Second-order Stream Research Articles

Varying redox potential affects P release from stream bank sediments.

Sediments in streams that drain agricultural watersheds may be sinks that can adsorb P from the stream or sources that can release P to the stream. Sediment characteristics and environmental factors, including the oxidation-reduction (redox) potential of the water associated with the sediment, determine whether P will be adsorbed or released by the sediment. We investigated P adsorption and release by four sediments [three Holocene-age sediments (Camp Creek, Roberts Creek and Gunder) as well as Pre-Illinoian-age Till] that occur in Walnut Creek, a second-order stream in Jasper County, Iowa, that is representative of many small streams in the glaciated upper Midwest of the US. The effects of two redox potentials on phosphorus buffering capacity (PBC) and equilibrium phosphorus concentration (EPC) were evaluated in batch adsorption experiments. We also simulated aerobic and anerobic conditions over a 24-day period and measured solution-phase P concentrations in stirred systems where the sediments were isolated from the water by dialysis tubing. The batch experiment indicated that the EPCs of the three Holocene-age sediments were similar to one another and increased with decreasing redox potential. In the stirred flow reactors, more dissolved P was released from the Camp Creek and Roberts Creek sediments under anaerobic conditions than from the other sediments. This observation suggests that these two sediments, which are younger and higher in the stratigraphic sequence, are more likely to be P sources in suboxic settings. The P buffering capacity was greatest in the till. Where it is in contact with the stream water, the till is likely to serve as an adsorbing sink for P in the water column.

A comparison of the larvivorous habits of exotic Poecilia reticulata and native Aplocheilus parvus

BackgroundThe exotic fish Poecilia reticulata is promoted in the tropics as a biological control agent for aquatic pathogenic carriers, such as mosquitoes. Such control measures are often adopted blindly, ignoring the potential of native species and the adverse effects of introduced species. The present study was conducted to assess the diet composition of two species of fish, the native Aplocheilus parvus and exotic P. reticulata, and to assess the availability of food items in their natural environment in four types of aquatic systems. Diet composition was estimated using 24 h gut contents analysis, in a clay quarry pit and a perennial reservoir for A. parvus, and in a man-made canal and a second-order natural stream for P. reticulata. Food items in these environments were quantified by analyzing water samples collected every 2 h.ResultsThe diet of A. parvus in the clay quarry pit and reservoir consisted of adult or larval stages of Insecta, Maxillopoda and Malacostraca. In both habitats, A. parvus selectively fed on insect parts and insect larvae. The diet of P. reticulata consisted of filamentous algae, diatoms and detritus. The diet of A. parvus showed active selection of insectivore food items against their low availability. In contrast, the diet of P. reticulata showed consumption of food items in accordance with their availability in the environment. The highest mean number of food items in the gut for A. parvus was recorded around mid-day in the clay quarry pit, but no peak feeding time was identified in the perennial reservoir. For P. reticulata, peak feeding was recorded around mid-day in both the habitats.ConclusionIrrespective of the type of environment and rate of occurrence, A. parvus preferred insect and insect larvae, whereas P. reticulata consumed the most readily available food items. The active selection of insects by A. parvus suggests they may have value as a biological control agent.

Theory of pressure acoustics with viscous boundary layers and streaming in curved elastic cavities.

The acoustic fields and streaming in a confined fluid depend strongly on the viscous boundary layer forming near the wall. The width of this layer is typically much smaller than the bulk length scale set by the geometry or the acoustic wavelength, which makes direct numerical simulations challenging. Based on this separation in length scales, the classical theory of pressure acoustics is extended by deriving a boundary condition for the acoustic pressure that takes viscous boundary-layer effects fully into account. Using the same length-scale separation for the steady second-order streaming, and combining it with time-averaged short-range products of first-order fields, the usual limiting-velocity theory is replaced with an analytical slip-velocity condition on the long-range streaming field at the wall. The derived boundary conditions are valid for oscillating cavities of arbitrary shape and wall motion, as long as both the wall curvature and displacement amplitude are sufficiently small. Finally, the theory is validated by comparison with direct numerical simulation in two examples of two-dimensional water-filled cavities: The well-studied rectangular cavity with prescribed wall actuation, and a more generic elliptical cavity embedded in an externally actuated rectangular elastic glass block.

