The dilution effect was originally proposed to describe the negative effect of increased host diversity on parasite abundance; with greater host diversity, parasite levels per host are predicted to be lower due to a higher probability of dispersing parasites encountering non-competent hosts. Dilution effects could also occur in many mutualisms if dispersing symbionts encounter hosts that vary in their competency. The introduction of non-native hosts can change community competency of a local group of host species. Crayfish introductions are occurring world-wide and these introductions are likely disrupting native crayfish-symbiont systems. Branchiobdellidan symbionts declined on native Cambarus crayfish occurring in the presence and absence of non-native Faxonius crayfish in the New River, USA. We performed an experiment investigating the effect of host density (1 vs 2 native hosts) and host diversity (1 native host and 1 introduced host) on branchiobdellidan abundance. The introduced F. cristavarius is a non-competent host for these worms. Six C. ingens were stocked on a C. chasmodactylus in each treatment and worm numbers were followed over 34 days. Worm numbers decreased over time on C. chasmodactylus alone and in the treatment in which a C. chasmodactylus was paired with an F. cristavarius. Worm numbers remained highest in the 2 C. chasmodactylus treatment . There was no significant effect of host diversity on worm reproduction. Crayfish invasions may have negative effects on mutualistic symbionts depending on the competence of introduced hosts. Loss of native symbionts is one of the potential hidden, negative effects of invasions on native freshwater diversity.
Biological invasions are regarded as one of the largest threats to native biodiversity. The eradication of non-native parasites by culling of hosts are a controversial conservation strategy, particularly when using indiscriminate methods involving whole ecosystem collateral damage. While short-term effects are abundantly documented, long-term surveys are needed to detect potential wider ecosystem effects. Here, we report a six-year study on effects of the piscicide rotenone on invertebrate communities from a Norwegian water course using a Before-After-Control-Impact design. Kick-net samples of benthic invertebrates were collected from three lentic sampling stations and two lotic stations two to four times per year in both a control and a treated watershed. In general, only relatively minor short-term effects immediately after the treatment on species turnover, measured as temporal beta-diversity, of benthic invertebrates were observed both in lentic and lotic locations. However, the lotic fauna was temporarily severely negatively affected following a period of rotenone exposure from an upstream lake. Species turnover co-varied markedly between control and treatment locations, indicating that natural environmental variation override effects of rotenone treatment. Likewise, the abundance of invertebrate taxa varied considerably both over time and between control and treatment locations. Our study indicates minor short-term (i.e. < one month) or long-term (i.e. four years) effects of rotenone treatment on benthic invertebrates, but severe effects on the lotic fauna eight months after treatment. However, long-term effects are likely to be taxa-specific and vary depending on habitat connectivity and thus potential for re-colonization and will differ among locations and among taxa.
Widespread hydrologic alterations have simplified in-stream habitats in rivers globally, driving population declines and local extirpations of many native fishes. Here, we examine how rapid geomorphic change in a historically degraded desert river has influenced habitat diversification and ecosystem persistence. In 2010, a large reach of the degraded and simplified lower San Rafael River (SRR), Utah, was impacted by the formation of a valley plug and began to shift from a homogenous, single-thread channel to a complex, multi-threaded riverscape. We combined field measurements and drone-collected imagery to document habitat changes due to the valley plug. Our results demonstrate that in 2021, the valley plug reach was more diverse than any other stream reach along the SRR, containing 641% more diverse habitat (e.g., pools, riffles, backwaters) than what was measured in 2015. The plug reach also retained water for periods beyond what was expected during seasonal drying, with the total extent of inundation within the riverscape increasing by over 2,800%. Since the formation of the valley plug, riparian habitat has increased by 230% and channel networks have expanded to more than 50 distinct channels throughout the zone of influence. Our results provide evidence of successful self-restoration in a formerly highly degraded reach of desert river, and encourage new methods of desert river restoration. We aim to inform the use of large-scale, disruptive restoration actions like intentional channel occlusions, with the goal of mitigating the impacts of simplification and increasing habitat persistence in the face of exacerbated aridity in the desert Southwest.
