Adjacent Riparian Zone Research Articles

Grasslands of cleared woodlands have lower invertebrate diversity and different assemblages to remnant woodlands in grazed landscapes of eastern Australia

Clearing of woodlands is used by graziers to promote pasture production, even though understanding of impacts of clearing on native fauna is lacking. We evaluate impacts of clearing on biodiversity by comparing invertebrates associated with ground-layer vegetation of cleared woodlands (grasslands) to that of nearby uncleared woodlands. Two replicates of grasslands consisting of pastures dominated by introduced grasses were compared with two woodlands at each of four locations. The adjacent riparian forest to each grassland and woodland site allowed evaluation of the effect of woodland clearing on the adjacent riparian zone. All habitats were grazed. Invertebrates of ground-layer vegetation were sampled using three suction subsamples of 1m2 at each site. Grasslands had significantly lower order richness and abundance of herbivores, pollinators and macroinvertebrates (food for birds) than the woodlands, whereas the riparian forests closely resembled each other in all metrics. Invertebrate assemblages of grasslands also differed significantly from those of the woodlands. BEST analysis showed that groundcover and leaf-litter percentage cover correlated strongest with invertebrate composition. This study has demonstrated that grazing management relying on clearing of fertile grassy woodlands of the rangelands of Central Queensland alters invertebrate diversity and assemblage. Thus, tree clearing not only leads to biodiversity losses in the canopy layer, but also in the ground-layer vegetation.Implications for insect conservationPastoralists have the capacity to improve outcomes for invertebrate biodiversity by maintaining groundcover (ground-layer vegetation and litter cover) above 80%, by encouraging native pastures over introduced species such as Buffel Grass and by retaining native woodlands.

Validation of an analytical model of groundwater velocity based on laboratory test and numerical simulation

Surface water–groundwater exchange affects the material and energy transfer of rivers and adjacent riparian zones. As an intuitive carrier of energy, temperature can effectively reflect the spatial and temporal variation of surface water–groundwater exchange process. In this paper, the influence of water head variation and sand sample uniformity on its temperature field and seepage field is studied through a one-dimensional sand column laboratory test. To verify the accuracy of the one-dimensional vertical heat analysis model, the vertical seepage velocity measured in the indoor test is compared with the vertical submerged exchange rate calculated by the four analysis models. The results show that the Hatch analytical solution, Keery analytical solution, McCallum analytical solution and Luce analytical solution calculated by VFLUX2 through MATLAB are reliable for calculating the vertical undercurrent exchange rate of the heterogeneous sand column.

Insectivorous bat foraging tracks the availability of aquatic flies (Diptera)

AbstractRivers and their adjacent riparian zones are model ecosystems for observing cross‐ecosystem energy transfers. Aquatic insects emerging from streams, for example, are resource subsidies that support riparian consumers such as birds, spiders, lizards, and bats. We collaborated with recreational river runners in Grand Canyon, Arizona, USA, to record acoustic bat activity and sample riparian insects using light traps at dusk from April through October 2017–2020. River runners collected these data on 1,428 events over 611 sampling nights at 410 sites throughout a 470‐km segment of the Colorado River. We documented 71 insect taxa in light traps and recorded 19 bat species with acoustic detectors. We hypothesized that bat activity along this highly regulated river segment would be influenced primarily by variation in prey availability, as compared to other habitat descriptors. We predicted that bat activity would be positively related to aquatic insect catch rates and unrelated to terrestrial insect abundance. We fit Bayesian regression models to test these hypotheses and to quantify the relationship between bat activity and a suite of environmental variables: time of year, time of day, distance from perennial tributaries, distance from rapids, channel width, geomorphic reach, tall vegetation cover, air temperature, and lunar phase. Bat activity was positively related to the abundance of aquatic flies (Diptera), which outcompeted all other prey categories and structural habitat descriptors in our models. Within our dusk sampling period, activity of small myotis (California myotis [Myotis californicus] and Yuma myotis [M. yumanensis]) was high late in the evening and canyon bats (Parastrellus hesperus), conversely, were more active early in the evening. Activity of canyon bats varied seasonally, with peak activity in August. Our results support the hypothesis that aquatic prey, specifically aquatic flies, are key predictors of bat activity along a large, regulated river corridor and demonstrate the power of community science as a tool for ecosystem monitoring.

