Lowland River Research Articles

A large-scale passage evaluation for multiple fish species: Lessons from 82 fishways in lowland rivers and brooks

River networks worldwide are highly fragmented and this is particularly prevalent in the human-dominated lowland landscape of the Netherlands. Fishways function as measures to facilitate passage alongside barriers for fish communities and have been evaluated mostly in single case studies. We compared fish monitoring data of 82 fishways with data of fish observed in adjacent water stretches, conducting the first nationwide study on passage for the full spectrum of fish species present. In total, 35 out of 38 (92%) native species recorded in fishways' surroundings used fishways, while per fishway the median was 59% related to a variety of factors (fish behaviour, fishway type, monitoring design). The species using fishways most frequently were perch Perca fluviatilis (71/78 of fishways present), roach Rutilus rutilus (70/79) and gudgeon Gobio gobio (68/77). Logistic regression models showed the effect of monitoring duration and timing in detecting specific fish species ascending fishways. This large-scale analysis highlights the need to consider all native fish species during the design and monitoring of fishways. The obtained information from this study can be used by water managers for improving monitoring schemes and river connectivity which is an essential component for achieving the European Water Framework Directive goals.

Drones in fish fauna assessment of rivers

The purpose of the study was to develop the method of using unmanned aerial vehicles (drones) to map fish fauna habitats in a large lowland river. The intention was to acquire and process spatial data to implement it in the Mesohabitat Simulation system (MesoHABSIM).At three different water levels, remote sensing data was acquired by unmanned aerial vehicles on the lower Vistula River section. In parallel, depths and velocities of flowing water were measured. Orthophotomosaics were created from the collected images, overlaid with depth and velocity observations to identify hydromorphological units. Maps of fish fauna habitats under different water levels were obtained. The application of the developed procedure algorithm proved successful in habitat mapping studies on a large, deep river with opaque water. The significance of using orthophotomosaics presenting the river under well-defined flow conditions was proven to achieve high accuracy in mapping habitat conditions and reduce errors in habitat usability analyses by fish community.The superiority of using unmanned aerial vehicles over other methods of collecting data on river habitats and depth and velocity measurements using the Acoustic Doppler Current Profiler (ADCP) method in large rivers with non-clarity water was confirmed. In addition, it was pointed out that drones provide high-resolution images under well-defined flow conditions, which are impossible to obtain using available cartographic materials. The validity of using the incomparably cheaper ADCP technology relative to laser technology, whose application in rivers with opaque water is very complicated, was also confirmed.

The patchy distribution of groundwater copepods in the lowland river valley

Our research provides valuable insights into the uneven distribution of groundwater copepods in the lowland river valley ecosystem using an integrative taxonomy approach. The study was conducted in 101 wells in the Biebrza River Valley (northeastern Poland), which is one of the largest and best-preserved lowland rivers in Europe. It stands out for its exceptional biodiversity and pristine natural landscapes. Groundwater copepods were found in 49 of the 101 analyzed wells. We identified ten species of Copepoda and eight species of Cladocera. The most frequent copepods were Diacyclops bicuspidatus, Diacyclops crassicaudis, Canthocamptus staphylinus, Paracyclops cf. fimbriatus, Diacyclops bisetosus, and Eucyclops serrulatus. The aforementioned species were categorized as stygophiles, and no stygobionts were detected. Our findings suggest that the groundwater Copepoda community in this region is relatively stable and composed mostly of stygophiles along with stygoxenes. We did not find any significant impact of environmental parameters or different aquifers on the distribution of copepods, suggesting a patchy distribution of groundwater copepods in the lowland river valley. The relatively high presence of stygoxenes suggests that the exchange of organisms between surface water and groundwater plays a vital role in maintaining the diversity of microcrustaceans in lowland river valleys. Our study contributes to filling the knowledge gap regarding groundwater fauna in lowland Europe, particularly in areas affected by Pleistocene glaciations.

