Large Floodplain Research Articles

Freshwater protists: unveiling the unexplored in a large floodplain system.

Protists play a fundamental role in all ecosystems, but we are still far from estimating the total diversity of many lineages, in particular in highly diverse environments, such as freshwater. Here, we survey the protist diversity of the Paraná River using metabarcoding, and we applied an approach that includes sequence similarity and phylogeny to evaluate the degree of genetic novelty of the protists' communities against the sequences described in the reference database PR2 . We observed that ~28% of the amplicon sequence variants were classified as novel according to their similarity with sequences from the reference database; most of them were related to heterotrophic groups traditionally overlooked in freshwater systems. This lack of knowledge extended to those groups within the green algae (Archaeplastida) that are well documented such as Mamiellophyceae, and also to the less studied Pedinophyceae, for which we found sequences representing novel deep-branching clusters. Among the groups with potential novel protists, Bicosoecida (Stramenopiles) were the best represented, followed by Codosiga (Opisthokonta), and the Perkinsea (Alveolata). This illustrates the lack of knowledge on freshwater planktonic protists and also the need for isolation and/or cultivation of new organisms to better understand their role in ecosystem functioning.

Longitudinal drifting pattern of larval assemblages in the lower reach of the Yangtze River: Impact of the floodplain lakes and conservation implementation

AbstractFloodplain lakes are significant lateral habitats that play an important role in structuring larval fish assemblages in large river–floodplain ecosystems. The abundance of floodplain lakes, including both naturally connected and main stem‐isolated lakes, declines in a downstream direction in the lower reach of the Yangtze River, forming a longitudinal reduction gradient. We investigated whether this longitudinal reduction gradient of floodplain lakes is an essential driver of the larval fish assemblage patterns in the river. Drifting larval fish were investigated weekly during the prevailing spawning season both in 2014 and 2015 among three locations at Wangjiang (WJ), Zhenjiang (ZJ) and Nantong (NT). Thirty‐four species belonging to 28 genera and eight families were identified. The number and abundances of collected species, the reproduction duration for dominant species and the niche breadth of dominant species all exhibit a longitudinal decrease (WJ > ZJ > NT) in the downstream direction. Additionally, the bloom of larval fish tended to occur earlier at WJ than at ZJ and NT. We suggest that these results reflect the crucial effect of floodplain lake distribution in shaping the longitudinal distribution pattern of larval assemblages. Both natural connected lakes and isolated lakes (with irregular connectivity patterns) play a crucial role in recruitment success of freshwater fish in the system. Hence, conservation and management approaches for fish resources should be concentrated on restoring and maintaining river–lake connectivity in the upstream sections of the lower reach of the Yangtze River.

Biogeochemical processes create distinct isotopic fingerprints to track floodplain rearing of juvenile salmon.

Floodplains represent critical nursery habitats for a variety of fish species due to their highly productive food webs, yet few tools exist to quantify the extent to which these habitats contribute to ecosystem-level production. Here we conducted a large-scale field experiment to characterize differences in food web composition and stable isotopes (δ¹³C, δ¹⁵N, δ³⁴S) for salmon rearing on a large floodplain and adjacent river in the Central Valley, California, USA. The study covered variable hydrologic conditions including flooding (1999, 2017), average (2016), and drought (2012-2015). In addition, we determined incorporation rates and tissue fractionation between prey and muscle from fish held in enclosed locations (experimental fields, cages) at weekly intervals. Finally, we measured δ³⁴S in otoliths to test if these archival biominerals could be used to reconstruct floodplain use. Floodplain-reared salmon had a different diet composition and lower δ13C and δ³⁴S (δ¹³C = -33.02±2.66‰, δ³⁴S = -3.47±2.28‰; mean±1SD) compared to fish in the adjacent river (δ¹³C = -28.37±1.84‰, δ³⁴S = +2.23±2.25‰). These isotopic differences between habitats persisted across years of extreme droughts and floods. Despite the different diet composition, δ¹⁵N values from prey items on the floodplain (δ¹⁵N = 7.19±1.22‰) and river (δ¹⁵N = 7.25±1.46‰) were similar, suggesting similar trophic levels. The food web differences in δ13C and δ³⁴S between habitats were also reflected in salmon muscle tissue, reaching equilibrium between 24-30 days (2014, δ¹³C = -30.74±0.73‰, δ³⁴S = -4.6±0.68‰; 2016, δ¹³C = -34.74 ±0.49‰, δ³⁴S = -5.18±0.46‰). δ³⁴S measured in sequential growth bands in otoliths recorded a weekly time-series of shifting diet inputs, with the outermost layers recording time spent on the floodplain (δ³⁴S = -5.60±0.16‰) and river (δ³⁴S = 3.73±0.98‰). Our results suggest that δ¹³C and δ³⁴S can be used to differentiate floodplain and river rearing habitats used by native fishes, such as Chinook Salmon, across different hydrologic conditions and tissues. Together these stable isotope analyses provide a toolset to quantify the role of floodplains as fish habitats.