Characterization of an AFFF impacted freshwater environment using total fluorine, extractable organofluorine and suspect per- and polyfluoroalkyl substance screening analysis

The vast number of per- and polyfluoroalkyl substances (PFASs) that are in global commerce (n > 4700) pose immense challenges for environmental monitoring. The large discrepancy between this large number and the few PFASs usually monitored suggest that environmental exposure might be substantially underestimated. This study applied a workflow, which included analysis of total fluorine (TF), extractable organofluorine (EOF), 24 target PFASs and suspect screening. The workflow aimed to close the organofluorine mass balance and to tentatively identify overlooked PFASs in various matrices from an aqueous film forming foam (AFFF) contaminated pond and its adjacent riparian zone. PFAS target analysis revealed that water, aquatic invertebrates as well as emergent aquatic insects had high concentrations with up to 2870 ng L−1, 9230 ng g−1 dry weight (dw) and 1470 ng g−1 dw ∑24PFASs, respectively. The EOF mass balance could be explained by target PFAS analysis for most biota samples such as aquatic invertebrates, emergent aquatic insects and terrestrial spiders and earthworms (i.e. EOF ≈ ∑24PFASs). In the pond surface water, 42–58% of the EOF was not explained by target PFASs. However most new tentatively identified PFASs (n = 25) were detected in water, which could contribute to the unknown EOF. Nine suspects could be further identified, which where perfluoroalkyl sulfonamide-based compounds and derivatives that all have been found in historical AFFFs produced by electrochemical fluorination. One suspect, F5S-PFOS, was also detected for the first time in aquatic and terrestrial invertebrates.

Effects of land‐cover transitions on emerging aquatic insects and environmental characteristics of headwater streams in an agricultural catchment

AbstractStreams and their adjacent riparian zones are increasingly viewed as interdependent systems linked by reciprocal exchanges of energy, organisms, and materials. We assessed potential associations between the emerging aquatic insect flux and transitions between agricultural land and forest fragments to better understand these stream‐riparian linkages in managed landscapes. We sampled stream environmental conditions and emerging insects at 28 sites distributed along three streams flowing through agriculture‐forest‐agriculture transitions in central Ohio, USA, in the summer of 2012. Ephemeroptera and Trichoptera (ET) taxa had higher flux densities in forests (mean and 95% CI: 0.3 insects m−2 d−1 [0.1, 0.8]) compared to agriculture (mean and 95% CI: 0 insect m−2 d−1 [0, 0.1]; p = .004), and ET taxa were found in 67% of forested sites compared to only 15% of agricultural sites. In contrast, Dolichopodidae were more strongly associated with agricultural land (mean and 95% CI: 0.6 insect m−2 d−1 [0.3 to 1.2]) than forests (mean and 95% CI: 0.1 insects m−2 d−1 [0.1, 0.2]; p = .002). Although Chironomidae were the most numerically abundant, ET taxa were among the larger bodied insects and comprised >30% of the total biomass flux, illustrating the importance of taxonomic traits in mediating flux dynamics. Mechanisms driving emerging insect flux were related to substrate grain‐size distribution, channel width, and nutrient concentrations. Overall, our results demonstrate that small forest fragments are strongly related to the aquatic‐to‐terrestrial insect flux and thus have important implications for terrestrial biodiversity and food webs in agricultural landscapes.