Influence of Vertical Hydrologic Exchange Flow, Channel Flow, and Biogeochemical Kinetics on CH4 Emissions From Rivers

AbstractCH4 emissions from inland water are highly uncertain in the current global CH4 budget, especially for rivers and streams due to sparse measurements and the uncertainty of measurements caused by turbulent water flow. A previous study has revealed that vertical hydrologic exchange flow (VHEF) is the main regulator of CH4 emissions from riverbed sediments. However, to what extent the understanding obtained from the plot‐scale can be extended to the reach scale and basin scale remains unknown. To address this challenge, we developed a process‐based model to estimate CH4 flux at the air‐water interface using the attributes available in the national hydrography data set. It calculates the annual mean flux of VHEF, CH4 production in sediments, and CH4 transport in the river channel in a sequential manner. Model performance is evaluated by CH4 efflux observed at the Hanford reach of the Columbia River. We show that reach‐wise sediment hydrologic and biogeochemical conditions estimated from the national hydrography data set could serve as a good indicator of CH4 emissions from rivers. Aerobic methane oxidation and export to the downstream are the dominant ways of total CH4 loss for the large lowland river. The hotspots of CH4 emissions are likely to be at the reaches with fine sediments and slow channel velocity. This study demonstrates the possibility of quantifying CH4 emissions at the reach scale and the modeling framework has the potential to be extended to the basin scale to improve estimates of CH4 emissions from lotic inland water.

Identification of catchment areas with nitrogen pollution risk for lowland river water quality

Identification of catchment areas with nitrogen pollution risk for lowland river water quality

Evaluating indicators of hydrologic alteration to demonstrate the impact of open-pit lignite mining on the flow regimes of small and medium-sized rivers

Our study provides a retrospective evaluation of the perturbances of the opencast mining on the small river. Based on a standard 'before-after-control-impact' approach, we assess the impact of disturbances on aquatic ecosystems. For this purpose, we used IHA (Index of Hydrological Alteration) statistics to fully quantify the impact of open-pit mine on flow regimes of small-to-medium-size rivers. The analysis results in flow data captured in the 33 parameters of the range variability approach (RVA) and five ecological flow components (EFC). This approach helped us comprehensively evaluate the alteration in the flow regime in areas of open-pit mining on the example of the Noteć River, a typical lowland river. We conclude that the level integrated hydrologic alterations to this river under the influence of the groundwater depression cone and water from the mine dewatering are high, as assessed by the RVA and EFC methods. Specifically, this work uses the IHA framework to verify the necessity of including a specific group of indicators. Our analysis shows that for the post-impact period on the investigated river, the functioning of the river ecosystem was degraded. One of the EFC components completely disappeared at both water gauge stations. There were also profound hydrologic alterations, such as increased frequency or duration of “extreme low flows” at one station; at the second station, “high flow pulses” occurred more frequently. More than 65 % of the altered parameters in five IHA groups were modified drastically. In the river section affected by the groundwater depression cone, both the minimum and maximum flows decreased by 20 % to 55 %. In the section where water from the mine dewatering is fed in, the minimum flows changed by + 170 to + 230 %. The use of the IHA tool provides an opportunity for environmental flow restoration of water conditions in degraded post-mining areas.

Machine learning insights of anthropogenic and natural influences on riverbed deformation in a large lowland river

