Assessing discharge periodicity in mountain catchments using classified environmental conditions (Tatra Mountains, Poland)

The periodicity of a river expressed in cycles of various lengths (monthly, seasonal, multiannual) is a result of climatic factors and overlapping environmental conditions within its catchment. In uncontrolled or poorly surveyed catchments, it is very difficult to determine the duration of a stream’s hydrological activity. This is especially relevant for catchments with complicated water circulation in karstic rocks. The present study concerns the small catchment of the Strązyski Potok river located in the area of the Tatra National Park, in the Western Tatras. The observation period covered the 2015 hydrological year, which differed hydrologically from average conditions. This study aims to develop a simple method to explain the processes shaping the mountain stream discharge periodicity. The research employed periodic field observations linked with climatic and non-climatic factors. Environmental conditions were assessed as four classes reflecting their influence on appearance or disappearance of mountain stream water. Class boundaries were the values of quartiles. The degree of correspondence between environmental factors and stream field observations was described via the Index DC (Degree Correspondence Index) approach. Complete correspondence (Index DC =0) was found in 23% catchments, a weak relationship between conditions favouring discharge and actual condition (Index DC= -1, +1) was noted within 11 catchments, while in 9 catchments, no such relationship was found (Index DC = -2, +2). The obtained results indicate a correspondence or lack thereof between the environmental potential of the catchment and its discharge periodicity. The discrepancies between the assessment of the influence of climatic and non-climatic factors and the data collected during field observations provide a basis for more detailed studies. Continuation of these studies based on the proposed classifications will allow for a more complete explanation of water disappearance in river channels and the determination of their short- and long-term discharge periodicity.

The Morphological Evolution of a Step–Pool Stream after an Exceptional Flood and Subsequent Ordinary Flow Conditions

Mountain streams are frequently characterized by step–pool morphology that provides stability and energy dissipation to the channel network. Large flooding events can overturn the equilibrium of the step–pool condition by altering the entire configuration. This work focuses on the impact of the “Vaia” storm (27–30 October 2018) on a step–pool mountain stream (Rio Cordon, Northeast Italy) and on its evolution after two years of ordinary flow conditions. To achieve the aims, this work uses both remote sensing data (LiDAR and UAV) and direct field measurements (i.e., longitudinal profiles and grain sizes distributions) performed pre-event, post-event, and 2 years later (current conditions). The results show a significant widening (width +81%, area +68%) and the creation of a new avulsion after the storm and a substantial change between the number of units (51 in the pre-event, 22 post-event, and 51 in the current conditions) and characteristics of step–pool sequences between pre- and post-conditions. Furthermore, it proves the ongoing processes of morphological stabilization since the current step–pool sequences parameters are heading back to the pre-event values. Such results suggest clear susceptibility of step–pool to exceptional events and fast recovery of such setting during barely two years of ordinary flow conditions.

Testing the Sensitivity and Limitations of Frequently Used Aquatic Biota Indices in Temperate Mountain Streams and Plain Streams of China

Different biological groups show biased responses to similar or different environmental stressors on different scales. The selection of bioindicators based on pressure characteristics is the basis for accurately assessing ecological quality. In this study, we investigated the responses of common bioindicators, namely, macroinvertebrates and fish, to multiple stressors in temperate mountain and plain streams of northeast China. We used 56 indices, including the single biological evaluation index and biological evaluation index system, to analyze and compare characteristic response to different scales under varying environmental stressors. The principal component analysis (PCA) showed that PCA axis 1 in the catchment scale explained 83.6% and 96.1% of the variance in mountain and plain rivers, respectively, which characterized the comprehensive pressure gradient integrated by land-use development and water pollution. PCA axis 1 explained 40.7% and 53.9% of variance in mountain and plain rivers on the reach scale and 63.1% and 61.8% of variance on the site scale. The correlation analysis showed responses of different indices to abiotic variables which did not overlap. Macroinvertebrate and fish indices successfully explained the change in water chemistry on a small scale, whereas fish indices additionally explained the change in land use on a large scale. Macroinvertebrate and fish indices were recommended because of their rich responses to environmental stressors, particularly in plain rivers. For mountain stream biomonitoring programs, especially in the Taizi River of northeastern China, we suggest that macroinvertebrates and fish should be used separately or jointly according to the actual capacity and cost, Moreover, compared with the possible differences in the evaluation results of different single biological evaluation indexes, the biological evaluation index system shows more stable monitoring results, and the single sensitivity index is more significant in biological evaluation, and more sensitive to some special environmental factors.