Stream Resource Gradients Drive Consumption Rates of Supplemental Prey in the Adjacent Riparian Zone

Decades of research have revealed the crucial roles of cross-system energy flows (spatial subsidies) in mediating trophic interactions in recipient systems. Food web theory predicts that the responses of subsidized consumers are a key to understanding the net impacts of spatial subsidies on in situ prey/resources of recipient systems. However, less is known about the factors triggering the cascading biotic interactions across coupled ecosystems. Here, we quantify how riverine productivity (donor system) mediates terrestrial food web interactions through spatial subsidies to simplified gravel bar communities. Our comparative study in Japan indicated that higher algal biomass in aquatic systems led to increased supplies of emerging aquatic insects, which were associated with greater densities of terrestrial consumers (Carabid beetles) and enhanced consumption rates of supplemental in situ prey on gravel bars. Our results highlight the potential of donor productivity to drive cascading biotic interactions across coupled ecosystems. Because cross-system energy flows should originate, at least in part, from primary producers of donor systems, our fundamental finding may form the basis of future studies exploring the driving factors of cross-system trophic interactions.

Identification of runoff formation with two dyes in a mid-latitude mountain headwater

Abstract. Subsurface flow in peat bog areas and its role in the hydrologic cycle has garnered increased attention as water scarcity and floods have increased due to a changing climate. In order to further probe the mechanisms in peat bog areas and contextualize them at the catchment scale, this experimental study identifies runoff formation at two opposite hillslopes in a peaty mountain headwater; a slope with organic peat soils and a shallow phreatic zone (0.5 m below surface), and a slope with mineral Podzol soils and no detectable groundwater (> 2 m below surface). Similarities and differences in infiltration, percolation and preferential flow paths between both hillslopes could be identified by sprinkling experiments with Brilliant Blue and Fluorescein sodium. To our knowledge, this is the first time these two dyes have been compared in their ability to stain preferential flow paths in soils. Dye-stained soil profiles within and downstream of the sprinkling areas were excavated parallel (lateral profiles) and perpendicular (frontal profiles) to the slopes' gradients. That way preferential flow patterns in the soil could be clearly identified. The results show that biomat flow, shallow subsurface flow in the organic topsoil layer, occurred at both hillslopes; however, at the peat bog hillslope it was significantly more prominent. The dye solutions infiltrated into the soil and continued either as lateral subsurface pipe flow in the case of the peat bog, or percolated vertically towards the bedrock in the case of the Podzol. This study provides evidence that subsurface pipe flow, lateral preferential flow along decomposed tree roots or logs in the unsaturated zone, is a major runoff formation process at the peat bog hillslope and in the adjacent riparian zone.

AN ABANDONED-CHANNEL FILL WITH EXQUISITELY PRESERVED PLANTS IN REDBEDS OF THE CLEAR FORK FORMATION, TEXAS, USA: AN EARLY PERMIAN WATER-DEPENDENT HABITAT ON THE ARID PLAINS OF PANGEA

Abstract A well preserved plant assemblage at the Colwell Creek Pond locality of Leonardian (Kungurian) age provides an opportunity to evaluate taphonomic conditions in a dryland alluvial setting. A narrow channel body incised to 5 m depth through red paleo-Vertisols contains 2 m of varicolored laminated mudstone with graded layers and plant material. X-ray diffraction analysis of individual laminae indicates the presence of chlorite, illite, kaolinite, and mixed-layer clay, with hematite in red and gray layers and goethite in yellow-brown laminae. No carbonate was identified, and the total organic carbon content is minimal. The fine sediment accumulated in a shallow abandoned channel from suspension and gentle underflows, probably linked to seasonal inflow, and analysis of lamina thickness suggests that standing water may have persisted for up to a few millennia. The preservation of lamination is attributed to a lack of bioturbation, possibly linked to a paucity of subsurface oxygen, low productivity, elevated salinity, rapid deposition, or a combination of these factors; minimal bioturbation may also reflect the limited use of freshwater ecospace during the Early Permian. Clay-rich paleo-Vertisols complete the fill, with drab root traces that indicate growth of vegetation in a strongly seasonal setting. Abundant plant material in the laminated beds includes branches of walchian conifers, the possible cycadophyte Taeniopteris spp., and the comioid, possible peltasperm, Auritifolia waggoneri. They were derived from an adjacent riparian zone and preserved as 3D goethite petrifactions. Much of the foliage shows evidence of arthropod herbivory. Although a humid climatic episode cannot be ruled out, the exceptional abundance and preservation of the plants probably reflects the persistence of an oxbow lake on a relatively arid alluvial plain, where riparian plants experienced periodic moisture stress but had access to groundwater nearly year round. Rapid burial in standing water, the lack of bioturbation in the laminated sediments, and early biomineralization probably explain the exceptional preservation of the plant remains.