As escalating environmental pressures threaten the world's river systems, understanding the driving factors influencing their geomorphological changes is of critical global importance. This study illuminates the complex interaction between natural and anthropogenic factors that drove geomorphological changes along the upper Apalachicola River (UAR) from 1960 to 2010. This study utilized LiDAR point cloud data and hydrographic surveys conducted between 1960 and 2010 to identify the primary factors driving riverbed changes. A triangular irregular network (TIN) was used to convert the combined datasets from each survey, following coordinate transformation, into a 1.5 m-resolution digital elevation model (DEM) raster. The DEM of difference (DEMoD) was computed by subtracting the 1960 DEM from that of 2010, and the total sediment loss and gain were then determined by multiplying the DEMoD by the DEM resolution. A random forest (RF) regression model was used to determine the most influential factors contributing to riverbed deformation. The results revealed substantial differences in sediment loss and gain across the river. The UAR, divided into three segments, experienced sediment loss of about −6.56 × 106 m3 between 1960 and 2010, with the farthest upstream section (RM 106 to 98) losing −3.14 × 106 m3, which is ~48 % of the sediment losses in the study area. Some sections of the channel have deepened significantly, with a pronounced depth difference of −17.20 m between RM 92 and 94, potentially caused by the presence of limestone and the enlargement of a sinkhole or spring on the riverbed. The RF model identified the dam (>28 %) as the most crucial anthropogenic factor in riverbed deformation, followed by the presence of dikes (~17—18 %), then historical dredging (~7—8 %), highlighting the substantial impact of artificial structures on riverbed deformation. In contrast, rock removal, while potentially impactful in certain locations, had negligible effects (>0.5 %) at the system scale. This study highlights the interaction of natural and anthropogenic factors in river geomorphology and the potential for machine learning to identify these dynamics and inform data-driven river management and conservation strategies.

Impact of anthropogenic infrastructure on aquatic and avian predator–prey interactions in a modified lowland river

Abstract The relationship between aquatic and avian predators and prey is a fundamental process that influences the ecological dynamics of freshwater communities; a landscape of fear underpins spatial and temporal habitat use of prey, i.e., non‐consumptive predation effects. For example, complex marginal vegetation and other natural in‐river refuges are known to be important for prey to manage predation risk and predators must alter their behaviour in response to habitat patches prey occupy. However, it is unclear how prey respond to predators, and vice versa, in heavily modified degraded lowland rivers with a high degree of river maintenance measures; a component critical for flood risk management globally. Such modifications could lead to prey seeking refuge at hazardous anthropogenic infrastructure, but a robust quantification of predator–prey interactions in this context is required to develop this understanding. Using multi‐beam sonar (Dual‐Frequency Identification Sonar), we non‐invasively and simultaneously quantified the temporal rate of predator–prey interactions, the attack behaviour of predators, and the refuge seeking behaviour of prey at a pumping station intake during winter in a heavily modified lowland river. Prey fish experienced temporally dynamic, density‐dependant, and species‐specific predation risks from two dissimilar predators (i.e., aquatic vs. avian); pike (Esox lucius) and cormorant (Phalacrocorax carbo). Generalised linear modelling revealed that prey refuge use was positively associated with predator attack rate. Non‐consumptive effects were evidenced by quantified changes to shoal structure (density, area), shoaling (group aggregation), and schooling (coordinated directional movement), including diurnal migrations to and from the pumping station intake for refuge. Our results show that in the absence of natural refuge habitats the natural landscape of fear shifted and speculate that prey fish were paradoxically dependant on hazardous anthropogenic infrastructure, i.e., a pumping station intake, for refuge from predators in a degraded lowland river. These findings strongly enhance our understanding of the impact of anthropogenic infrastructure on predator–prey interactions by demonstrating how flood risk management, including river maintenance measures, and associated anthropogenic infrastructure can impact the behavioural game played between aquatic and avian predators and their prey.

The Use of UAV for Measuring the Morphology of Ice Cover on the Surface of a River: A Case Study of the Low Head Dam and Fishway Inlet Area in the Odra River

The application of UAV to acquire data on the morphometry of frazil ice floe in motion is demonstrated in the measurements conducted in the area of the Wrocław Water Junction at the Opatowice weir on the Odra River (Poland). Image processing was performed using open-source software dedicated to image analysis. The methodology presented in the publication offers a cost-effective and low-overhead technique for describing ice phenomena in lowland rivers. The focus of the methodology was on measuring the area, average size, perimeter, and circularity of frazil ice floe. The measurements were carried out for individual frames captured by a UAV, and the results were analyzed using statistical techniques. In prior research, the team effectively assessed ice velocity on an identical test sample. Deriving the average velocity, surface area, and fundamental morphological traits of frazil ice facilitates the automated segmentation, classification, and prediction of potential risks related to ice blockages on water routes. These risks encompass potential waterway obstructions, as well as infrastructure impairments, and may pose a danger to human safety.