Financing organizational changes from without

PurposeAchieving specific changes within autonomous organizations is often a necessary condition for the success of strategic public policy. Wherever it is impossible to induce such changes by regulations, a frequently used tool is inducing their occurrence with financial stimuli. This practice appears to have been fully substantiated by the early systems-evolutionary understanding of the relationship between organizations and their environment, whose peak popularity in the 1950s and 1960s coincided with the appearance of new international organizations formulating strategic policies on a previously unprecedented scale. The conceptual framework available at that time failed, however, to provide a solid ground for operationalization and systemic evaluation of such interventions. As a result, even though it was implicitly presumed that policy implementation depended on organizational changes taking place in a large number of organizations, a conceptualization of the exact ways of how to ensure and assess such changes was hardly pronounced. This paper aims to uncover the problematique of that missing conceptualization.Design/methodology/approachIn this paper, the author draws on the second-order stream of systems thinking, arguing that without such a deliberate operationalization, it seems much more likely that the external financing of organizational changes functions merely as organizational “perturbations” which do not crystallize into lasting changes, as they are mitigated by equally potent “compensation” to cancel out the perturbations. Using the theory of social system’s autopoiesis, the author posits that adaptive fluctuations evoked in organizations by the interferences of the policymakers may thus be considered “change” just as well as non-change.FindingsOnce the behavior of an autopoietic organizational system is seen as a continuous perpetuation of its own identity pattern, fashioned discursively as the organization’s self-description, then the only change which seems worthy of the publicly assigned resources and efforts is a shift in that pattern.Originality/valueIt is argued that the assessment of whether target organizations are indeed implementing or only superficially performing (and instantly compensating for) the desired changes should be inferred from a qualitative analysis of the daily discursive practices that forge the target domains rather than by a comparison of the measurable parameters, which are currently dominating in the evidence-based paradigm.

Assessment of landslide-triggering factors and occurrence using morphometric parameters in Geyraz Basin, Tokat, Northern Turkey

The Geyraz Stream is one of the tributaries of the Yesilirmak River, located within the southern of Middle Black Sea section, and enters the main river at Tokat city centre. Geyraz is a basin that has experienced numerous landslide events. In this study, morphometric parameters of areas prone to landslide occurrence were investigated and computed in GIS. These parameters included the Dissection Index, hypsometric integral, drainage density, Stream Power Index, and Compound Topographic Index, first and second orders. In addition, rocks, slopes and frequencies of first- and second-order streams in subbasins were considered and their correlations were examined. The moderate (15–20%) and high slope (20–25%) values with primarily clayey rocks (schist and flysch) correspond to high Di and Hi values, a maximum number of first- and second-order streams, and high landslide occurrence, while very steep slopes lead to low Di values and a low density of landslides. Results show that former landslides occurred in areas with Di values of 0.1–0.15, moderately steep and steep slopes, a maximum number of first- and second-order streams, high stream frequency on clayey rocks (schist and flysch), CTI values of 60–100, and SPI values of 75–85. In other words, landslide events are triggered by higher drainage frequency on primarily clayey rocks, intense fluvial incision and medium–high steep slopes. The location of agricultural land in the landslide area, reflecting lack of vegetation cover, is another landslide-triggering factor. Thus, the study area was ranked in six different classes and risk was rated as very low, low, medium, medium to high, high and very high. Determination of areas prone to landslide occurrence and their triggering factors is important for regional planning with respect to basin management and hazard prevention.