Riparian zones represent an important ecosystem providing a range of functions and services important to humans—e.g., biodiversity support, a reduction in erosion risk, or the transport of pollutants from the surrounding landscape to watercourses. At the same time, it is, unfortunately, an environment that has been often subjected to significant pressure during the agricultural cultivation of the landscape or the development of industrial and residential activities of human society. Thus, a large number of riparian ecosystems have disappeared or degraded. The assessment of the overall ecological status of riparian habitats constitutes an important source of information for the needs of watercourse management and landscape planning in the riparian landscape, the aim of which should be to maintain good status or to improve the current unsatisfactory state of these habitats. However, in order to reliably evaluate the current ecological status of the landscape, it is necessary to have information on the reference status, i.e., a potentially natural status that would prevail without human influence. For this purpose, a methodology that can determine the potential natural status of riparian zones in Central European conditions was developed. In this study, it was found that approximately a quarter (26 %) of all river basins in the Czech Republic reach very low environmental values of the potential natural status of riparian zones and, conversely, approximately 29 % of river basins are expected to develop significantly above average riparian zone quality if we neglect human impact.
Riparian vegetation provides many noteworthy functions in river and floodplain systems including its influence on hydrodynamic processes. Traditional methods for predicting hydrodynamic characteristics in the presence of vegetation involve the application of static roughness ( ns) values, which neglect changes in roughness due to local flow characteristics. The objectives of this study were to: (1) implement numerical routines for simulating dynamic hydraulic roughness ( nd) in a two-dimensional (2D) hydrodynamic model; (2) evaluate the performance of two dynamic roughness approaches; and (3) compare vegetation parameters and hydrodynamic model results based on field-based and remote sensing acquisition methods. A coupled vegetation-hydraulic solver was developed for a 2D hydraulics model using two dynamic approaches, which required vegetation parameters to calculate spatially distributed, dynamic roughness coefficients. Vegetation parameters were determined by field survey and using airborne LiDAR data. Water surface elevations modeled using conventional and the proposed dynamic approaches produced similar profiles. The method demonstrates the suitability in modeling the system where there is no calibration data. Substantial spatial variations in both n and hydraulic parameters were observed when comparing the static and dynamic approaches. Thus, the method proposed here is beneficial for describing the hydraulic conditions for the area having huge variation of vegetation. The proposed methods have the potential to improve our ability to simulate the spatial and temporal heterogeneity of vegetated floodplain surfaces with an approach that is more physically-based and reproducible than conventional “look up” approaches. However, additional research is needed to quantify model performance with respect to spatially distributed flow properties and parameterization of vegetation characteristics.
Spatial and temporal heterogeneity, or messiness, is a broadly desirable characteristic of river corridors and an indicator of many of the geomorphic processes that sustain fluvial ecosystems. However, quantifying geomorphic heterogeneity is complicated by a lack of consistent metrics, classification schemas for dividing the river corridor into the patches that form the basis for those metrics, and guidance on interpreting metrics. Drawing from both geomorphic and landscape ecology concepts, we offer ideas and guidance intended to help investigators, from researchers to restoration practitioners, more effectively and reliably use heterogeneity to describe river corridor processes and characteristics. We define geomorphic heterogeneity both spatially and temporally. Spatially, heterogeneity can be described by diversity, or the evenness and richness of geomorphic units, and spatial configuration, or the arrangement and shape of geomorphic units. Temporally, heterogeneity can be described by turnover rate, or the rate of change of geomorphic units. Interpretation of heterogeneity metrics depends integrally on the definition of the geomorphic unit schema on which metrics are based. Contextual information, such as measurements of process space (i.e., how much room a river has to move), disturbance frequency, and geomorphic trajectory, can also be key to interpreting measurements of heterogeneity. Geomorphic applications of heterogeneity require carefully defined geomorphic unit schemas that reflect processes and characteristics of interest, robust metrics of heterogeneity whose meaning is appropriate to the question at hand, and interpretation of those metrics based on the context of expected geomorphic processes and the disturbance regime.
Hydropeaking operation leads to fluctuations in wetted area between base and peak flow and increases discharge-related hydraulic forces (e.g., flow velocity). These processes promote macroinvertebrate drift and stranding, often affecting benthic abundance and biomass. Our field experimental study – conducted in three hydropeaking-regulated Swiss rivers – aimed to quantify (i) the short-term effects of the combined increase in flow amplitude and up-ramping rate based on macroinvertebrate drift and stranding, as well as (ii) long-term effects based on the established community composition. Hydropeaking led to increased macroinvertebrate drift compared to base flow and to unaffected residual flow reaches. Moreover, stranding of macroinvertebrates was positively related to drift, especially during the up-ramping phase. Flow velocity and up-ramping rate were identified as major determinants for macroinvertebrate drift, while flow ratio and down-ramping rate for stranding. Particularly high sensitivity towards HP was found for Limnephilidae, whereas Heptageniidae seemed to be resistant in respect to short and long-term hydropeaking effects. In the long-term, hydropeaking did not considerably reduce benthic density of most taxa, especially of some highly resistant and resilient taxa such as Chironomidae and Baetidae, which dominated the community composition even though they showed comparably high drift and stranding responses. Therefore, we argue that high passive drift and/or stranding, especially of individual-rich taxa, does not necessarily indicate strong hydropeaking sensitivity. Finally, our results demonstrate the necessity to consider the differences in river-specific morphological complexity and hydropeaking intensity, since these factors strongly influence the community composition and short-term drift and stranding response of macroinvertebrates to hydropower pressure.