Detecting Enigmatic Declines of a Once Common Salamander in the Coastal Plain of Georgia

Abstract For amphibian species suspected of undergoing enigmatic declines, it is important to determine the effort required to confidently establish species absence. Desmognathus auriculatus (Southern Dusky Salamander) has purportedly gone from being quite common throughout the southeastern US Coastal Plain to now being enigmatically rare. We used repeated standardized surveys of 5 historically occupied streams and their adjacent riparian zones between 2007 and 2010 to estimate detection rate of Southern Dusky Salamanders. We detected Southern Dusky Salamanders at 3 of 5 historic sites. Mean detection rate across streams known to be occupied at least once during the study was moderately low (mean ± 1 SE = 0.20 ± 0.12 for a double-sampled 50-m survey), improved at 2 sites with increasing time since drought, and varied among streams. For comparison, we evaluated detection rates of several other stream salamanders and found those rates to range from 0.37 (± 0.07) for Eurycea quadridigitata (Dwarf Salamander)...

Terrestrial and aquatic invertebrates in the riverbed of an intermittent river: parallels and contrasts in community organisation

Summary Although intermittent rivers (IRs) are coupled aquatic and terrestrial ecosystems, their constituent communities are generally considered separately by ecologists. Yet, both communities are disturbed by alternating flowing and dry phases. IRs are thus good systems in which to explore the parallels and contrasts in community organisation in response to disturbance, and gain understanding in the drivers of these responses. We compared the structure and composition of aquatic and terrestrial communities in the Albarine River, France. We collected aquatic and/or terrestrial invertebrates in the riverbed and adjacent riparian zone at four intermittent reaches and aquatic invertebrates in the riverbed at three perennial reaches over 1 year. A total of 175 taxa were collected at intermittent reaches, including 62 aquatic and 113 terrestrial taxa, contrasting with the low biodiversity generally associated with IRs. Aquatic and terrestrial communities were mainly subsets of those in perennial reaches and in the riparian zone, respectively, indicating that the riverbed of intermittent reaches is strongly linked to perennial reaches and the riparian zone by dispersal. Total taxonomic richness in the riverbed (i.e. γ diversity) of intermittent reaches was mainly due to among‐reach variation for terrestrial invertebrates, but to within‐reach variation for aquatic invertebrates. For both communities, among‐reach dissimilarity was mainly due to spatial turnover rather than community nestedness. Spatial turnover was higher for terrestrial communities than for aquatic communities. Our results indicate that aquatic and terrestrial communities have different organisation patterns in this intermittent river; probably in response to the contrasting spatial arrangements of colonist sources. Heterogeneous sources of terrestrial colonists along the riparian zone compared to more homogeneous sources of aquatic colonists from perennial reaches could explain the relative contribution of nestedness versus turnover.