Carbon dioxide degassing at the groundwater-stream-atmosphere interface: isotopic equilibration and hydrological mass balance in a sandy watershed

Streams and rivers emit significant amounts of CO2 and constitute a preferential pathway of carbon transport from terrestrial ecosystems to the atmosphere. However, the estimation of CO2 degassing based on the water-air CO2 gradient, gas transfer velocity and stream surface area is subject to large uncertainties. Furthermore, the stable isotope signature of dissolved inorganic carbon (δ13C-DIC) in streams is strongly impacted by gas exchange, which makes it a useful tracer of CO2 degassing under specific conditions. For this study, we characterized the annual transfers of dissolved inorganic carbon (DIC) along the groundwater-stream-river continuum based on DIC concentrations, stable isotope composition and measurements of stream discharges. We selected a homogeneous, forested and sandy lowland watershed as a study site, where the hydrology occurs almost exclusively through drainage of shallow groundwater (no surface runoff). We observed the first general spatial pattern of decreases in pCO2 and DIC and an increase in δ13C-DIC from groundwater to stream orders 1 and 2, which was due to the experimentally verified faster degassing of groundwater 12C-DIC compared to 13C-DIC. This downstream enrichment in 13C-DIC could be modelled by simply considering the isotopic equilibration of groundwater-derived DIC with the atmosphere during CO2 degassing. A second spatial pattern occurred between stream orders 2 and 4, consisting of an increase in the proportion of carbonate alkalinity to the DIC accompanied by the enrichment of 13C in the stream DIC, which was due to the occurrence of carbonate rock weathering downstream. We could separate the contribution of these two processes (gas exchange and carbonate weathering) in the stable isotope budget of the river network. Thereafter, we built a hydrological mass balance based on drainages and the relative contribution of groundwater in streams of increasing order. After combining with the dissolved CO2 concentrations, we quantified CO2 degassing for each stream order for the whole watershed. Approximately 75% of the total CO2 degassing from the watershed occurred in first- and second-order streams. Furthermore, from stream order 2–4, our CO2 degassing fluxes compared well with those based on stream hydraulic geometry, water pCO2, gas transfer velocity, and stream surface area. In first-order streams, however, our approach showed CO2 fluxes that were twice as large, suggesting that a fraction of degassing occurred as hotspots in the vicinity of groundwater resurgence and was missed by conventional stream sampling.

Distribution, body size, genetic structure and conservation of Siphlaenigma janae (Insecta: Ephemeroptera)

ABSTRACTDistribution, body size and genetic structure of the poorly known New Zealand mayfly Siphlaenigma janae were investigated to improve understanding of its conservation status. It has now been recorded from 42 locations, 22 of which are reported for the first time. The distribution of S. janae extends from Northland to the central North Island, and also into the northwestern South Island. Population structure consisted of three distinct haplotype networks; two in the North Island and one in the South Island. Maximum uncorrected genetic distance was 6.1% but no strong evidence for the presence of sibling species was found. The specialist nymphs predominantly live in low-gradient first- and second-order forested streams, a habitat that is increasingly being threatened by land-use modification. We recommend S. janae remain classified as Nationally Vulnerable due to its unique taxonomic position, regionally distinct genetic structure and fragmented population that is likely experiencing ongoing decline.

Urban riparian systems function as corridors for both native and invasive plant species

Riparian areas are often the only green areas left in urban and suburban landscapes, providing opportunities for conservation and connectivity of both aquatic and terrestrial organisms. While city planners and land managers often tout the importance of riparian networks for these uses, it is not well established if urban riparian plant communities are actually functioning as connected assemblages. Furthermore, urban riparian zones are well known to be highly invaded by non-native plant species and may be functioning to increase the spread of non-native species across the landscape. Here we examine connectivity of plant assemblages in riparian networks within an extensively urbanized landscape. We sampled riparian plant communities at 13 sites along three second-order streams of the Rahway River watershed, New Jersey. We also characterized propagule dispersal at each site by sampling litter packs on the river banks five times between March–October 2011 and identifying germinants from litter packs after cold stratification. Species turnover of both riparian and litter vegetation was more strongly associated with flow distance, particularly for native species, indicating that riverine systems are important for promoting connectivity of native plant assemblages in urban landscapes. However, non-native germinants significantly dominated propagule dispersal along the stream reaches, particularly early in the growing season, suggesting spread utilizing the river system and preemption may be an important mechanism for invasion success in this system. Our data show that management of invasive species should be planned and implemented at the watershed scale to reduce spread via the river system.

