ions in stream waters remained strongly chemostatic throughout the event. This was particularly the case for Ca2+ and Mg2+, as well as the alkalinity by association, and suggests that chemical regulation of these particular cations occurs through rapid equilibration processes. DOC concentrations increased during the storm pulse, leading to a shift in alkalinity partitioning from bicarbonate to organic anions and a decrease in pH, mediated by the CO2 saturation levels in the stream water. Our results suggest that alkalinity generation (Ca2+ and Mg2+ acquisition) and partitioning (DOC export) are decoupled processes that may respond differently to repeat storm events depending on the capacity of these systems to provide these constituents. Under extreme case scenarios depletion of DOC (at constant alkalinity) would lead to a smaller pH dip during a storm pulse, while depletion of alkalinity would lead to a larger pH dip, with buffering controlled by free acid. We have not identified the mechanism for the chemostasis of Ca2+ and Mg2+ (and therefore alkalinity) in this work, but this will be critical to understanding the capacity of these peatlands to respond to repeat and more intense storm events.

Hydrology controls dissolved organic matter export and composition in an Alpine stream and its hyporheic zone.

Streams and rivers transport dissolved organic matter (DOM) from the terrestrial environment to downstream ecosystems. In light of climate and global change it is crucial to understand the temporal dynamics of DOM concentration and composition, and its export fluxes from headwaters to larger downstream ecosystems. We monitored DOM concentration and composition based on a diurnal sampling design for 3 years in an Alpine headwater stream. We found hydrologic variability to control DOM composition and the coupling of DOM dynamics in the streamwater and the hyporheic zone. High‐flow events increased DOM inputs from terrestrial sources (as indicated by the contributions of humic‐ and fulvic‐like fluorescence), while summer baseflow enhanced the autochthonous imprint of DOM. Diurnal and seasonal patterns of DOM composition were likely induced by biological processes linked to temperature and photosynthetic active radiation (PAR). Floods frequently interrupted diurnal and seasonal patterns of DOM, which led to a decoupling of streamwater and hyporheic water DOM composition and delivery of aromatic and humic‐like DOM to the streamwater. Accordingly, DOM export fluxes were largely of terrigenous origin as indicated by optical properties. Our study highlights the relevance of hydrologic and seasonal dynamics for the origin, composition and fluxes of DOM in an Alpine headwater stream.

Predicting alpine headwater stream intermittency: a case study in the northern Rocky Mountains

This investigation used climatic, geological, and environmental data coupled with observational stream intermittency data to predict alpine headwater stream intermittency. Prediction was made using a random forest classification model. Results showed that the most important variables in the prediction model were snowpack persistence, represented by average snow extent from March through July, mean annual mean monthly minimum temperature, and surface geology types. For stream catchments with intermittent headwater streams, snowpack, on average, persisted until early June, whereas for stream catchments with perennial headwater streams, snowpack, on average, persisted until early July. Additionally, on average, stream catchments with intermittent headwater streams were about 0.7°C warmer than stream catchments with perennial headwater streams. Finally, headwater stream catchments primarily underlain by coarse, permeable sediment are significantly more likely to have intermittent headwater streams than those primarily underlain by impermeable bedrock. Comparison of the predicted streamflow classification with observed stream status indicated a four percent classification error for first-order streams and a 21 percent classification error for all stream orders in the study area.

Nutrient dynamics in an alpine headwater stream: use of continuous water quality sensors to examine responses to wildfire and precipitation events

Stream water quality can change substantively during diurnal cycles, discrete flow events, and seasonal time scales. In this study, we assessed event responses in surface water nutrient concentrations and biogeochemical parameters through the deployment of continuous water quality sensors from March to October 2011 in the East Fork Jemez River, located in northern New Mexico, USA. Events included two pre-fire non-monsoonal precipitation events in April, four post-fire precipitation events in August and September (associated with monsoonal thunderstorms), and two post-fire non-monsoonal precipitation events in October. The six post-fire events occurred after the Las Conchas wildfire burned a significant portion of the contributing watershed (36%) beginning in June 2011. Surface water nitrate (NO3N) concentrations increased by an average of 50% after pre-fire and post-fire non-monsoonal precipitation events and were associated with small increases in turbidity (up to 15 NTU). Beginning 1 month after the start of the large regional wildfire, monsoonal precipitation events resulted in large multi-day increases in dissolved NO3N (6 × background levels), dissolved phosphate (100 × background levels), specific conductance (5 × background levels), and turbidity (>100 × background levels). These periods also corresponded with substantial sags in dissolved oxygen (<4 mg l−1) and pH (<6.5). The short duration and rapid rates of change during many of these flow events, particularly following wildfire, highlight the importance of continuous water quality monitoring to quantify the timing and magnitude of event responses in streams and to examine large water quality excursions linked to catchment disturbance. Copyright © 2015 John Wiley & Sons, Ltd.

