Small run-of-river hydropower dams and associated water regulation filter benthic diatom traits and affect functional diversity

Habitat preference drives the community composition, beta diversity and assembly processes of benthic diatoms: A case study of a wetland cluster in a cold region

The loss of biodiversity in urban wetlands has become increasingly severe due to urbanization. Based on their attachment patterns, benthic diatoms can be divided into several types according to habitat (i.e., epilithic, epiphytic, and epipelic). Diatoms are often used as ecological indicators due to their sensitivity to environmental changes. However, details concerning the composition, temporal dynamics, and assembly mechanisms of benthic diatom communities in urban wetland habitats remain unclear. In this study, we systematically evaluated the composition and seasonal dynamics of benthic diatoms among three different habitats and validated the applicability of eDNA for studies of diatoms. The relationship between the taxonomic and functional diversity of diatom communities was examined, and the assembly mechanisms of diatom communities were explored. An analysis of 249 benthic diatom samples and 27 water samples from nine sites over eight months revealed differences in the composition of diatom communities among epilithic, epiphytic, and epipelic habitats. In diatoms from different hydrological periods, the contribution of turnover to total taxonomic beta diversity was much larger than that of nestedness, while the nestedness components of functional diversity were comparatively large. Nutrients, total dissolved solids, and water temperature were the main variables that affected the taxonomic and functional beta diversity of the benthic diatom communities. The results suggested that benthic diatom community assembly in the three habitats and eDNA was primarily driven by deterministic processes. This study has demonstrated how habitat preference affects the composition, beta diversity and assembly processes of benthic diatom communities and provided novel insights into the conservation of biodiversity in urban wetlands.

Ruin-of-the-rivers? A global review of run-of-the-river dams

The classification of a hydropower scheme as run-of-the-river (or run-of-river; ROR) evokes an image of a low-impact installation; however, examination of eight case studies worldwide shows that substantial negative societal and ecological impacts are tied to them, albeit in somewhat different ways. We conclude that ROR dams not only potentially displace communities, disrupt livelihoods, and degrade environments in surrounding areas, but they also divert water from areas of need, impact aquatic ecology through habitat destruction and disruption of fish migrations, emit non-trivial amounts of greenhouse gases over the lifespan of the project, and disrupt streamflow in downstream river sections. While these negative impacts vary on a case-by-case basis, medium and large ROR dams consistently have multiple and cumulative impacts, even when not having appreciable reservoirs. We contend that many impactful dams do not qualify as low-impact ROR projects, despite being defined as such. Such mislabeling is facilitated in part by the ambiguous definition of the term, which risks the ROR concept being used by proponents of impactful structures to downplay their negative effects and thus mislead the public or gain status, including within the Clean Development Mechanism in relation to mitigating climate change.

Differential impacts of small hydropower plants on macroinvertebrate communities upstream and downstream under ecological flow

Hydropower dams influence freshwater biodiversity by altering river flow patterns and habitat conditions. With the global surge in small hydropower plants (SHPs), their impacts on aquatic ecosystems have become increasingly recognized. However, most previous studies did not consider the recently implemented ecological flows. Consequently, the effects of SHPs under ecological flow conditions on aquatic organisms, such as macroinvertebrate communities, remain unclear. We surveyed 15 SHPs in the Oujiang region, establishing sampling sites upstream of the intake dams (S1), in dam-induced reservoirs (S2), in dewatered sections downstream of the dams with ecological flows (S3), and in sections with restored natural flow (S4). By comparing macroinvertebrate community composition, diversity, functional feeding groups, and network structures in these areas, we assessed the ecological response of macroinvertebrates to SHPs under ecological flows. Our research found that SHPs significantly impact macroinvertebrate communities. Specifically, at site S2, stagnant water species replaced those typically found in flowing conditions, resulting in a marked difference in species composition between S2 and other sites. Compared to S1 and S4, diversity indices at S2 and S3 were lower, with filterers and collectors dominating the functional feeding groups at S2 and S3. Co-occurrence network analysis revealed that network complexity at S2 and S3 was lower than at S1 and S4. Additionally, S3 was less affected by SHPs than S2, underscoring the importance of ecological flow replenishment. Overall, our research confirmed the remarkable influence of SHPs on S2 macroinvertebrate community, and emphasized the importance of maintaining sufficient ecological flow to the downstream aquatic organism of S3 reach. We suggest a comprehensive assessment of the potential environmental impacts of SHPs, particularly the negative effects caused by insufficient ecological flow, to ensure the sustainable development of ecosystems.