In-stream habitat predicts salamander occupancy and abundance better than landscape-scale factors within exurban watersheds in a global diversity hotspot

The southern Appalachian Mountains have experienced rapid human population growth rates since the 1980s. Land used practices are shifting from rural to residential. The majority of development has been low density, and is often near biologically diverse areas such as National Forests and National Parks. The long-term effects of urbanization in the southeastern Appalachian Mountains are not clearly understood and even less is known with respect to stream salamander response to urbanization. In order to determine the temporal influence of exurban housing on southern Appalachian streams we sampled 27 first- and second-order streams in watersheds containing exurban developments ranging in age from 4 to 44 years, along with eight forested streams, over the course of two summers. We sought to determine if the relative age of an exurban development related to occupancy and abundance of southern Appalachian stream salamanders. Age of exurban development and other watershed scale variables were not top predictors of salamander assemblages, while local site variables such as salinity and undercut banks predicted the abundance of several species of salamander. Our results suggest local habitat improvements can be used to better conserve salamanders and stream ecosystems in an increasingly urbanized region.

Exclusion size and material have minimal effects on stream benthic algae and macroinvertebrate colonization within submerged cages

Despite their widespread use in grazer–biofilm studies, stream exclusion cages have inherent physical properties that may alter benthic organism colonization and growth. We used laboratory studies and a field experiment to determine how exclusion cage design (size and material) alters light availability, water velocity, and benthic organism colonization. We measured light reduction by various plastic cage materials and flow boundary layer thickness across a range of exclusion cage sizes in the laboratory. We also deployed multiple exclusion cage designs based on commonly available materials into a second-order stream to assess algae and macroinvertebrate colonization differences among exclusion cages. All plastics reduced some light (190–700 nm wavelengths) and blocked more ultraviolet light than photosynthetically active radiation. Exclusion cage size did not influence flow boundary layer thickness, but larger exclusions tended to have higher velocity at the substrata surface. Despite light and water velocity differences, algal biomass, macroinvertebrate density, and community composition were similar between exclusion cage types. However, algal assemblages outside exclusion cages differed in composition and had higher biomass compared to inside exclusion cages. In terms of algal and macroinvertebrate colonization, plastic exclusion cage size and material appear to be flexible within the sizes tested, but differences can still exist between exclusion cage communities and those within the stream. Overall, artifacts of screened exclusion cages do not appear to introduce large bias in results of grazer–biofilm studies, but efforts to design exclusion cages that better mimic the natural system should continue.

The transition from riparian to upland forest plant communities on headwater streams in the southern Sierra Nevada, California, United States1,2