Leaf Litter Decomposition and Invertebrate Colonization in Alpine Environments Above the Tree Line: an Experimental Study

ABSTRACT: Recent studies suggested a general warming trend in the Alps, resulting in a significant migration of forests to altitudes higher than the usual, regional tree line. As a consequence, some headwater streams will likely receive more allochthonous organic matter. For this reason, the dynamics of decomposition of terrestrial leaf detritus in stream reaches that naturally lacked this resource represents a subject of considerable interest, on which no information is currently available. The aim of this study was to analyse breakdown and macroinvertebrate colonisation of leaf bags in an Alpine headwater stream above the tree line. Results of this study indicate that decomposition of terrestrial leaves in a lotic alpine environment above the tree line takes place through a process similar to what happens at lower altitudes, but with some differences. The reduced rate of decomposition observed may be due to lower temperatures. At lower altitudes, tree cover provides a supply of organic material sufficie...

The mass distribution of coarse particulate organic matter exported from an Alpine headwater stream

Abstract. Coarse particulate organic matter (CPOM) particles span sizes from 1 mm, with a dry mass less than 1 mg, to large logs and entire trees, which can have a dry mass of several hundred kilograms. Pieces of different size and mass play different roles in stream environments, from being the prime source of energy in stream ecosystems to macroscopically determining channel morphology and local hydraulics. We show that a single scaling exponent can describe the mass distribution of CPOM heavier than 0.1 g transported in the Erlenbach, a steep mountain stream in the Swiss pre-Alps. This exponent takes an average value of −1.8, is independent of discharge and valid for particle masses spanning almost seven orders of magnitude. Similarly, the mass distribution of in-stream large woody debris (LWD) in several Swiss streams can be described by power law scaling distributions, with exponents varying between −1.8 and −2.0, if all in-stream LWD is considered, and between −1.3 and −1.8 for material locked in log jams. We found similar values for in-stream and transported material in the literature. We had expected that scaling exponents are determined by stream type, vegetation, climate, substrate properties, and the connectivity between channels and hillslopes. However, none of the descriptor variables tested here, including drainage area, channel bed slope and the percentage of forested area, show a strong control on exponent value. Together with a rating curve of CPOM transport rates with discharge, the scaling exponents can be used in the design of measuring strategies and in natural hazard mitigation.

Exposure of dissolved organic matter to UV-radiation increases bacterial growth efficiency in a clear-water Alpine stream and its adjacent groundwater

In many streams, microbial growth largely relies on terrigenous organic carbon that has been initially stored in soils and that is generally believed to be recalcitrant to microbial metabolism. The various mechanisms that underlie the availability of terrigenous organic carbon as it enters streams remain poorly understood. One possible mechanism can be photodegradation of terrigenous dissolved organic carbon (DOC) upon exposure to sunlight in streamwater. To explore this, we experimentally exposed streamwater, shallow groundwater and soil water from a clear-water Alpine headwater stream, and both soil and algal extracts, to UV-radiation and studied the effects on DOC optical properties and implications for microbial growth. Our results on the apparent quantum yield suggest that DOC from groundwater and soil water is more resistant to photodegradation than DOC in the streamwater itself. This would highlight the relevance of the exposure history of DOC to sunlight. Overall, UV-radiation decreased the aromaticity (as SUVA254) of the DOC and reduced its molecular weight as indicated by the slope ratio, S R (S 275–295/S 350–400). UV-treatment significantly increased bacterial growth rate and bacterial growth efficiency in the streamwater, soil water, groundwater and soil extract but not in algal extract. Our findings suggest photodegradation as one mechanism that contributes to the microbial utilisation of terrigenous DOC even in clear-water streams.

Alpine headwater streams as reservoirs of remnant populations of the Danubian clade of brown trout

Summary1. The watercourses of Austria are mainly part of the Danube drainage system. Nonetheless, only few brown trout (Salmo trutta) populations of the Danubian phylogenetic lineage have been found in this region, not being introgressed by hatchery‐reared fish of the allochthonous Atlantic lineage. The present study was aimed at identifying further waterbodies that might harbour the non‐admixed gene pool of the Danubian clade trout.2. In a first step, the complete mitochondrial DNA control region of 447 specimens from 20 sampling sites was sequenced to screen for the presence of specific haplotypes. In a second step, 332 individuals from 15 populations were analysed at 10 microsatellite DNA loci.3. Although sampling concentrated on remote Alpine headwater streams, introgression of hatchery strains was more common than expected from existing stocking records. For seven populations, however, no evidence of recent hatchery trout introgression was found and it is suggested that they might represent the indigenous gene pool.4. Populations showing no evidence of hatchery introgression displayed lower levels of intrapopulation variability and higher degrees of differentiation compared to purported admixed populations.5. Our study demonstrates the necessity of combining different molecular markers to reveal the impact of introgression into wild populations, since at some sampling sites admixture would have remained undetected if only a single molecular technique was applied.6. The identification and characterisation of non‐introgressed populations provides important information for setting conservation priorities and preventing further loss of genetic integrity of a unique element of the native fish fauna.