Hydrologic and biogeochemical influences on carbon processing in the riparian zone of a subarctic stream

Small streams and their adjacent riparian zones contribute disproportionately to the processing and transport of C compared to the catchment area and, thereby, may alter the flux of dissolved organic C (DOC) from aquatic ecosystems. We examined the factors influencing C uptake and CO2 production in the riparian zone of a boreal stream. We tested the hypothesis that enhanced water velocity increases the supply of DOC and nutrients to microorganisms and, therefore, enhances DOC mineralization in the riparian zone. We measured in situ DOC mineralization rate in the riparian zone of a headwater stream at locations that varied with respect to the water velocity of shallow ground water. We added acetate as a tracer to measure C uptake in shallow ground water and monitored CO2 production during tracer experiments. We assessed relationships of these measures of C dynamics with the time that the tracer remained in shallow ground water surrounding the injection site (plume residence time), pre-injection DOC concentration, nutrient availability, and aromaticity of dissolved organic matter. Acetate uptake rate varied 2 orders of magnitude from −0.002 to −0.19/min, whereas CO2 production rate varied 5-fold from 0.01 to 0.06/min. Acetate uptake rate was negatively correlated with mean plume residence time, indicating that rapid water velocity increased C decomposition in the riparian zone. Moreover, Damköhler numbers (Da) were generally <1, indicating that substrate uptake was slower than hydrologic transport. Neither acetate nor CO2 rates were correlated with pre-injection riparian groundwater chemistry. Spatial heterogeneity of water velocity in the subsurface has a critical influence on the processing and subsequent export of C and nutrients from the riparian zone to streams.

Drying of a temperate, intermittent river has little effect on adjacent riparian arthropod communities

Summary Rivers and their adjacent riparian zones are intimately linked by fluxes of water, air masses, nutrients and organisms. Intermittent rivers constitute ideal systems to examine these linkages because they are characterised by alternating wet and dry phases, and hence, natural and contrasting variations occur in aquatic resources and environmental conditions. We addressed the effects of river drying on riparian communities by collecting ground‐dwelling arthropods at three perennial sites and at four intermittent sites during a wet and a dry phase in the Albarine River (France). We first predicted that during dry phases, declines in aquatic resources and loss of water would create harsh environmental conditions and alter community structure and composition, due to decreases in predators and hygrophilous taxa. Second, we predicted that alternating wet and dry phases would enhance cumulative taxonomic richness over time owing to an increase in species turnover. We found similar decreasing temporal patterns in taxonomic richness and abundance at intermittent and perennial sites, indicating that river drying had no effect on the structure of riparian arthropod communities. Community composition at intermittent and perennial sites differed only during the dry phase, indicating that intermittent and perennial sites gained and lost different sets of taxa as the river dried. Nevertheless, species turnover did not differ between sites, while taxonomic richness was always higher at intermittent sites. Although some taxa were sensitive to river drying, the Albarine River supports rich and abundant riparian arthropod communities. While several studies have reported a strong dependence of riparian populations on aquatic resources, our results indicate that the effects of river drying on riparian arthropods at the community level are weak and discrete in time in the Albarine River.

Stable isotope evidence indicates the incorporation into Japanese catchments of marine‐derived nutrients transported by spawning Pacific Salmon