Fixed-width buffer zones on rivers and streams are designed to protect the diverse riparian community and its important function in the ecosystem. However, recent data suggest that riparian areas of some western forests have become more fire prone because of restrictions on fuel reduction treatments within buffer zones. Surprisingly little is known about where and how the plant community transitions from riparian to upland vegetation, but understanding that transition would inform the restoration of riparian forest structure and function and its associated management applications (e.g., prescribed fire or mechanical thinning) that may be necessary to achieve restoration. Using data collected from the Kings River Experimental Watersheds, we assessed the transition in plant structure and composition from riparian near-stream areas to upland locations, in mixed-conifer and red fir forests on headwater streams of the southern Sierra Nevada, CA. Our data strongly support the conclusion that the riparian zone, as evidenced by the riparian plant community, extends beyond 10 m from the stream on these narrow, first- and second-order streams. The herbaceous community at 10 m from the stream was distinct from the upland community and had greater similarity to locations closer to the stream in mixed-conifer forest. Species richness was three to four times greater in riparian areas compared with upland areas, and there was little overlap of the more-abundant herbaceous species. In addition, riparian forests were generally denser, with smaller trees in mixed-conifer forest, but had similar stand structure to upland forests in red fir forest. Differences between mixed-conifer and red fir forests may reflect different departures from the historical fire regime in these two community types. Compared with riparian areas of wetter climates, riparian areas of dry climates, such as the Sierra Nevada, may harbor even greater species diversity relative to nearby upland areas, indicating that buffer zones of restricted management may be justified in these forests if the goal is the preservation of biodiversity. However, perhaps more important, these findings, along with recent literature, highlight the need to identify site-specific goals when undertaking restoration of riparian forests in the western USA: herbaceous biodiversity, fuels reduction, historic tree composition and structure, and/or water quality.

The significance of small streams

Headwaters, defined here as first- and secondorder streams, make up 70%‒80% of the total channel length of river networks. These small streams exert a critical influence on downstream portions of the river network by: retaining or transmitting sediment and nutrients; providing habitat and refuge for diverse aquatic and riparian organisms; creating migration corridors; and governing connectivity at the watershed-scale. The upstream-most extent of the channel network and the longitudinal continuity and lateral extent of headwaters can be difficult to delineate, however, and people are less likely to recognize the importance of headwaters relative to other portions of a river network. Consequently, headwaters commonly lack the legal protections accorded to other portions of a river network and are more likely to be significantly altered or completely obliterated by land use.

Detrital-Zircon Mixing and Partitioning In Fluvial To Deep Marine Systems, Central California, U.S.A.

The increasing use of detailed detrital-zircon provenance data calls for an understanding of the controls that sedimentary processes exert on changing detrital-zircon provenance signatures in time and space in addition to the well-established influence of tectonics. Our U-Pb detrital-zircon data from seventeen samples taken across a series of modern second-order rivers to deep-marine depositional systems in central California show that marine mixing processes and sediment pathway partitioning exert important controls on detrital-zircon provenance signatures. The second-order rivers that input sediment into this system produce differentiable detrital-zircon provenance signatures as a function of the unique geology of their drainage areas. The mixing of sediment through the marine realm can be traced through the evolution of distinct detrital-zircon signatures as sediment is transported from rivers down system through longshore drift into submarine canyons. Sediment input from second-order streams is mixed in the littoral and upper submarine-canyon environments, yielding provenance signatures that are not thoroughly mixed until the lower reaches of submarine canyons. Sediment partitioning in these dispersal systems results in distinct detrital-zircon provenance signatures for different dispersal systems over short length scales (< 50 km). These data suggest that sediment mixing in the marine realm is likely an important control on detrital-zircon provenance signatures in any system in which sediment is transported through longshore drift and sediment pathways are distinctly partitioned. Changes in the way sediment pathways are partitioned through time has the potential to create heterogeneous detrital-zircon provenance signatures in stratigraphic successions through time and between closely spaced deposits of the same age. When seen in an ancient system, this heterogeneity may be mistakenly attributed to tectonic controls without the recognition of the influence of sedimentary process.

Effects of inset floodplains and hyporheic exchange induced by in-stream structures on nitrate removal in a headwater stream