Japanese Hops (Humulus japonicus) Control and Management Strategies in Large River Floodplains

AbstractJapanese hops (Humulus japonicus) is an invasive vine that establishes in open areas in riverine habitats and suppresses tree regeneration and native vegetation. This study evaluated the use of herbicides and tree plantings to control and manage Japanese hops on five Mississippi River islands over a four-year period. Herbicide treatments included a preemergent (sulfometuron methyl), a postemergent (glyphosate), and a combination of both. Tree plantings used containerized and bareroot American sycamore (Platanus occidentalis) and eastern cottonwood (Populus deltoides) trees. Japanese hops biomass was significantly lower in all herbicide treatments in 2012 and 2015, but the preemergent treatment was less effective than other treatments in 2012 and 2014. After two years, average survivorship of containerized trees was 20%–42%, whereas bareroot seedlings had near 100% mortality. Results indicate that postemergent treatments are effective for short-term control, but large floods reestablish Japanese hops in treated areas. Reforestation, if combined with herbicide treatments and active management, may be a promising approach in large river floodplains that experience frequent flooding, but low tree survivorship presents challenges to reforestation.

The changing face of floodplains in the Mississippi River Basin detected by a 60-year land use change dataset

Floodplains provide essential ecosystem functions, yet >80% of European and North American floodplains are substantially modified. Despite floodplain changes over the past century, comprehensive, long-term land use change data within large river basin floodplains are limited. Long-term land use data can be used to quantify floodplain functions and provide spatially explicit information for management, restoration, and flood-risk mitigation. We present a comprehensive dataset quantifying floodplain land use change along the 3.3 million km2 Mississippi River Basin (MRB) covering 60 years (1941–2000) at 250-m resolution. We developed four unique products as part of this work, a(n): (i) Google Earth Engine interactive map visualization interface, (ii) Python code that runs in any internet browser, (iii) online tutorial with visualizations facilitating classroom code application, and (iv) instructional video demonstrating code application and database reproduction. Our data show that MRB’s natural floodplain ecosystems have been substantially altered to agricultural and developed land uses. These products will support MRB resilience and sustainability goals by advancing data-driven decision making on floodplain restoration, buyout, and conservation scenarios.

Effects of river-lake disconnection and eutrophication on freshwater mollusc assemblages in floodplain lakes: Loss of congeneric species leads to changes in both assemblage composition and taxonomic relatedness

River floodplain ecosystems host one of the highest freshwater molluscan biodiversity on Earth. However, multiple human disturbances, such as loss of hydrological connectivity and deterioration of water quality, are seriously threatening most floodplain lakes throughout the world. Given the high imperilment rate of freshwater molluscs but the scarcity of studies examining the anthropogenic effects on this fauna, we test the response of mollusc assemblages to river-lake disconnection and eutrophication in 30 lakes in the Yangtze River floodplain, China. The species richness of entire Mollusca, Gastropoda and Bivalvia and 6 dominant families were all much lower at disconnected lakes than that in connected lakes, and decreased with increasing water eutrophication. The assemblage structure differed significantly among four lake groups for datasets based on entire Mollusca, Gastropoda and Bivalvia, indicating the serious impacts of hydrological disconnection and eutrophication. Moreover, the connected lakes showed significantly lower values of average taxonomic distinctness (Δ+) but higher values of variation in taxonomic distinctness (Λ+) than disconnected lakes. Such variations were triggered by the extirpation of congeneric and endemic species (mainly from families Unionidae and Viviparidae), which giving a waring of the loss of mollusc endemism in this region. In general, the present study showed that river-lake disconnection and deterioration of water quality resulted in serious biodiversity declines of both gastropods and bivalves in the Yangtze River floodplain lakes. A systematic approach including restoration of river-lake connectivity and habitats and improvement of water quality should be implemented in the conservation planning in this large river floodplain.