Summary Pacific salmon (Oncorhynchus spp.) transport marine‐derived nutrients (MDN) and organic matter to freshwater ecosystems, which enhances the productivity of North Pacific ecosystems. Relatively few studies, however, have evaluated the MDN subsidy to both the aquatic system and the terrestrial catchment simultaneously. Using stable isotope analysis, we tested how the dynamics of MDN differed between the river and adjacent riparian forest in rivers of the Shiretoko World Natural Heritage Site in eastern Hokkaido (Japan). In addition, we accounted for temporal and spatial variations in the stable isotope signatures of freshwater organisms due to the presence or absence of spawning salmon. We analysed carbon and nitrogen stable isotopes (δ13C and δ15N) of biofilm, invertebrates, fish, riparian plants and brown bear (Ursus arctos) in the Rusha River during the pre‐spawning and spawning periods and in the Akai River (where there are no salmon). Willow leaves were collected along the 50‐m transects to evaluate how far MDN are incorporated within the riparian area. We counted the number of pink salmon (O. gorbuscha) carcasses in riparian areas and categorised their mode of transport. In addition, we examined the stomach contents of Dolly Varden (Salvelinus malma). The δ13C and δ15N of aquatic organisms increased by 1–4‰ and 1–6‰, respectively, with the arrival of salmon spawners. Aquatic organisms incorporated 23% of their nitrogen from salmon (range: 7–46%). The diet of Dolly Varden switched from aquatic invertebrates to salmon eggs during the salmon spawning run. More salmon carcasses were transported from the stream to riparian areas by flooding than by brown bears. The δ13C and δ15N of blowflies (Calliphora spp.) and brown bears increased significantly during the spawning run. Riparian vegetation, with the exception of Manchurian alder (Alnus hirsuta), incorporated 25% of its nitrogen from salmon. The δ15N values of riparian willow (Salix spp.) were correlated negatively with distance from the stream. The proportion of MDN incorporated in the freshwater biota was lower than that reported for North American rivers, potentially due to the influence of dams and modification of the river environment in this Japanese example. The riparian forest incorporated a relatively high fraction of MDN, however, mainly due to the transport of salmon carcasses from the channel by brown bears and, particularly, flooding. The dynamics of salmon‐derived nutrients thus differed between river and adjacent riparian zones. These results suggested the importance of linkages between freshwater and riparian ecosystems for the extent of the marine nutrient subsidy.

Investigating riparian groundwater flow close to a losing river using diurnal temperature oscillations at high vertical resolution

Abstract. River-water infiltration is of high relevance for hyporheic and riparian groundwater ecology as well as for drinking water supply by river-bank filtration. Heat has become a popular natural tracer to estimate exchange rates between rivers and groundwater. However, quantifying flow patterns and velocities is impeded by spatial and temporal variations of exchange fluxes, insufficient sensors spacing during field investigations, or simplifying assumptions for analysis or modeling such as uniform flow. The objective of this study is to investigate lateral shallow groundwater flow upon river-water infiltration at the shoreline of the riverbed and in the adjacent riparian zone of the River Thur in northeast Switzerland. Here we have applied distributed temperature sensing (DTS) along optical fibers wrapped around tubes to measure high-resolution vertical temperature profiles of the unsaturated zone and shallow riparian groundwater. Diurnal temperature oscillations were tracked in the subsurface and analyzed by means of dynamic harmonic regression to extract amplitudes and phase angles. Subsequent calculations of amplitude attenuation and time shift relative to the river signal show in detail vertical and temporal variations of heat transport in shallow riparian groundwater. In addition, we apply a numerical two-dimensional heat transport model for the unsaturated zone and shallow groundwater to obtain a better understanding of the observed heat transport processes in shallow riparian groundwater and to estimate the groundwater flow velocity. Our results show that the observed riparian groundwater temperature distribution cannot be described by uniform flow, but rather by horizontal groundwater flow velocities varying over depth. In addition, heat transfer of diurnal temperature oscillations from the losing river through shallow groundwater is influenced by thermal exchange with the unsaturated zone. Neglecting the influence of the unsaturated zone would cause biased interpretation and underestimation of groundwater flow velocities. The combination of high resolution field data and modeling shows the complex hydraulic and thermal processes occurring in shallow riparian groundwater close to losing river sections as well as potential errors sources for interpreting diurnal temperature oscillations in such environments.