Small hydropower plants affect aquatic community diversity: A longitudinal study under ecological flow

Small hydropower plants (SHPs) play a crucial role in clean energy production, yet they also disrupted river ecosystems. To achieve a balance between energy production, biodiversity conservation, and ecosystem integrity, it is essential to study how aquatic organisms respond to SHP operations. Prior researches had shown that SHP operations have the most significant impact in dewatering sections, but studies often overlook the influence of ecological flows. Therefore, our study focused on the Oujiang river basin, where SHPs are prevalent, to investigate the effects of SHP operations on riverine algae under ecological flow and conditions of intensive exploitation. We compared species composition, traits composition, and diversity indices across different river sections and used multiple linear regression models to identify the main drivers influencing algal communities. The results showed: 1) SHP operations significantly altered hydrological and physicochemical conditions in reservoir sections, leading to distinct differences in algal community composition and traits; 2) physicochemical factors had the greatest influence on diversity indices, driving the observed patterns; and 3) SHP operations indirectly affected algal communities through interspecific interactions, particularly with macroinvertebrates like Scrapers. These findings emphasize the need for stronger governance and enforcement to ensure adequate ecological flow releases by SHPs, especially under growing environmental and climate challenges.

Size-dependent effects of dams on river ecosystems and implications for dam removal outcomes

Understanding the relationship between a dam's size and its ecological effects is important for prioritization of river restoration efforts based on dam removal. Although much is known about the effects of large storage dams, this information may not be applicable to small dams, which represent the vast majority of dams being considered for removal. To better understand how dam effects vary with size, we conducted a multidisciplinary study of the downstream effect of dams on a range of ecological characteristics including geomorphology, water chemistry, periphyton, riparian vegetation, benthic macroinvertebrates, and fish. We related dam size variables to the downstream-upstream fractional difference in measured ecological characteristics for 16 dams in the mid-Atlantic region ranging from 0.9 to 57 m high, with hydraulic residence times (HRTs) ranging from 30 min to 1.5 years. For a range of physical attributes, larger dams had larger effects. For example, the water surface width below dams was greater below large dams. By contrast, there was no effect of dam size on sediment grain size, though the fraction of fine-grained bed material was lower below dams independently of dam size. Larger dams tended to reduce water quality more, with decreased downstream dissolved oxygen and increased temperature. Larger dams decreased inorganic nutrients (N, P, Si), but increased particulate nutrients (N, P) in downstream reaches. Aquatic organisms tended to have greater dissimilarity in species composition below larger dams (for fish and periphyton), lower taxonomic diversity (for macroinvertebrates), and greater pollution tolerance (for periphyton and macroinvertebrates). Plants responded differently below large and small dams, with fewer invasive species below large dams, but more below small dams. Overall, these results demonstrate that larger dams have much greater impact on the ecosystem components we measured, and hence their removal has the greatest potential for restoring river ecosystems.

Existing levels of biodiversity and river location may determine changes from small hydropower developments

Global ecosystems are facing anthropogenic threats that affect their ecological functions and biodiversity. However, we still lack an understanding of how biodiversity can mediate the responses of ecosystems or communities to human disturbance across spatial gradients. Here, we examined how existing, spatial patterns of biodiversity influence the ecological effects of small hydropower plants (SHPs) on macroinvertebrates in river ecosystems. This study found that levels of biodiversity (e.g., number of species) can influence the degrees of its alterations by SHPs occurring along elevational gradients. The results of the study reveal that the construction of SHPs has various effects on biodiversity. For example, low-altitude areas with low biodiversity (species richness less than 12) showed a small increase in biodiversity compared to high-altitude areas (species richness more than 12) under SHP disturbances. The increases in the effective habitat area of the river segment could be a driver of the enhanced biodiversity in response to SHP effects. Changes in the numerically dominant species contributed to the overall level of community variation from disturbances. Location-specific strategies may mitigate the effects of SHPs and perhaps other disturbances.

Dam-induced flow alternations drive the regime shift towards a cyanobacteria-dominated microbiota state in the Yangtze River