Warmer Winters Increase the Biomass of Phytoplankton in a Large Floodplain River

AbstractWinters are changing rapidly across the globe but the implications for aquatic productivity and food webs are not well understood. In addition, the degree to which winter dynamics in aquatic systems respond to large‐scale climate versus ecosystem‐level factors is unclear but important for understanding and managing potential changes. We used a unique winter data set from the Upper Mississippi River System to explore spatial and temporal patterns in phytoplankton biomass (chlorophyll a, CHL) and associated environmental covariates across 25 years and ∼1,500 river km. To assess the role of regional climate versus site‐specific drivers of winter CHL, we evaluated whether there were coherent long‐term CHL dynamics from north to south and across lotic‐lentic areas. We then estimated the degree to which these patterns were associated with climate variability (i.e., the Multivariate El Nino‐Southern Oscillation Index), winter severity (freezing degree days), river discharge, or site‐specific environmental variables (ice depth, snow depth, and nutrient concentrations). We found that winter CHL was typically highest in ice‐free reaches and backwater lakes, occasionally exceeding summer values. We did not find highly synchronous CHL dynamics across the basin, but instead show that temporal trends were independent among river reaches and lotic‐lentic areas of the river. Moreover, after accounting for these spatial dynamics, we found that CHL was most responsive to winter air temperature, being consistently higher in years with warmer winters across the basin. These results indicate that although productivity dynamics are highly dynamic within large river ecosystems, changes in the duration and severity of winter may uniformly increase wintertime productivity.

Grain yield optimisation in the Plain of Reeds in the context of climate variability

ABSTRACT The Plain of Reeds is a large floodplain in the Mekong Delta, where natural disasters such as droughts, off-season rainfall and floods have dramatically increased, leading to declining in crop yields. The objective of this study was to evaluate the adverse impacts of climate factors (ICF) on grain yield of the main growing crops in the Plain of Reeds to define the suitable cultivation period (SCP) for rice-growing areas as an adaptation solution to minimise the adverse impacts of climate factors. To conduct this research, a crop model was applied to define the suitable cultivation periods based on simulating the grain yields of each rice-growing season. When the suitable cultivation periods were deployed, the grain yield of all simulated growing crops improved significantly compared to the current cultivation periods (baseline), which, for the main growing crops in the Plain of Reeds, are no longer suitable for the current weather conditions that have been deeply affected by climate variability in recent years.

Sizable carbon emission from the floodplain of Ob River

The Ob River floodplain is the second largest floodplain in the world. Despite its vast area, estimates of carbon (C) emissions from the Ob River floodplain are largely absent. Here we present seasonal C emission and water area extent from the main channel and the floodplain along a ~4 km reach in the boreal zone of the Ob River. We found strong seasonality in water area extent of the Ob main channel (~1.8 km2) and floodplain (~3 km2) with water covering 34% of land during flood and subsequently declining to ~16% and ~14% during summer and autumn baseflow, respectively. The C emissions also varied seasonally over the open water period, ranging from −0.1 to 0.6 g C m−2 d−1 for the Ob main channel and from 0 to 9 g C m−2 d−1 for the floodplain. The dissolved organic carbon positively affected CO2 concentrations and fluxes in the floodplain during all seasons, whereas pH and oxygen concentration negatively impacted CO2 concentrations and fluxes. Some nutrients (ammonia and phosphate) positively correlated with CO2 and CH4 concentrations in summer. The total C emission from the study reach (1.8 km2 main channel, 3 km2 floodplain) during moderate flooding was 236 ± 51 tons C yr−1 (>99% CO2, <1% CH4) with the floodplain accounting for ~65%. The contribution of the floodplain to the net river C evasion may be even greater during years of high flooding and in northern regions of the Ob River basin, where floodplain soils are more C-rich and are underlain by permafrost, and in years with more extensive flooding.