Terrestrial invertebrates of dry river beds are not simply subsets of riparian assemblages

Dry river beds are common worldwide and are rapidly increasing in extent due to the effects of water management and prolonged drought periods due to climate change. While attention has been given to the responses of aquatic invertebrates to drying rivers, few studies exist on the terrestrial invertebrates colonizing dry river beds. Dry river beds are physically harsh and they often differ substantially in substrate, topography, microclimate and inundation frequency from adjacent riparian zones. Given these differences, we predicted that dry river beds provide a unique habitat for terrestrial invertebrates, and that their assemblage composition differs from that in adjacent riparian zones. Dry river beds and riparian zones in Australia and Italy were sampled for terrestrial invertebrates with pitfall traps. Sites differed in substrate type, climate and flow regime. Dry river beds contained diverse invertebrate assemblages and their composition was consistently different from adjacent riparian zones, irrespective of substrate, climate or hydrology. Although some taxa were shared between dry river beds and riparian zones, 66 of 320 taxa occurred only in dry river beds. Differences were due to species turnover, rather than shifts in abundance, indicating that dry river bed assemblages are not simply subsets of riparian assemblages. Some spatial patterns in invertebrate assemblages were associated with environmental variables (irrespective of habitat type), but these associations were statistically weak. We suggest that dry river beds are unique habitats in their own right. We discuss potential human stressors and management issues regarding dry river beds and provide recommendations for future research.

Abundance, diversity and functional gene expression of denitrifier communities in adjacent riparian and agricultural zones

Lands under riparian and agricultural management differ in soil properties, water content, plant species and nutrient content and are therefore expected to influence denitrifier communities, denitrification and nitrous oxide (N(2) O) emissions. Denitrifier community abundance, denitrifier community structure, denitrification gene expression and activity were quantified on three dates in a maize field and adjacent riparian zone. N(2) O emissions were greater in the agricultural zone, whereas complete denitrification to N(2) was greater in the riparian zone. In general, the targeted denitrifier community abundance did not change between agricultural and riparian zones. However, nosZ gene expression was greater in the riparian zone than the agricultural zone. The community structure of nirS-gene-bearing denitrifiers differed in June only, whereas the nirK-gene-bearing community structure differed significantly between the riparian and the agricultural zones at all dates. The nirK-gene-bearing community structure was correlated with soil pH, while no significant correlations were found between nirS-gene-bearing community structure and soil environmental variables or N(2) O emissions, denitrification or denitrifier enzyme activity. The results suggested for the nirK and nirS-gene-bearing communities different factors control abundance vs. community structure. The nirK-gene-bearing community structure was also more responsive than the nirS-gene-bearing community structure to change between the two ecosystems.

A conceptual model for the longitudinal distribution of wood in mountain streams

AbstractWood load, channel parameters and valley parameters were surveyed in 50 contiguous stream segments each 25 m in length along 12 streams in the Colorado Front Range. Length and diameter of each piece of wood were measured, and the orientation of each piece was tallied as a ramp, buried, bridge or unattached. These data were then used to evaluate longitudinal patterns of wood distribution in forested headwater streams of the Colorado Front Range, and potential channel‐, valley‐ and watershed‐scale controls on these patterns. We hypothesized that (i) wood load decreases downstream, (ii) wood is non‐randomly distributed at channel lengths of tens to hundreds of meters as a result of the presence of wood jams and (iii) the proportion of wood clustered into jams increases with drainage area as a result of downstream increases in relative capacity of a stream to transport wood introduced from the adjacent riparian zone and valley bottom. Results indicate a progressive downstream decrease in wood load within channels, and correlations between wood load and drainage area, elevation, channel width, bed gradient and total stream power. Results support the first and second hypotheses, but are inconclusive with respect to the third hypothesis. Wood is non‐randomly distributed at lengths of tens to hundreds of meters, but the proportion of pieces in jams reaches a maximum at intermediate downstream distances within the study area. We use these results to propose a conceptual model illustrating downstream trends in wood within streams of the Colorado Front Range. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Phylogeography of an Island Endemic, the Puerto Rican Freshwater Crab (Epilobocera sinuatifrons)