Headwater streams can constitute up to 80% of river channel length and are vulnerable to anthropogenic pressures due to their high connectivity to adjacent land, large relative catchment size and low dilution capacity. In these environments unrestricted cattle access is a potential significant cause of water quality deterioration, resulting from increases in stream bank erosion, riparian damage and sediment deposition among others. Several studies have reported improvements in physico-chemical and hydromorphological conditions of streams following elimination of cattle access; few, however, have focussed on the ecological impacts of such management practices. Here, such impacts are assessed. We look at the short-term effects by comparing habitat condition, sediment deposition, and instream macroinvertebrate communities upstream and downstream of cattle access points prior to, and one year following exclusion via fencing. The long-term effects are also measured by reassessing a small stream catchment entirely fenced off from cattle access in 2008 under a concerted management effort. In the short term, cattle exclusion led to reduction in deposited sediment downstream of cattle access points and a related homogenisation of macroinvertebrate community structure between upstream and downstream sampling points. Increased abundances of specific indicator taxa ( Ancylus fluviatilis, Glossosomatidae and Elmidae) in the fenced catchment following 9 years of exclusion highlight the long-term ecological benefits of such mitigation practices. These findings highlight the importance of incentivised agri-environment schemes in reducing the negative impacts of cattle access to these vulnerable ecosystems.
Although the dispersal is important for riparian plants, few studies have evaluated the patterns in species richness and composition of propagules deposited by different dispersal types. In the present study we evaluate the temporal and spatial patterns in the diversity of propagules deposited by hydrochory and by other types of dispersal along mountain rivers. To do this, we sampled the propagules deposited in the riparian zone in a distance gradient with respect to the site of origin of the rivers in two seasons. Regarding the temporal analyses, we found no differences in the number of propagules between seasons. In the rainy season we observed a greater number of species deposited by hydrochory, while for other types of dispersal there was a greater number of species in the dry season. Differences in composition were observed for hydrochory but not for other types of dispersal. Regarding the spatial analyses, there were no changes in the number of propagules deposited along the river by hydrochory, while for other types of dispersal an increase was observed in the dry season. A lower number of species deposited by hydrochory along the river in the rainy season was observed. Finally, we observed that turnover increases by other types of dispersal in the dry season but not for hydrochory. The results underline the importance of types of dispersal other than hydrochory in the contribution of propagules dispersed and the temporal and spatial particularities of the hydrochory in mountain rivers.
Blue sucker ( Cycleptus elongatus) occurs in the Mississippi River and Gulf of Mexico drainages of North America and is negatively affected by habitat fragmentation and flow regime alteration caused by dams. During fish assemblage surveys in August of 2022, we collected five specimens of juvenile blue sucker (312-428 mm total length) in the Angelina River upstream of Sam Rayburn Reservoir in east Texas (46,335-hectare surface area) where the occurrence of the species was previously unconfirmed. Given this unexpected finding, we (1) analyzed blue sucker mesohabitat associations to compare habitats we sampled with reports in the literature, and (2) reviewed blue sucker occurrence in state, national, and global databases across historical (1950-1980) and contemporary (1981-2022) periods to assess occurrence across gradients of habitat fragmentation and streamflow regulation. The blue sucker population in the Angelina River upstream of Sam Rayburn Reservoir was previously unconfirmed but is within the native range of the species. Mesohabitats occupied by blue sucker were consistent with literature reports, including fast velocity, shallow depth, and coarse substrates. The low degree of regulation (19% of natural runoff stored by upstream reservoirs) and a high degree of habitat connectivity (287 rkm of mainstem habitat) for the Angelina River upstream of Sam Rayburn Reservoir matched range-wide patterns of persistence within relatively intact (unfragmented and unregulated) or remnant (fragmented but unregulated) riverscapes. Our review reveals that blue sucker populations might persist (1) in remnant river fragments where local habitat templates are appropriate and (2) where effects of habitat fragmentation and flow regulation are not coupled.