From drought to deluge: spatiotemporal variation in migration routing, survival, travel time and floodplain use of an endangered migratory fish

We developed a novel statistical model to relate the daily survival and migration dynamics of an endangered anadromous fish to river flow and water temperature during both extreme drought and severe flooding in an intensively managed river system. Our Bayesian temporally stratified multistate mark–recapture model integrates over unobserved travel times and route transitions to efficiently estimate covariate relationships and includes an adjustment for telemetry tag battery failure. We applied the model to acoustic-tagged juvenile Sacramento River winter-run Chinook salmon (Oncorhynchus tshawytscha) and found that survival decreased with decreasing river flows and increased water temperatures. We found that fish were likely to enter a large floodplain during flood conditions and that survival in the floodplain was comparable to the mainstem Sacramento River. Our study demonstrates the response of an endangered anadromous fish population to extreme spatial and temporal variability in habitat accessibility and quality. The general model framework we introduce here can be applied to telemetry of migratory fish through systems with multiple routes to efficiently estimate spatiotemporal variation in survival, travel time, and routing.

Effects of a Proposed Hydraulic Project on the Hydrodynamics in the Poyang Lake Floodplain System, China.

Knowledge of dam construction in floodplain systems and its hydrodynamic effects plays a critical role in managing various kinds of floodplains. This study uses 3D floodplain hydrodynamic modeling to explore the possible effects of a proposed hydraulic project in Poyang Lake (PLHP) on the hydrodynamics, exemplified by a large floodplain system. Simulations showed that the water levels across most lake regions presented more significant changes than in the floodplain areas during the study period. The increased water levels upstream from the PLHP (~1.0 m) were distinctly higher than that downstream (~0.1 m). The PLHP may decrease the magnitude of the water velocities in the main channels of the lake, whereas velocities may experience mostly minor changes in the floodplains, depending upon the altered flow dynamics and transport. On average, the water temperature may exhibit mostly minor changes (~<1.0 °C) for both the horizontal and vertical scales within the flood-pulse-influenced lake system. Additionally, the model results indicated that the outflow process caused by the PLHP may be altered from the natural discharge into the Yangtze River to frequent backflow events during the storage period, demonstrating the non-negligible effect of the PLHP on the water supply for the downstream Yangtze River in the future.

Simple modelling for a large-scale assessment of total phosphorus retention in the floodplains of large rivers

Floodplains provide a multitude of ecosystem functions and services with water purification being one of them. For this study we modelled the retention of total phosphorous (TP) in the floodplains of the river Rhine and the river Elbe, looking at sediment deposition as the main process responsible for removing TP from rivers during inundation events. We applied two different approaches: a proxy-based approach (PBA) and a one-dimensional model based approach (MBA). We used both to calculate the yearly TP retention and compared it with the annual TP load in the rivers. Compared to the transported river load the Elbe floodplains investigated retained approx. 4.9% TP resp. 1.4% (PBA vs. MBA) while in the floodplains of the river Rhine about 1.8% vs. 0.3% TP was retained. We found that the greatest difficulty in quantifying TP retention in floodplains is due to the lack of spatial detail on the hydrological connectivity between rivers and their adjacent floodplains and that a sound validation of the results is absolutely necessary. Long-term monitoring data for floodplains, especially on hydrological connectivity, are of crucial importance in this respect.