Flow resistance at lowland and mountainous rivers

Abstract This study initially examines the various sources of flow resistance in sand-bed (lowland) and gravel-bed (mountainous) rivers along with the limitations of traditional estimation methods. The nondimensional hydraulic geometry approach, relating dimensionless flow discharge (q *) to the Darcy-Weisbach friction factor (f), has demonstrated good performance for both river types, covering shallow to moderately deep flows. However, accuracy in estimating f is affected by simplifications like assuming uniform and deep flow, neglecting bed load transport and vegetation effects, which require further evaluation. To address these issues, the proposed method is evaluated using data from four sand-bed rivers in Slovakia (with vegetation), and three gravel-bed rivers in Iran (dominated by cobbles and boulders). Bedforms prove to be significant resistance sources in all studied rivers. The approach yields separate predictors for each river type, showing a satisfactory agreement between observed and calculated values within a maximum deviation of ±20% error bands. These predictors are further validated using field data and established equations from rivers with similar physiographic characteristics. Results indicate the method performs well in predicting flow resistance in sand-bed rivers, slightly overestimating overall (+40%). It effectively captures riverbed features and vegetation influence under small-scale roughness conditions. However, the predictor’s validity for gravel-bed rivers is somewhat limited due to high variability in water-surface profiles, making it challenging to accurately capture flow dynamics under large-scale roughness conditions. Addressing complex characteristics of gravel-bed riverbeds, including boulders and local energy extraction, is crucial for improving the estimation of water-surface profile variations and flow resistance using the hydraulic geometry approach.

Characterizing nitrogen dynamics and their response to sediment dredging in a lowland rural river

It is challenging to quantify nitrogen (N) balance in a lowland rural river covered by macrophytes due to its multiple N sources and complex cycling processes. In order to address the challenge, we developed a N-cycling model (River-N) for lowland rural rivers covered by macrophytes, and coupled it with Nitrogen Dynamic Polder (NDP) and Environmental Fluid Dynamics Code (EFDC) model. A two-year (2020–2021) dataset was used to calibrate and validate the coupled model. The coupled model was used to investigate a lowland rural river’s N imports, exports and its response to sediment dredging, and resulted in an acceptable performance for total N concentrations (TN) (R2 = 0.57). Our investigation conducted in the study river revealed a considerable N retention (difference between imports and exports) ability (955.5 kg yr−1). The river’s N imports were mainly from connected-river import (55.6 %), surrounding-polder export (31.4 %), sediment N release (7.4 %). N exports were mainly from connected-river export (59.0 %), denitrification (31.7 %) and macrophyte uptake (4.4 %). Compared with scenario without dredging, the rates of particulate nitrogen (PN) resuspension and settling increased, and the rates of ammonia nitrogen (NH4+) release, denitrification and macrophyte uptake decreased under the scenario with dredging. During dredging period, the rates of NH4+ release, PN resuspension, PN settling were 2.6, 7.0 and 4.3 times higher than that during pre-dredging period, respectively. The denitrification rate during dredging period was decreased to 43.5 % of that during pre-dredging period. Our investigation results implied that sediment dredging can be a pathway to remove high-N sediment, but did not necessarily decrease TN in river due to external N imports. Considering the critical role of macrophytes in N-cycling, macrophytes should be a critical component requiring our attention in water management practices.