Stream restoration efforts in the United States are increasingly aimed towards water quality improvement, yet little process-based guidance exists to compare pollutant removals from different restoration techniques for variable site conditions. Excess nitrate (NO3−) is a frequent pollutant of concern due to eutrophication in downstream waterbodies such as the Chesapeake Bay. We used MIKE SHE to simulate hydraulics and NO3− removal in a 90m restored reach of Stroubles Creek, a second-order stream in Blacksburg, Virginia. Site specific geomorphic, hydrologic, and hydraulic data were used to calibrate the model. We evaluated in-stream structures that induce hyporheic zone denitrification during baseflow and inset floodplains that remove NO3− during storm flows. We varied hydraulic conditions (winter baseflow, summer baseflow, storm flow), biogeochemical parameters (literature hyporheic zone denitrification rates and newly available inset floodplain removal rates) and boundary conditions (upstream NO3− concentration), sediment conditions (hydraulic conductivity), and stream restoration design parameters (inset floodplain length). Our results indicate that NO3− removal rates within the 90m reach were minimal. Structure-induced hyporheic zone denitrification did not exceed 3.1% of mass flowing in from the upstream channel, was achieved only during favorable background groundwater hydraulic conditions (i.e. summer baseflow), and was transport-limited such that non-trivial removal rates were achieved only when the streambed hydraulic conductivity (K) was at least 10−4m/s. Inset floodplain nitrogen removal was limited by floodplain residence time and NO3− removal rate, and did not exceed 1% of inflowing mass. Summing these removals for both restoration practices over the course of the year based on the frequency of storm and summer baseflow conditions yielded ∼2.1% annual removal. Achieving 30% NO3− removal required increasing the length of stream reach restored to 0.9km–819km (depending on hydraulic conductivity) and 3.8–46km (depending on inset floodplain length and nitrogen removal rate) for in-stream structures during baseflow and inset floodplains during storm flow, respectively. In one of the first comparisons of process-based modeling to the Chesapeake Bay Program stream restoration guidance, we found that the guidance overestimated hyporheic NO3− removal for our modeled reach, but correctly estimated inset floodplain removal. Overall, our results indicate that in-stream structures and inset floodplains can improve water quality, but overall required level of effort may be high to achieve desired results.

Levels, sources and risk assessment of PAHs in multi-phases from urbanized river network system in Shanghai

Spatial-temporal distributions, sources identification and risk assessment of polycyclic aromatic hydrocarbons (PAHs) in overlying water and surface sediments in urban river networks of Shanghai were studied. Analytical results showed that there was a significant seasonal variation in concentrations of ∑16PAHs in water, suspended particulate matter (SPM) and sediment phases in this study area. The PAHs pollution in these multi-phases were in the medium level compared with other areas around the world, and the levels of PAHs contamination in SPM and sediment phases in hierarchical rivers showed TS (the third-order stream) > FS (the first-order stream) > SS (the second-order stream). Two manners of isomer ratios and principal component analysis (PCA) were used to identify PAHs origins, and suggested that combustion processes are dominant for PAHs sources. The ratios of PAHs origins by fossil fuels combustion, coke burning and crude oil in hierarchical rivers were determined with FS > SS > TS in SPM and sediment phases, and the ratio of PAHs origins by traffic emissions was analyzed with TS > SS > FS. PAHs in water samples have a certain impact on aqueous ecological system especially due to the fact that the ∑ceq values of nine PAHs were calculated from 0.715 to 15.831 μg/L in winter, which inferred serious ecological risk to some special aquatic organisms. The calculations of MERMQ in sediment samples showed that the MERMQ values ranged from 0.021 to 1.209 in winter and 0.019 to 0.643 in summer, which suggested high toxicity at six sampling sites in winter and only one location in summer due to high levels of PAHs. Furthermore, the toxicity degree of sediments were demonstrated with TS > FS > SS.

Restructuring of the Aquatic Bacterial Community by Hydric Dynamics Associated with Superstorm Sandy.