While satisfying the demands of social and economic development, dams act as physical barriers affecting both abiotic and biotic factors in large rivers. These altered factors can interact with each other and gradually reshape the local ecosystem state. The reshaped state may spread downstream and affect ecosystem states on a large scale. However, the spread extent and characteristics of ecosystem states along large rivers remain understudied. To address this problem, alternative microbiota states and their responses to environmental conditions in the Yangtze River were investigated, considering the preponderance of alternative stable states theory in explaining the response of ecosystem states as well as the role of benthic microorganisms in indicating ecosystem states. In this study, flow discharge was identified as the main hydrological factor that clustered benthic microbiota into two types, and these two microbiota types were bistable and characterized by differential enrichment of the Cyanobacteria phylum. Potential analysis demonstrated that reducing flow discharge beneath a threshold (i.e., flow discharge < 12,900 m3/s) could shift benthic microbiotas to a state where benthic cyanobacteria would become the dominant species (the Microbiota State B). In the bistable region (i.e., 12,900 < flow discharge < 28,000 m3/s), both the ecological resilience and the contribution of deterministic process were found weak by relative potential depth calculations and neutral community modeling, suggesting that this region is susceptible to the microbiota state of its upstream and thus deserves more scientific attention to prevent the unfavorable state from spreading downstream. In addition, high denitrification potential at sites of the Microbiota State B was likely responsible for the low N:P ratio, further benefiting the dominance of N-fixing cyanobacteria. This study empirically showed the response of alternative microbiota states to flow gradients, and explored the distribution and characteristics of the microbiota states along the mainstream of the Yangtze River, therefore providing insights into environmental flow design and reservoir regulation of large rivers.

The Global Trend of Microplastic Research in Freshwater Ecosystems

The study of microplastics and their impact on aquatic ecosystems has received increasing attention in recent years. Drawing from an analysis of 814 papers related to microplastics published between 2013 and 2022 in the Web of Science Core Repository, this paper explores trends, focal points, and national collaborations in freshwater microplastics research, providing valuable insights for future studies. The findings reveal three distinct stages of microplastics: nascent development (2013-2015), slow rise (2016-2018), and rapid development (2019-2022). Over time, the focus of research has shifted from "surface", "effect", "microplastic pollution", and "tributary" to "toxicity", "species", "organism", "threat", "risk", and "ingestion". While international cooperation has become more prevalent, the extent of collaboration remains limited, mostly concentrated among English-speaking countries or English and Spanish/Portuguese-speaking countries. Future research directions should encompass the bi-directional relationship between microplastics and watershed ecosystems, incorporating chemical and toxicological approaches. Long-term monitoring efforts are crucial to assessing the sustained impacts of microplastics.

Responses of macroinvertebrate functional trait structure to river damming: From within-river to basin-scale patterns

Revealing how aquatic organisms respond to dam impacts is essential for river biomonitoring and management. Traditional examinations of dam impacts on macroinvertebrate assemblages were frequently conducted within single rivers (i.e., between upstream vs. downstream locations) and based on taxonomic identities but have rarely been expanded to level of entire basins (i.e., between dammed vs. undammed rivers) and from a functional trait perspective. Here, we evaluated the effects of dams on macroinvertebrate assemblages at both the within-river and basin scales using functional traits in two comparable tropical tributaries of the Lancang-Mekong River. At different scales, maximum body size, functional feeding groups (FFG), voltinism and occurrence in drift respond significantly to dam impact. Armoring categories varied significantly between downstream sites and upstream sites, and oviposition behavior, habits and adult life span significantly differed between rivers. The key traits at the within-river scale resembled to those at the between-river scale, suggesting that within-river trait variation could further shape functional trait structure at the basin scale in dammed rivers. Furthermore, water nutrients and habitat quality induced by dams showed the most important role in shaping trait structure, although trait-environment relationships varied between the two different scales. In addition, the trait-environment relationships were stronger in the dry season than in the wet season, suggesting a more important role of environmental filtering processes in the dry season compared with the wet season. This study highlights the utility of the trait-based approach to diagnose the effects of damming and emphasizes the importance of spatial scale to examine dam impacts in riverine systems.

Reducing the Risk of Benthic Algae Outbreaks by Regulating the Flow Velocity in a Simulated South-North Water Diversion Open Channel

The reduction in open-channel flow velocity due to China's South-to-North Water Diversion Project (SNP) increases the risk of benthic algal community blooms resulting in drinking water safety issues. Consequently, it has attracted attention from all walks of life. However, regulatory measures to mitigate the risk of algal blooms and the main risk-causing factors are unclear. This study simulated the river ecosystem of the SNP channel through water diversion. Simulated gradient-increasing river flow velocity affects environmental factors and benthic algal alterations, and can be used to explore the feasibility of regulating the flow velocity to reduce the risk of algal blooms. We found that the algal biomasses in the velocity environments of 0.211 and 0.418 m/s decreased by 30.19% and 39.88%, respectively. Community structure alterations from diatoms to filamentous green algae were 75.56% and 87.53%, respectively. We observed significant differences in biodiversity, especially in terms of richness and evenness. The α diversity index of a species is influenced by physical and chemical environmental factors (especially flow velocity). Our study revealed that flow velocity is the main factor affecting the growth and outbreak of benthic algae. The risk of algal blooms in open channels can be effectively mitigated by regulating the flow velocity. This provides a theoretical basis for ensuring the water safety of large-scale water conservancy projects.