Spatial variation in the recovery potential of freshwater macroinvertebrate assemblages: Moving towards spatially defined assemblage vulnerability to chemicals

The vulnerability of freshwater biodiversity to chemical stressors is dependent on its ability to resist chemical stress and recover from any stress-induced effects. Spatial variation in recovery has the potential to exacerbate or mitigate assemblage vulnerability but this has not been explored in detail. By combining information on assemblage-specific recovery potential with information on assemblage-specific chemical sensitivity, we have demonstrated that the vulnerability of 3307 macroinvertebrate assemblages to 18 different chemicals is spatially dependent and that recovery potential may reduce chemical risk. The recovery potential of each assemblage was quantified based on trait information and landscape factors using a weighted sum method, but it did not consider succession processes. Recovery potential varied by river type with assemblages in mid-altitude siliceous rivers with small catchments in the west of England having the lowest recovery potential. For 17 or the 18 chemicals investigated, there was a positive correlation between the recovery potential and sensitivity and this was strongest for assemblages exposed to metals. More sensitive assemblages had a higher recovery potential and were therefore potentially less vulnerable than would be expected based on sensitivity alone. Assemblages in rivers with small catchments were the most vulnerable to chemical exposure. Furthermore, assemblages with high vulnerability to insecticide exposure were more prevalent in mid-altitude rivers with siliceous geology in the west of England, whereas assemblages with high vulnerability to metals were more prevalent in lowland rivers with calcareous or mixed geology in the midlands. This study: (i) highlights the importance of spatial context in determining the risk of chemical pollution to freshwater biodiversity; (ii) demonstrates how spatial variation in taxonomic composition influences both the internal and external recovery of assemblages and how landscape factors modify trait-based recovery capabilities; (iii) provides the foundations for spatially-defined vulnerability assessment by identifying ecological scenarios for assessing chemical risk.

Water level prediction using long short-term memory neural network model for a lowland river: a case study on the Tisza River, Central Europe

BackgroundPrecisely predicting the water levels of rivers is critical for planning and supporting flood hazard and risk assessments and maintaining navigation, irrigation, and water withdrawal for urban areas and industry. In Hungary, the water level of rivers has been recorded since the early nineteenth century, and various water level prediction methods were developed. The Discrete Linear Cascade Model (DLCM) has been used since 1980s. However, its performance is not always reliable under the current climate-driven hydrological changes. Therefore, we aimed to test machine learning algorithms to make 7-day ahead forecasts, choose the best-performing model, and compare it with the actual DLCM.ResultsAccording to the results, the Long Short-Term Memory (LSTM) model provided the best results in all time horizons, giving more precise predictions than the Baseline model, the Linear or Multilayer Perceptron Model. Despite underestimating water levels, the validation of the LSTM model revealed that 68.5‒76.1% of predictions fall within the required precision intervals. Predictions were relatively accurate for low (≤ 239 cm) and flood stages (≥ 650 cm), but became less reliable for medium stages (240–649 cm).ConclusionsThe LSTM model provided better results in all hydrological situations than the DLCM. Though, LSTM is not a novel concept, its encoder–decoder architecture is the best option for solving multi-horizon forecasting problems (or “Many-to-Many” problems), and it can be trained effectively on vast volumes of data. Thus, we recommend testing the LSTM model in similar hydrological conditions (e.g., lowland, medium-sized river with low slope and mobile channel) to get reliable water level forecasts under the rapidly changing climate and various human impacts.Graphical

Trophic relationships of translocated and indigenous chub Squalius cephalus populations with trophically analogous fishes

The translocation of non-indigenous fishes into lowland rives can result in invasive populations establishing and dispersing. Because non-indigenous fishes can cause ecological harm, it is important to understand their trophic relationships and the effects they may have on native fishes. We assessed the trophic ecology of a translocated chub Squalius cephalus population in the River Frome, a lowland chalk stream in Southern England, using bulk stable isotope (δ13C, δ15N) metrics, and compared the results with those derived from indigenous chub populations. The isotopic niche sizes of all fishes in the River Frome were substantially larger in the tidal versus non-tidal site, with the δ13C values suggesting some fish were foraging further downstream of their sampling point in areas that had greater tidal and salinity influences. Inter-specific comparisons of isotopic niches revealed a consistent pattern of similar niche size and overlap between chub and the trophically analogous dace Leuciscus leuciscus. These isotopic relationships between chub and dace were then also apparent in the indigenous populations of these fishes. These results suggest that the colonisation of this lowland river by translocated chub is being facilitated by their isotopic relationships with other fishes that are similar to those observed in their native range.