Bacterial community composition and longitudinal fluctuations were monitored in a riverine system during and after Superstorm Sandy to better characterize inter- and intracommunity responses associated with the disturbance associated with a 100-year storm event. High-throughput sequencing of the 16S rRNA gene was used to assess microbial community structure within water samples from Muddy Creek Run, a second-order stream in Huntingdon, PA, at 12 different time points during the storm event (29 October to 3 November 2012) and under seasonally matched baseline conditions. High-throughput sequencing of the 16S rRNA gene was used to track changes in bacterial community structure and divergence during and after Superstorm Sandy. Bacterial community dynamics were correlated to measured physicochemical parameters and fecal indicator bacteria (FIB) concentrations. Bioinformatics analyses of 2.1 million 16S rRNA gene sequences revealed a significant increase in bacterial diversity in samples taken during peak discharge of the storm. Beta-diversity analyses revealed longitudinal shifts in the bacterial community structure. Successional changes were observed, in which Betaproteobacteria and Gammaproteobacteria decreased in 16S rRNA gene relative abundance, while the relative abundance of members of the Firmicutes increased. Furthermore, 16S rRNA gene sequences matching pathogenic bacteria, including strains of Legionella, Campylobacter, Arcobacter, and Helicobacter, as well as bacteria of fecal origin (e.g., Bacteroides), exhibited an increase in abundance after peak discharge of the storm. This study revealed a significant restructuring of in-stream bacterial community structure associated with hydric dynamics of a storm event. In order to better understand the microbial risks associated with freshwater environments during a storm event, a more comprehensive understanding of the variations in aquatic bacterial diversity is warranted. This study investigated the bacterial communities during and after Superstorm Sandy to provide fine time point resolution of dynamic changes in bacterial composition. This study adds to the current literature by revealing the variation in bacterial community structure during the course of a storm. This study employed high-throughput DNA sequencing, which generated a deep analysis of inter- and intracommunity responses during a significant storm event. This study has highlighted the utility of applying high-throughput sequencing for water quality monitoring purposes, as this approach enabled a more comprehensive investigation of the bacterial community structure. Altogether, these data suggest a drastic restructuring of the stream bacterial community during a storm event and highlight the potential of high-throughput sequencing approaches for assessing the microbiological quality of our environment.

Weathering assessment in the Achala Batholith of the Sierra de Comechingones, Córdoba, Central Argentina. II: major hydrochemical characteristics and carbon dynamics

La Trucha is a granitic, second-order catchment (~1.9 km2, average slope 5 %) placed at ~1400-m elevation in the Sierra de Comechingones (Argentina, 31°54′ S, 64°45′W; 31°53′, 64°44′W). The main geochemical characteristics were studied during a hydrological year (March 2005–February 2006), attending mainly seasonal and spatial variability and weathering-induced carbon consumption. Subjected to a weathering-limited denudation regime, the catchment is representative of hundreds of the second-order streams that dissect the Achala Batholith. Water chemistry is typical of streams draining F− rich granites. High discharges (austral summer) determine the dilution of Na+, Ca2+, Mg2+, HCO3 −, SO4 2−, and F−. Conversely, high summer rainfall increases Cl− concentrations. The rainfall-induced mobilization of K+ is likely masked by its affinity for adsorption onto fine-grained particles and that of SiO2 by its biological consumption. La Trucha’s upper catchment shows that steep slopes and a shallow regolith layer result in a more diluted water flow, chemically closer to rainfall. Subsurface flow increases the downriver positive rate of La Trucha’s electrical conductivity twice as much in winter (6.7 µS cm−1 per 1000 m) than during wet summer (3.9 µS cm−1 per 1000 m). PHREEQC allowed the calculation of free CO2 and the likelihood of its evasion in different seasons and areas of the catchment. A mean carbon efflux of ~180 mg C m−2 year−1 has been estimated for La Trucha catchment. The first- and second-order streams, which occupy ~62.5 km2 of the Achala Batholith’s surface area, are significant sources in the Sierra de Comechingones’ carbon cycle.