Understanding Constraints on Submersed Vegetation Distribution in a Large, Floodplain River: the Role of Water Level Fluctuations, Water Clarity and River Geomorphology

Aquatic vegetation is a key component of large floodplain river ecosystems. In the Upper Mississippi River System (UMRS), there is a long-standing interest in restoring aquatic vegetation in areas where it has declined or disappeared. To better understand what constrains vegetation distribution in large river ecosystems and inform ongoing efforts to restore submersed aquatic vegetation (SAV), we delineated areas in ~1200 river km of the UMRS where the combined effects of water clarity, water level fluctuation, and bathymetry appeared suitable for establishment and persistence of SAV based on a 22-year dataset for total suspended solids (TSS), water surface elevation, and aquatic vegetation distribution. We found a large increase in suitable area downstream from a large natural riverine lake near the northern end of the UMRS (river km 1230) that functions as a sink for suspended material. Downstream from river km 895, there was much less suitable area due to decreased water clarity from tributary input of suspended material, changes in river geomorphology, and increased water level fluctuation. A hypothetical scenario of 75% reduction in TSS resulted in only minor increases in suitable area in the southern portion of the UMRS, indicating limitations by water level fluctuation and/or bathymetry (i.e., limited shallow area). These results improve our understanding of the structure and function of large river systems by illustrating how water clarity, fluctuations in water level, and river geomorphology interact to create complex spatial patterns in habitat suitability for aquatic species and may help to identify locations most and least likely to benefit from management and restoration efforts.

Coherent Satellite Monitoring of the Water Cycle Over the Amazon. Part 2: Total Water Storage Change and River Discharge Estimation

AbstractIn companion paper 1, the SAtellite Water Cycle (SAWC) satellite data set integration approach was presented. SAWC accounts for (1) the closure of the water budget at the sub‐basin scale by (2) using upstream/downstream dependencies. Here, the SAWC database is used to reconstruct a missing water component. The total water storage change (dS) can be reconstructed prior to the Gravity Recovery and Climate Experiment (GRACE) period. In terms of seasonal and interannual variations, SAWC provides long‐term dS estimates that are highly consistent with the GRACE estimate (2002–2015). When compared to the extent of inundation, correlation of anomalies reaches 0.6 over sub‐basins presenting large floodplain. A strong relationship (correlation of 0.7) exists between the monthly dS and the El Niño index. The river discharge R can also be estimated using the SAWC approach during the GRACE period. A scheme was developed to estimate R continuously along the river through the indirect interpolation of discharge station measurements by using satellite estimates of dS, precipitation P, and evaporation E. These R estimates along the river were evaluated with 80 independent measurements and had a correlation of 0.86 and a Kling‐Gupta efficiency index of 0.71. A comprehensive monthly river discharge estimate was proposed for the first time across the Amazon basin over 13 years (2002–2015) at a 0.25° resolution. The SAWC framework is a pure observation tool for integrating a large range of satellite estimates, potentially facilitating the assimilation of such data into land surface models.

Hydrogeomorphic indicators of wetland health inferred from multi-temporal remote sensing data for a new Ramsar site (Kaabar Tal), India

Wetlands form an important and dynamic ecosystem, and therefore, need continuous monitoring. We have developed a framework to assess the status of wetland health based on hydrogeomorphic characteristics and vegetation dynamics and have implemented it on a newly designated Ramsar site, Kaabar Tal, a large floodplain wetland in Ganga Plains, eastern India. The study has attempted to integrate the hydrological, geomorphological, and ecological concepts and tools to develop the protocols for a hydrogeomorphic assessment of wetland health. We have used multi-temporal Landsat imageries to define several health indicators based on water-spread area, geomorphology, and vegetation and have integrated them to compute wetland health index (WHI) scores. These health indicators correspond to a range of spatial scales – landscape (wetland) scale, ecosystem (sub-wetland) scale, and local (pixel) scale and individually represent different hydrogeomorphic and ecological functions as well as dynamics of a wetland. We have also presented four different wetland health scenarios based on dominating health indicator and two integrated scenarios representing the best and the worst WHI scores in all four scenarios. Our results show that the Kaabar Tal is in a degraded state and the degree of degradation varies spatially within the wetland. Agriculture is the prime factor for its degradation, augmented by decreasing rainfall and anthropogenic drainage reorganization. Based on our data analysis, we have suggested several recommendations to restore the health of this wetland. The framework presented in this work has a potential to understand the relationship between hydrodynamics and ecological functions in wetland systems in different hydro-climatic settings.