F28: A novel coupling strategy for 1D/2D hydraulic models for flood risk assessment of the Mekong Delta

Abstract Coupling models of different dimensions is one of the most important yet under-represented challenges. This paper introduces a new modeling strategy to streamline a more flexible and effective integrated one-dimensional (1D)/two-dimensional (2D) model for floodplains along lowland rivers. The 1D model, utilizing the finite volume method, solves the Saint–Venant equations, while the 2D mesh employs unstructured quadrilateral elements. The two strategies couple the 1D/2D models: direct 1D/2D connection by the law of mass conservation at supernode, and lateral 1D/2D model connection by spillways at riverbank. The coupling strategy in F28 guarantees the water balance and the conservation of momentum at the integrated 1D/2D nodes. The model was applied to the Mekong Delta to address the capacity of hydrodynamic simulations integrating various water infrastructures. Results showed that the developed model has a strong potential to be applied to other lowland rivers worldwide with complex infrastructures.

Flow event size influences carbon, nutrient and zooplankton dynamics in a highly regulated lowland river

AbstractRiver regulation and water extraction has significantly altered flow regimes and reduced flood events in many inland river systems. Environmental flows have been adopted in many systems to mitigate the ecological impacts of river regulation, however a lack of knowledge regarding the interrelationship between flow regimes, carbon transport and instream productivity make prioritising water management difficult. To address this knowledge gap, we conducted a study on the Namoi River in the Murray-Darling Basin, Australia, monitoring changes in dissolved organic carbon (DOC), nutrient dynamics and planktonic food web structure during a period of variable flows. Nutrient and DOC concentrations were positively correlated with river discharge and zooplankton concentrations were highest post flow events. Planktonic chlorophyll-a, increased DOC concentration and higher discharge were the most influential drivers of change in zooplankton communities. Further, our results indicated that flow events increased production through both heterotrophic and autotrophic pathways, significantly boosting zooplankton concentration compared to base flow conditions across all measured flow events. We suggest even small in-channel flow events can be important for increasing basal and zooplankton production in rivers, and therefore should be protected or promoted by environmental flow management, particularly during drought conditions.

Checklist, distribution, diversity, and rarity of mayflies (Ephemeroptera) in Slovakia.

Despite the essential role of mayflies (Ephemeroptera) in freshwater ecosystems and their long-term use in research and routine biomonitoring in the Carpathian and Pannonian ecoregions, their distribution data are fragmentary and outdated. All published and unpublished data on mayflies from Slovakia was gathered and a database of > 15,000 species records from 2206 localities built with the aims (i) to critically revise available data and assess the completeness of the species inventory, (ii) to identify hotspots of species diversity, and (iii) to provide a benchmark for assessment of species rarity and conservation status in the region. After the critical revision of the data covering more than 100 years, the occurrence of 109 mayfly species in Slovakia was confirmed. The species inventory appears to be nearly complete, as evidenced by the rarefaction curve and a nonparametric species richness estimator. The highest mayfly gamma diversity was recorded below 500 m a.s.l. and in streams of the fifth order, which can be considered hotspots of mayfly diversity in the region. Six species were last recorded before 1990 and thus can be considered extinct in Slovakia. Twenty-nine species could be classified as very rare, with their occurrence frequency decreasing with increasing altitude and most of them being restricted to large lowland rivers and stagnant water habitats in their floodplains. In conclusion, our study provides comprehensive data on key freshwater bioindicators and suggests increasing conservation priorities, especially in lowland river floodplains occupied by several very rare mayfly species.