Visual Harmony of Engineering Structures in a Mountain Stream

This study uses the cognitive factor of “visual harmony” to assess the visual quality of stream engineering in a mountainous region. Images of engineering structures such as revetments and submerged dams in the mountain streams of Taiwan were collected. Three image groups with different structures invaded by vegetation were used for a questionnaire survey, which yielded 154 valid samples. We used statistical analysis to develop a model of visual harmony H with respect to the percentage of visible greenery GR, that is, the perceived curve of vegetation change. A comparison of our data with the literature determined the upper and lower bound curves of the relationship between H and GR. We found that the physical elements of “softscape” and “hardscape”—namely, percentage of visible water WR, visible structure IR, and visible natural material on the structure NR—affected this relationship. Results show that H is equivalent to visual preference P, and both can be improved by better green visibility (increasing GR and GR < 50%), avoiding low water visibility (WR < 10%), or increasing the amount of visible natural material (NR > 0.9). High visibility of the structures (IR > 0.3) may decrease H and P. We ultimately propose a visual harmony or preference model concerning a combined physical indicator that comprises GR, WR, IR and NR. Results of this study could be helpful to improve or access the aesthetics of stream engineering design.

The joint contributions of environmental filtering and spatial processes to macroinvertebrate metacommunity dynamics in the alpine stream environment of Baima Snow Mountain, Southwest China

As a rapidly growing field of community ecology, the study of meta-communities provides an effective framework to unravel community assembly mechanisms by focusing on the relative contributions of environmental screening and spatial processes. While macroinvertebrates have been extensively investigated in many river ecosystems, meta-community ecology perspectives in high mountain stream networks are very limited. In this study, we assessed the role of ecological determinants and temporal dynamics in the macroinvertebrate meta-community assembly of an alpine stream situated in a dry-hot valley of Baima Snow Mountain, Northwest Yunnan. Our results show significant differences in the macroinvertebrate community composition across time periods. Spatial structuring and environmental filtering jointly drive the configuration of macroinvertebrate meta-community, with relative contributions to the variance in community composition varying over time. Redundancy Analysis (RDA) and variation partitioning indicate that environmental variables are the most important predictors of community organization in most scenarios, whereas spatial determinants also play a significant role. Moreover, the explanatory power, identity, and the relative significance of ecological indicators change over time. Particularly, in the years 2018 and 2019, stronger environmental filtering was found shaping community assembly, suggesting that deterministic mechanisms predominated in driving community dynamics in such a specific environment of the stream. However, spatial factors had a stronger predictive power on meta-community structures in 2017, implying conspicuous dispersal mechanisms which may be owing to increased connectivity amongst locations. Thereby, we inferred that the stream macroinvertebrate metacommunity composition can be regulated by the interaction of both spatial processes and environmental filtering, with relative contributions varying over time. Based on these findings, we suggest that community ecology studies in aquatic systems should be designed beyond single snapshot investigations.

Coarse and Fine Particulate Organic Matter Transport by a Fourth-Order Mountain Stream to Lake Bourget (France)

Transport of coarse particulate organic matter (CPOM) derived from forest litterfall has been hardly studied in rivers, unlike fine particulate organic matter (FPOM) or dissolved organic matter (DOM). Yet, many rivers are dammed or run into lakes, and there is growing evidence that CPOM accumulation in river delta participates substantially in ecological processes such as greenhouse gas emissions of lakes and reservoirs. We investigated the transport of CPOM and FPOM by the Leysse River (discharge from 0.2 to 106 m3 s−1) to Lake Bourget (France) in relation to aerial litter deposition, river network length, and discharge. Over a 19-month study period, the volume-weighted mean CPOM and FPOM concentrations were 1.3 and 7.7 g m−3, respectively. Most CPOM and FPOM transport occurred during major flood events, and there were power relationships between maximum discharge and particulate organic matter (POM) transport during these events. The annual export of CPOM (190 t AFDM) was 85% of the litter accumulation in autumn on permanent sections of the riverbed (224 t AFDM), which suggests that export is a major process compared to breakdown. Export of CPOM was 1.25 t yr−1 km−2 of the forested catchment area. This study highlights the need to account for long-range CPOM transport to describe the fate of litter inputs to streams and to quantify the organic matter input and processing in lakes and reservoirs.