Emergence and Flight Activity of Alpine Stream Insects in Two Years with Contrasting Winter Snowpack

Flight of alpine stream insects has not been well studied but is an important ecological process that ensures successful mating and allows gene flow among relatively isolated populations. In this study, we collected actively flying insects along a perpendicular transect from an alpine headwater stream in the Colorado Rocky Mountains (U.S.A.) during the summer emergence season in two consecutive years with contrasting hydrology: 2002 had minimal snowfall the previous winter, while 2003 snowfall was above average. Flight activity patterns among four common stream taxa were similar to previously reported results from streams below treeline: Ephemeroptera and Plecoptera declined as an inverse power function, Trichoptera declined as a negative exponential function, and Simuliidae did not decrease with lateral distance. Sex ratios typically were strongly biased, possibly a result of the harsh terrestrial environment negatively influencing the naturally more sedentary sex (which varies among taxa). In 2003, the majority of common species emerged approximately one month later than in 2002, and abundance and diversity were greater in 2003 than 2002, patterns potentially attributable to increased snowpack amount and duration. Late-emerging species, by contrast, were less abundant in 2003, likely because that year emergence was delayed to later in the season, when cooler air temperatures reduce flight activity. Our results suggest that alpine streams are sensitive to interannual variation in snowpack, and therefore more research will be needed to address the potential effects of climate change and associated winter snowfall trends on these unexpectedly diverse aquatic systems.

Hydrological behaviour of sub-Mediterranean alpine headwater streams in a badlands environment

Summary Investigations of flash floods generation processes are very few compared with studies in humid areas. The rapidity and intensity of these processes make difficult any data acquisition on the field. The present study aims to improve our knowledge on these environments by using the facilities of the Draix ERBs (Southern French Alps) where well-documented black marls show a typical badlands morphology with V-shaped gullies. Time-varying components of total flow and flow sources have been investigated from a long-term isotopic and hydrochemical survey of rainwaters, overland waters, soil waters and streamwaters. We present in greater detail two flood events which occurred in three adjacent basins and had contrasting antecedent soil moisture conditions. Isotopic data were used to apply time-based hydrograph separations. Due to the very short transfer time of waters, convolution of the rainfall isotopic signal with a two-parameter transfer function (dispersive transfer function) proved the best way to assess the event isotopic concentration. End Members Mixing Analysis (EMMA) has been carried out using non reactive chemical species to get source areas of runoff. New water participation by direct surface transfer was the main mechanism at the peak discharge. This contribution was found to dominate irrespective of whether the antecedent soil moisture was dry or wet. Nevertheless, up to 20% of subsurface water has been observed at peak discharge for dry pre-event conditions. During the flow recession phases, this subsurface water contribution was greater for wet antecedent conditions. In dry initial conditions, subsurface water was mostly new water, whereas in wet conditions pre-event water could contribute to the flow. A deep groundwater reservoir may take part in the pre-event component in the larger basin but in the smaller basins pre-event water was mostly made of short residence time water (from previous rainfall events) originating from sporadic, highly transmissive shallow hillslope reservoirs (loose detrital cover and upper regolith). Differences regarding the hierarchy of the processes were not found to be simply proportional to the basins’ area, nor were they clearly related to the mean slope gradients or to the percentage of vegetal cover. It is suggested that controls on streamflow generation occur at local scale and consist in the spatial organisation and the gradual accumulation downwards of first order basins with their respective physical specificities (mostly orientation and slope gradient).

Enhancement of invertebrate food resources by bryophytes in New Zealand alpine headwater streams

Abstract The variability and abundance of potential invertebrate foods in two contrasting alpine streams were investigated. Potential foods examined included periphyton and detritus on artificial substrates, and naturally occurring organic matter in gravel and bryophyte habitats. Artificial substrates simulating rocks (plain tiles), and bryophytes (tiles covered with grass carpet) were used to sample periphyton. Chlorophyll a levels were higher at the unshaded site (3.42 μg cm−2 of tile) than the shaded (0.56 μ.g cm−2 of tile) and were also higher on artificial bryophytes (3.61 μg cm−2 of tile) than rocks (0.38 μg cm−2 of tile). Periphyton colonising artificial bryophytes at the unshaded site also had a much lower temporal variability than that colonising plain tiles (coefficient of variation for total periphyton pigments = 5.7% on artificial bryophytes; 10.8% on plain tiles). Variation in periphyton biomass was also higher at the unshaded site (CV = 8.3%) than the shaded site (CV = 4.4%). This reflects t...