River Habitat Survey: Does This Help to Explain the Nature of Water Mite (Acari and Hydrachnidia) Assemblages?

In the European Union, assessments of the quality of the aquatic environment based on aquatic invertebrates are mandatory. Biological methods are supplemented with hydromorphological assessments of watercourses. There are many studies analysing the relationships between aquatic invertebrates and the hydromorphological assessment of the environment by the River Habitat Survey (RHS) method, but thus far, there has been no detailed study including water mites (Acari, Hydrachnidia) and the application of this method. In the present study, the following research hypothesis was put forth: a hydromorphological characterization of habitats is a significant element explaining the nature of water mite communities, and the RHS method can be used to predict the characteristics of Hydrachnidia populations in a river. The research was carried out in a small lowland river, the River Krąpiel (north-western Poland). Six locations were selected as representative of some state of habitat modification for the assessment of the hydromorphological conditions of the river and the collection of biological samples. The following conclusions can be drawn from the research: (1) the biology and ecology of water mites make them suitable as bioindicators of the environment, including hydromorphological modifications, and (2) the hydromorphological characteristics of habitats explain the nature of water mite communities in the river at the level of general population parameters (number of specimens and species), while at the species level, general regularities in water mite fauna distribution in river ecosystems, the continuity of the river ecosystem, and characteristics at a smaller spatial scale (habitat scale) better explain water mite community structure than the hydromorphological indices determined for a given site or section of the river.

Zooplankton as an indicator of hydrological connectivity of the main channel and the floodplain in a large river system

AbstractThe connectivity of the river–floodplain system during floods plays a decisive role in organisms/matter contribution to the ecosystems of large lowland rivers. We hypothesized that the direction of water-level trends during flood was crucial for zooplankton release from the floodplain into the river channel. The seven-year study of zooplankton in the large plain Ob river suggest that at the inundated floodplain even a slight water level fall brings to plankton release from the floodplain into the river thus causing its abundance boom in the main channel. At water-level stabilization or rise, zooplankton communities of floodplain become isolated from the channel that ceases the recruits’ inflow from the floodplain and brings to a sharp drop in number of river zooplankton. Therefore, it is necessary to take into account dynamics trends (i.e., fall or rise) in river water level when assessing zooplankton density in large lowland rivers. The analysis of such trends should become the basis for predicting of river zooplankton abundance and assessing changes in feeding conditions of fish during the growing season.

Habitat aging and degradation in terrestrialized floodplains: a need to rejuvenate processes for sustaining freshwater mussel populations

Progressive floodplain terrestrialization leads to the deterioration of floodplain ecosystems. Although the importance of hydrological connectivity between floodplain habitats and river channels for floodplain‐dependent organisms has been emphasized, less attention has been paid to the temporal relationship between habitat degradation and aquatic organisms. In this study, we examined temporal changes in mussel population structures and the hydrological connectivity of pond‐like floodplain water bodies (FWBs) in the terrestrialized floodplains of a lowland river in Japan. We tested the hypothesis that FWB habitats for mussels degrade over time, while newly formed FWBs contribute to providing recruitment sites for mussels. The habitat age for 53 FWBs was determined as of 2018, using historical aerial photographs. Inundation frequency, mussel abundance, and size structures (shell length) were also investigated in the FWBs in 2007 and 2018. Mussel abundance decreased with FWB aging (6–36 years), which coincided with a decrease in inundation frequency. In most existing FWBs, mussel abundance in all size classes decreased over the decade (2007–2018). In contrast, small individuals occurred in the newly formed FWBs at age 6, and in the existing FWBs where inundation frequency slightly increased over the decade (2007–2018), such FWBs were rare. In conclusion, the “young” and “rejuvenated” FWBs were found to be critically important for the recruitment and persistence of mussel populations. This suggests that the floodplain management strategy of “cyclic floodplain rejuvenation” could be applicable for mussel conservation in terrestrialized floodplains of lowland rivers.