Discharge-driven shifts in the wetted area of streams can modify the amount of leaf litter resources available to stream consumers as well as the physical conditions to which leaf litter is exposed. The consequences of this continual movement from wet to dry storage for rates of organic matter processing and resource availability to benthic communities are poorly understood. We used a 30-day field experiment during the period of maximum stream contraction to examine the effects of stranding on black cottonwood ( Populus trichocarpa ) leaf litter decomposition rates and associated changes in microbial respiration in a forested stream in western Montana. Leaf litter was enclosed in both coarse and fine mesh bags and moved from the wetted area of the stream to the stream bank in six treatments designed to mimic a gradient of dry exposure due to stranding. We also measured existing accumulations of organic material in quadrats placed in both wet and dry areas of the stream. The total storage of litter resources (ash-free dry mass, g m 2 ) retained on dry stream banks increased steadily as stream flow decreased, resulting from reductions in wetted width and continuous inputs from terrestrial zones. In contrast, total mass of stored litter submerged in the stream channel remained relatively constant. Leaf decomposition rates increased as a function of time inundated and were fastest in the presence of macroinvertebrates. Our results suggest that prolonged stranding can alter fundamental processes and energy pathways in stream food webs by shifting pools of resources from the active channel to dry storage on riverbanks where decomposition is driven primarily by microbial processes. Since the length of time that leaf litter is inundated prior to stranding alters decomposition rates, changes in stream hydrograph variability (as a consequence of land management practices or incipient climate change) has the potential to alter energy flow through stream systems. In particular, dry storage may function as a type of ‘temporal subsidy’ for stream organisms particularly if slowly decomposing stranded leaf litter is re-entrained during periods when in-stream detrital resources are otherwise scarce.
Quantifying spatial patterns of physical and biological features is essential for managing aquatic systems. To meet broad-scale habitat assessment and monitoring needs, we evaluated the feasibility of predicting 25 instream physical habitat measures for wadeable stream reaches in Wisconsin and northern Michigan using geographic information system (GIS) derived stream network and landscape data. Using general additive modeling and boosting variable selection, predictions of reasonable accuracy were obtained for 10 widely used in-stream habitat measures, including bankfull depth and width, conductivity, substrate size, sand substrate, thalweg water depth, wetted width, water depth, and widthto- depth ratio. Biased predictions were obtained for habitat measures such as bank erosion, large woody debris, fish cover, canopy shading, and substrate embeddedness. Model predictions for many commonlyused habitat variables were judged acceptable based on several criteria, including correspondence between prediction errors and observed interannual and inter-site variability in habitat measures and agreement in correlation analyses of fish assemblage metric data with both predicted and observed values. Prediction of physical habitat variables from widely available GIS datasets represents a potentially powerful and cost-effective approach for broad-scale (e.g., multi-state, national) assessment and monitoring of in-stream conditions, for which direct measurement is largely impractical because of resource limitations.
The well-documented re-colonisation of the French large river basins of Loire and Rhone by European otter and beaver allowed the analysis of explanatory factors and threats to species movement in the river corridor. To what extent anthropogenic disturbance of the riparian zone influences the corridor functioning is a central question in the understanding of ecological networks and the definition of restoration goals for river networks. The generalist or specialist nature of target species might be determining for the responses to habitat quality and barriers in the riparian corridor. Detailed datasets of land use, human stressors and hydro-morphological characteristics of river segments for the entire river basins allowed identifying the habitat requirements of the two species for the riparian zone. The identified critical factors were entered in a network analysis based on the ecological niche factor approach. Significant responses to riparian corridor quality for forest cover, alterations of channel straightening and urbanisation and infrastructure in the riparian zone are observed for both species, so they may well serve as indicators for corridor functioning. The hypothesis for generalists being less sensitive to human disturbance was withdrawn, since the otter as generalist species responded strongest to hydro-morphological alterations and human presence in general. The beaver responded the strongest to the physical environment as expected for this specialist species. The difference in responses for generalist and specialist species is clearly present and the two species have a strong complementary indicator value. The interpretation of the network analysis outcomes stresses the need for an estimation of ecological requirements of more species in the evaluation of riparian corridor functioning and in conservation planning.
The central reach of the Ebro River, one of the largest rivers in the Mediterranean region, is characterised by a meandering channel that develops into a wide floodplain. In the present study, we analysed the evolution of the landscape structure and function of riparian forests in the Middle Ebro River (~ 250 km long) in 1927, 1957 and 2003 in order to evaluate the forest dynamics in this river corridor. In the 20th century, the Ebro floodplain suffered a dramatic hydromorphological transformation as a result of urban and agricultural encroachment on the river territory and modifications to its flow regime. This study assesses the overall morphology and connectivity of riparian forest patches in the study area using a wide range of parameters. The influence of the hydromorphological changes of the river system on the general status of the riparian forests was then determined. The analysis revealed a profound trend toward the homogenisation and isolation of forest patches. Habitat loss and landscape fragmentation were the dominant processes in the study area and were especially intense in some river segments, where large forest patches and high connectivity once prevailed. Landscape modification and overall homogenisation intensified during the second half of the last century. The results for the entire set of parameters can be used to identify guidelines for the effective attenuation of these trends and for the progressive rehabilitation of the dynamics of the Middle Ebro landscape.
The aim of this study was to determine the sources of water uptake for two common riparian tree species found in the southeastern United States, Acer negundo and Betula nigra . The study site was located within a riparian zone typical of this region and those of the temperate USA. Water sources were determined by analyzing signatures of stable isotopes found naturally in water, 2 H and 18 O. Samples from surface water, groundwater, and soil, plus woody tissue from mature individuals of each species, were taken once each month during the 2011 summer growth season. Both species relied strongly on an unidentified ground water source, although A. negundo also showed a strong correlation with deep soil moisture (r 2 = 0.972). Sampling limitations did not permit an accurate determination of the fractional contribution of each source to plant water, limiting the strength of the results. The evidence collected leads to conclusions comparable to those of studies that have been conducted in arid parts of North America, corroborating that streamside species may rely heavily on groundwater sources, not surface streamwater. Further studies are needed in this region to confirm the evidence reported here to establish a baseline for vegetation in these systems.