Salt dilution is a well-established streamflow measurement method in creeks, which works particularly well downstream of turbulent flow sections as mixing of the salt tracer is enhanced. Usually salt dilution measurements are performed manually, which considerably limits the observations of rare peak flow events. However, these events are particularly important for constructing robust rating curves and avoiding large uncertainties in the extrapolation of river discharge values. An additional challenge is the variability of the river cross-section, especially after larger discharge events, leading to non-stationary rating curves. Therefore, discharge measurements well distributed over time are needed to both construct a reliable streamflow-water level relationship and to detect changes caused by erosion and deposition processes. To overcome these two issues, we used an automated streamflow measuring systems at three different sites in the Alps for event-based discharge measurements. This system allowed us to measure close to the highest peak flows at all three sites in the observation period (2020-2021) and to detect abrupt changes in the rating curve. Based on a very large data set of almost 300 measurements, we were able to evaluate the reliability of the system and to identify the main sources of uncertainty in the experimental setup. One key aspect was the site selection for the downstream electrical conductivity sensors as measurement location strongly controls the signal-to-noise ratio (SNR) in the recorded breakthrough curves.
Mapping fluvial hydromorphology is an important part of defining river habitat. Mapping via field sampling or hydraulic modelling is however time consuming, and mapping hydromorphology directly from remote sensing data may offer an efficient solution. Here we present a system for automated classification of fluvial hydromorphology based on a deep learning classification scheme applied to aerial orthophotos. Using selected rivers in Norway, we show how surface flow patterns (smooth or rippled surfaces versus standing waves) can be classified in imagery using a trained convolutional neural network. We show how integration of these classified surface flow patterns with information on channel gradient, obtained from airborne topographic LiDAR data, can be used to compartmentalize the rivers into hydromorphological units that represent the dominant flow features. Automated classifications were consistent with those produced manually. They were found to be discharge-dependent, showing the temporally dynamic aspect of hydromorphology. The proposed system is quick, flexible, generalizable, and free from researcher-subjectivity. The deep learning approach used here can be customized to provide more detailed information on flow features, such as delineating between standing waves and advective diffusion of air bubbles/foam, to provide a more refined classification of surface flow patterns, and the classification approach can be further advanced by inclusion of additional remote sensing methods that provide further information on hydromorphological features.
Centuries of human development have altered the connectivity of rivers, adversely impacting ecosystems and the services provided. Significant investments in natural resource projects are made annually with the goal to restore function to degraded rivers and floodplains and protect freshwater resources. Yet restoration projects often fall short of their objectives, in part, due to the lack of systems-based, strategic planning. To evaluate channel-floodplain (dis)connectivity and erosion/incision hazard at the regional scale, we calculate Specific Stream Power (SSP), an estimate of the energy of a river, using a topographically-based, low-complexity hydraulic model. Other basin-wide SSP modeling approaches neglect reach-specific geometric information embedded in Digital Elevation Models. Our approach leverages this information to generate reach-specific SSP-flow curves. We extract measures from these curves that describe (dis)connected floodwater storage capacity and erosion hazard at individual design storm flood stages and demonstrate how these measures may be used to identify watershed-scale patterns in connectivity. We show proof-of-concept using 25 reaches in the Mad River watershed in central Vermont and demonstrate that the SSP results have acceptable agreement with a well-calibrated process-based model (2D Hydraulic Engineering Center’s River Analysis System) across a broad range of design events. While systems-based planning of regional restoration and conservation activities has been limited largely due to computational and human resource requirements, measures derived from low complexity models can provide an overview of reach-scale conditions at the regional level and aid planners in identifying areas for further restoration and/or conservation assessments.
Floodplain restoration can enhance capacity for carbon sequestration by facilitating higher water tables, deposition of fine sediment, and increased input and residence time of organic matter. We measured floodplain soil organic carbon stocks in nine stream restoration projects across the western United States and compared them to nearby degraded and reference condition floodplains. Degraded floodplains had the lowest soil mean carbon stocks in the majority of floodplains measured (range 161-894 Mg C/ha), and reference stocks had the highest stocks (range 391-904 Mg C/ha) of those with statistically significant differences between the three categories. Across all sites measured, stream restoration sites, referred to as treatment sites, had stocks (range 203-1028 Mg C/ha) similar to degraded condition floodplains but the largest range. When modeled under degraded conditions, four out of nine of the treatment sites had significantly higher OC stocks than predicted. Climate and geologic variables are most influential in predicting carbon stocks, and floodplains in the interior western USA have the highest carbon stocks. As the demand for carbon sequestration increases due to climate change, ecologically responsible floodplain restoration provides a significant opportunity for carbon storage. However, despite the statistically significant relationships we observed in this dataset, the variations within the data in relation to degraded/treatment/reference categories illustrate the uncertainties in quantifying the effects of restoration on floodplain carbon stocks.