The endemic Puerto Rican crab, Epilobocera sinuatifrons (Pseudothelphusidae), has a freshwater-dependant life-history strategy, although the species has some capabilities for terrestrial movement as adults. In contrast to all other freshwater decapods on the island (e.g., caridean shrimp), E. sinuatifrons does not undertake amphidromous migration, and is restricted to purely freshwater habitats and adjacent riparian zones. As Puerto Rico has a dynamic geologic history, we predicted that both the life history of E. sinuatifrons and the geological history of the island would be important determinants of phylogeographic structuring in the species. Using a fragment of the cytochrome c oxidase subunit 1 mtDNA (mitochondrial DNA) gene, we tested for deviations from panmixia among and within rivers draining Puerto Rico and used statistical phylogeography to explore processes that may explain extant patterns of genetic variation in the species. While populations of E. sinuatifrons were significantly differentiated among rivers, they were likely to be recently derived because nested clade analysis (NCA) indicated evolutionarily recent restricted gene flow with isolation by distance (IBD) and contiguous range expansion at various spatial scales. Ongoing drainage rearrangements associated with faulting and land slippage were invoked as processes involved in sporadic gene flow among rivers throughout the Pleistocene. Patterns of genetic differentiation conformed to IBD and population demographic statistics were nonsignificant, indicating that although recently derived, populations from different rivers were in drift-mutation equilibrium. A shallow (0.6 million years ago), paraphyletic split was observed in the haplotype network, which NCA indicated arose via allopatric fragmentation. This split coincides with an area of high relief in central Puerto Rico that may have experienced relatively little drainage rearrangements. Shallow but significant genetic isolation of populations of E. sinuatifrons among Puerto Rican rivers suggests phylogeographic patterns that are intermediate to terrestrial habitat specialists (highly divergent populations) and other freshwater biota, such as amphidromous species and insects with aerial adult dispersal (highly connected populations).

ASSESSMENT OF NITRATE POLLUTION IN FIZES WATERSHED

River streams together with the adjacent riparian zones constitute essential multifunctional elements of the ecological network worldwide. Water Framework Directive demands a concerted approach towards the goal of achieving a good ecological state for all water bodies across Europe. Nitrate pollution is an issue that all Member States confront. It can be asserted that the sources of nitrate pollution are diffusing (multiple such discharges which are difficult to locate), while the main polluters (farms) are sensitive to everything that affects their economic viability. Fizes watershed, tributary of Somes catchment is located in the northern part of Romania and covers a surface of approximately 562 km2, with an average altitude of approximately 400 m. Nitrate concentration, together with other species of mobile nitrogen were measured in surface water along the Fizes watershed. The main pollution sources are represented by sediments generated by erosional processes and transported in the water bodies, and diffuse pollution sources originated from agricultural activities and the improper septic systems of the localities Through its features of relatively low antropic pressures and with little structural changes, Fizes watershed represents a natural laboratory for designing and implementing programs of restorations of watersheds in agricultural landscapes.

Riparian influence on hyporheic‐zone formation downstream of a small dam in the Blackland Prairie region of Texas

AbstractSmall‐order streams have highly variable flows that can result in large temporal and spatial variation of the hyporheic zone. Dam construction along these intermittent headwater streams alters downstream flow and influences the hydrologic balance between stream water and the adjacent riparian zone. A 3‐year site study was conducted along an impounded second‐order stream to determine the water balance between stream, unsaturated zone, groundwater and riparian vegetation. The presence of the upstream impoundment provided near‐perennial water flow in the stream channel. The observed woody plant transpiration accounted for 71% of average annual water loss in the site. The overall contribution of stream water via the hyporheic zone to site water balance was 73 cm, or 44% of total inputs. This exceeded both rainfall and upland subsurface contribution to the site. A highly dynamic hyporheic zone was indicated by high water use from woody plants that fluctuated seasonally with stream water levels. We found leaf area development in the canopy layer to be closely coupled with stream and groundwater fluctuations, indicating its usefulness as a potential indicator of site water balance for small dam systems. The net result of upstream impoundment increased riparian vegetation productivity by influencing movement of stream water to storage in the groundwater system. Copyright © 2006 John Wiley &amp; Sons, Ltd.