Hydrogeomorphic connectivity on roads crossing in rural headwaters and its effect on stream dynamics

Unpaved roads are ubiquitous features that have been transforming the landscape through human history. Unpaved roads affect the water and sediment pathways through a catchment and impacts the aquatic ecosystem. In this study, we describe the effect of unpaved road on the hydrogeomorphic connectivity at the rural headwater scale. Measurement was based on the stream crossing approach, i.e., road superimposing the drainage system. We installed a Parshall flume coupled with single-stage suspended sediment sampler at each stream crossing. In addition, we displayed our monitoring scheme with an upscaling perspective from second-order to third-order stream. We concluded that the road–stream coupling dramatically changed the stream dynamic. The increase of discharge caused by roads at the headwater was 50% larger compared to unaffected streams. Additionally, suspended sediment concentration enhancement at stream crossings ranged from to 413% at second-order streams to 145% at third-order streams. The landform characteristics associated with the road network produced an important hydrogeomorphic disruption in the landscape. As a result, the sediment filter function of the riparian zone was reduced dramatically. Therefore, we recommend that projects for aquatic system restoration or conservation in rural landscape consider the role of the road network on stream dynamics.

Diel variation in the structure of fish assemblages in south western Amazon streams

Abstract: Aim We investigate the influence of luminosity, habitat conservation and pluviometric periods in fish assemblages of in pasture and forest small streams in western amazon. Methods Sampling was conducted every two months from July 2013 to April 2014 in nine first- and second-order streams using seine nets and dip nets during the day and night. Fish composition, richness and total abundance were determined for each sampling period. The PERMANOVA was used to evaluate the effects of land use, season, and photoperiod, on fish assemblages. Fish assemblage structure for each stream in the presence and absence of photoperiod was ordered by NMDS analysis. Results In the light period, 3,484 specimens from 69 species were collected, while 4,574 specimens from 71 species where collected in the dark period. No significant differences in abundance and species richness were recorded between the presence and absence of luminosity periods, rainy and dry seasons and streams in forest and deforested areas. We found evidence of the dark phase composition and richness of exclusive species (22% of species collected were found at night), which were greater than in the light period (20% of species). Conclusion Despite our failure to identify any nycterohemeral segregation, the results complement existing knowledge of regional ichthyofauna and help provide a better understanding of the distributional, behavioral and functional ecological patterns of fish assemblages.

Leaf-Nets (LN): A New Quantitative Method for Sampling Macroinvertebrates in Non-Wadeable Streams and Rivers

The ecological knowledge of large rivers is still scarce or highly fragmented mainly because of complex, laborious and expensive procedures to collect informative samples from the benthic biota. Standard sampling protocols for macroinvertebrates were mainly developed and calibrated for wadeable streams, while a number of heterogeneous non-standard sampling procedures are available for large rivers. We propose the new, easy-to-build and cost-effective leaf-nets (LN) method to quantitatively sample benthic invertebrates in non-wadeable waterways. The LN method uses Phragmites australis leaves as substrate and combines the characteristics of the leaf-bags and the Hester–Dendy (HD) multiplates methods. We compared the effectiveness of the LN and HD methods in a near-pristine and in an impacted stream-reach (downstream an aquaculture plant) of a non-wadeable second-order stream of Central Apennines (Italy). Twenty-five of the 34 cumulatively collected macroinvertebrate taxa were common to both methods, while seven taxa were found only on LN and two only on HD. Taxonomic richness and total macroinvertebrate abundance were higher for LN assemblages. Number of Ephemeroptera, Plecoptera and Trichoptera taxa (EPT) also tended to be higher on LN. Assemblage composition was different on LN and HD. Both methods documented a significant decrease in EPT taxa and a concomitant increase in the total abundance of more pollution-tolerant taxa in the impacted stream-reach, but the LN method was more sensitive to impact-associated changes in macroinvertebrate assemblage structure. In contrast to the hardboard plates of HD, the assembled leaves of the LN may act as a direct or indirect food source and may better mimic the texture and composition of more heterogeneous natural substrates thus favouring the migration–colonization process from both bottom and littoral benthic invertebrates. The sampling efficiency, cost effectiveness and simplicity warrant the routine use of the new LN method in large-river ecological assessment. Copyright © 2015 John Wiley & Sons, Ltd.