The Clarence River (New South Wales, Australia) was the main transport corridor for the sugar cane industry operating in the area from the 1860s to the 1970s. Using archaeological, documentary and oral historical resources we explore some of the anthropogenic impacts of this industry upon river channels and hydrology, in particular through the deliberate abandonment of obsolete vessels. These deliberately discarded former cane barges have been used as erosion control devices in several areas around the Harwood Island sugar mill, resulting in the accumulation of sediments and the establishment of mangrove environments in what were degraded areas.
Over the last decade, rapid vegetation colonization and changes in channel morphology have been observed in the Naeseongcheon Stream in South Korea, which were linked to short-term hydrological fluctuations under a changing monsoon climate. The surface area covered by vegetation has been increasing; this increase intensified after the 2014–2015 drought, which provided a window of opportunity for vegetation establishment. During the drought, pioneer herbaceous vegetation densely colonized the lower floodplains, including bare sandbars and temporarily exposed riverbed. Although the colonized lower floodplain and river banks were partially rejuvenated by several subsequent floods, succession to woody vegetation continued, resulting in stable vegetation cover in areas that had previously been bare. Moreover, sediment carried by flood water was deposited on and around the vegetated areas, and the low-water channel was incised, causing vertical development of river topography. In addition, the main channel width decreased in previously relatively wide sites, and secondary channels formed. The results of this study show that river rejuvenation by floods may decrease owing to systemic changes in the river system. Therefore, we concluded that the Naeseongcheon watershed was primed by human-induced changes, which made the river system more susceptible to changes in rainfall and discharge due to climate change. Furthermore, after the initial vegetation colonization, changes in nutrients and temperature created a positive feedback loop, which reinforced vegetation establishment.
In riparian forests, clear-cutting causes long-lasting changes in both riparian and aquatic biota. In this work, we examined if past clear-cutting events occurred at different times have imprints on riparian forests in a Mediterranean river in central Italy. We carried out a randomized, plot-based vegetation survey of riparian forests in systematically sampled 500 m-long sectors along the whole river, dividing the riparian zone into two internal and two external strips. From historical aerial photos, past clear-cutting events within plots were detected and classified in age classes: i) cut in the past 8 years (recent); ii) cut between 8 and 19 years ago (intermediate); iii) no signs of clear-cutting in the last 19 years (distant). We analyzed the responses of vegetation to clear-cutting and strip position. Alien species richness increased and woody species richness decreased in recently clear-cut areas compared to those with a distant clear-cutting event. Moreover, recently cut woods showed an increase in alien and synanthropic species. Intermediate clear-cut areas had higher levels of invasion by alien species compared to areas with distant cut. Riparian forests of internal strips are impacted by clear-cutting, but seem to recover in 8-19 years thanks to their natural resilience. Conversely, recent or intermediate clear-cutting events did not affect any of the investigated vegetation attributes in the external strips since such forests were already invaded by Robinia pseudoacacia after human disturbance. Our results confirm that clear-cutting events can have long-lasting effects on Mediterranean riparian forests, confirming the vulnerability of river ecosystems to clear-cutting and suggesting the need for more caution in management practices to improve the conservation status of riparian forests.
Most studies exploring land use impacts have focused on taxonomic metrics, but interest in the functional approach has increased because it helps to understand the relationships between community structure and functioning of aquatic ecosystems. We evaluated how functional and taxonomic approaches contribute to assessing the effects of land use on macroinvertebrate assemblages of lowland streams. We also studied the relationships between both approaches. We sampled benthic macroinvertebrates in 17 sites with different land uses (agricultural, peri-urban, and extensive livestock). We computed the taxonomic metrics and biotic indexes as well as functional richness (FRic), divergence (FDiv), dispersion (FDis), and Rao diversity indexes for each site. We performed general linear mixed models to compare land-uses and also performed correlation analysis between taxonomic and functional indexes. Taxonomic richness was significantly higher in extensive livestock than in the other two land uses, while Shannon diversity was significantly different between land uses (extensive livestock>peri-urban>agriculture). FRic and FDiv were significantly lower in peri-urban land use than in agricultural and extensive livestock sites. Only taxonomic richness showed a significant and positive relationship with FRic, FDis, and Rao, but they fit better to a logarithmic function. Therefore, an increase in taxonomic richness and Shannon diversity did not necessarily imply an increase in the functional aspects of the macroinvertebrate assemblage. Using only one of these approaches could lead to partial evaluations and loss of information. Combining them could improve bioindication and predictive potential and help assess the effects of multiple stressors on freshwater ecosystems to improve biomonitoring.