Development of a site selection methodology for run-of-river hydroelectric power plants within the water-energy-ecosystem nexus

Even though water-energy systems and ecosystems have complex relationships, regional energy policies seldom consider this connection. The current regional energy policies are developed primarily based on technical and socio-economical aspects and lack ecological considerations that may deteriorate the ecosystems and sustainability of these policies. Hence, considering the water-energy-ecosystem nexus in the development stage of the energy policies can aid decision-makers in initiating successful multi-dimensional energy policies. This study proposes a novel approach to develop an environmental index to support the regional energy policies that rely on Run-of-river (RoR) hydroelectric power plants (HEPPs) with a nexus approach. First, significant environmental impacts of RoR HEPPs are identified as environmental flows, impacts on ecologically valuable areas such as heritage sites, wetlands, national parks, river water quality degradation, modification of habitat, and impact on ecosystems and biodiversity loss. Then these impacts are represented through indicators and are aggregated into an overall environmental performance index through a fuzzy multi-criteria decision-making approach. The proposed approach allows the integration of ecological dimensions into the evaluation of RoR HEPPs through easy-to-measure indicators, among which environmental flow is a critical component.

Dam-induced flow velocity decrease leads to the transition from heterotrophic to autotrophic system through modifying microbial food web dynamics

The impoundment of reservoirs changes the river from a riverine heterotrophic system to a lacustrine autotrophic system, which could be attributed to the shift of pelagic microbial food webs in response to the dam-induced disturbances. However, little is known about what is the key factor controlling this variation and how different underlying interactions affect the food web dynamics. This study investigated the effects of flow velocity and nutrient supply on microbial plankton using a microcosm experiment. The results showed that flow velocity decrease was the main factor inducing the detritus-based food web transformed to the autotroph-based food web, with heterotrophic bacteria and protozoan dominated at high velocity, whereas phytoplankton and metazoan were prevalent in the lentic environment. The lentic-acclimated genera, such as Chlorella sp., Mallomonas sp. and Microcystis sp., showed hysteresis after the velocity recovery, suggesting the potential of algae bloom in reservoirs and even downstream of dams. We further conducted a flow-velocity manipulating experiment and constructed a multi-trophic nitrogen cycling model to provide a mechanistic explanation for the microbial food web dynamics and the nitrogen transformation performances. As indicated in model prediction and sensitivity analysis, the abiotic and biotic variations were directly or indirectly controlled by nutrient utilization and predator-prey interactions. Quantification of these bottom-up and top-down forces revealed the buffer role of predators in mitigating the positive effects of nutrient availability on autotrophs at low velocity and on heterotrophs at high velocity, respectively. This study highlights the importance of mastering the whole information of different trophic levels, in order to better capture the complex microbial food web interactions and the consequent biogeochemical processes in river-reservoir systems.

Small Run-of-River Dams Affect Taxonomic and Functional β-Diversity, Community Assembly Process of Benthic Diatoms

Being increasingly constructed worldwide, dams are a main driver of flow regime change and biodiversity decline. Although small run-of-river dams have exceeded the number of large dams, their impacts on taxonomic and functional β-diversity as well as community assembly process of aquatic organisms have been largely neglected. Ninety sites within twenty three small run-of-river dams in the Xiangxi River were selected, and the hydrological and physicochemical variables for each site were measured. We analyzed the traits and β-diversity of benthic diatoms, and explored the key driving mechanism of benthic diatom community assembly. Our results indicated that the construction of small run-of-river dams could affect the β-diversity of benthic diatoms and the mechanism of community assembly. Specifically, we found that small run-of-river dams could change the relative contribution of nestedness components to the trait-based β-diversity of benthic diatoms, but generally the taxonomy-based β-diversity was relatively higher than the trait-based β-diversity. Furthermore, the community assembly process of benthic diatoms was also affected. In areas affected directly by small run-of-river dams, dispersal assembly was the key mechanism for community assembly. Compared to unregulated habitats, the dispersal assembly process between the impacted and the unregulated habitats has been enhanced. We advocate that this study can be expanded to other organisms (such as macroinvertebrates, phytoplankton, fish) in future to fully understand impacts of small run-of-river dams on biodiversity from a multi-trophic level aspect. Based on our results, we suggest that maintaining genetic and ecological connectivity based on an effective impact assessment in dry seasons is a potential solution to mitigate the impacts of such dams, as key to adaptive management and sustainability.