Importance of floodplains for water beetle diversity: a crucial habitat for the endangered beetle Graphoderus bilineatus in Southeastern Europe

Natural floodplains are rapidly disappearing ecosystems worldwide, primarily due to changing hydrology. Continental Croatia harbours some of the largest remaining and best preserved natural riverine floodplains in Europe. To establish conservation priorities, we surveyed water beetle assemblages in three large floodplains situated in the Danube, Drava, and Sava basins (Ramsar sites Kopacki rit and Lonjsko polje, and Spacva riparian forest), with special focus on the red-listed beetle Graphoderus bilineatus. The main aim of our study was to investigate the key environmental factors driving variation of water beetle assemblage structure and occurrences of G. bilineatus. Environmental variables describing habitat types and various habitat characteristics were measured and/or estimated for each sampling site. A total of 4.339 water beetle specimens belonging to 98 species were recorded. G. bilineatus was recorded at two out of the three investigated floodplains at 14 out of 30 sampling sites. We found significant differences in water beetle assemblages among the three localities based on the total species dataset, accompanied by different environmental background and vegetation composition. The density of riparian vegetation, permanent water durability, shading, type of bank, and plant communities best explained the distribution of the water beetle species. Based on the regression model, human impact and shading have significant effects on the abundance of G. bilineatus. Sites with G. bilineatus presence have significantly higher species richness compared to the sites without the species. Co-existence analysis indicates various types of associations between G. bilineatus and other water beetle species.

Refining the concept of hydrological connectivity for large floodplain systems: Framework and implications for eco-environmental assessments

Recent years, the hydrological connectivity has gained popularity in various research fields, however, its definition and threshold effects at a system scale have not received adequate attention. The current research proposes a promising framework to refine the concept of surface hydrological connectivity by combining hydrodynamic modeling experiments, threshold effects and geostatistical connectivity analysis, exemplified by the flood-pulse-influenced Poyang Lake floodplain system (China). To enhance the inherent linkage between hydrological connectivity and eco-environments, total connectivity (TC), general connectivity (GC), and effective connectivity (EC) were proposed to refine the metrics of hydrological connectivity. The results show that substantial differences between the three connectivity metrics are observed for all target directions, demonstrating that the joint role of water depth and flow velocity may produce more dynamic and complex influences on EC than the other two metrics of TC and GC. Topographically, the connectivity objects/areas within the flood pulse system reveal that the floodplain is a more sensitive area than the lake's main flow channels under different connectivity conditions. The modelling experimental studies show that variations in water depth thresholds are more likely to have a strong effect on connectivity for the dry, rising, and falling limbs, rather than the flooding period, while the flow velocity may exert an opposite threshold effect. The lake-floodplain system is characterized by a dynamic threshold behavior, with seasonally varying water depth and velocity thresholds. This study highlights the importance of redefined connectivity concept for facilitating scientific communication by combining hydrodynamic thresholds and offering recommendations for future connectivity assessments using our proposed metrics of TC, GC, and EC.

Land cover dynamics and their driving factors in a protected floodplain ecosystem

AbstractFloodplains are simultaneously the most productive and threatened ecosystems in the world, with land cover integrity compromised even in “protected” floodplains. Due to their rich biodiversity, monitoring of land cover and understanding the causative drivers becomes imperative in the protected floodplains. This work performs spatio‐temporal assessment of land cover changes and discovers the underlying processes in the largest protected floodplain of the Brahmaputra River (BR)—Kaziranga National Park (KNP). Remotely sensed images of 1988, 1998, 2008, and 2018 were analyzed using visual interpretation techniques coupled with class description keys in GIS platform along with ground‐based surveys. Results show a substantial area decline under Dense Forest (7.7%), Open Forest (10%), Waterlogged wetland (31.3%), and Marsh/Swamp (70.4%) land cover classes between 1988 and 2018. The considerable reduction in forests and wetlands is likely to impact the life support system of the park as they form the key habitats and food resources. Concurrently, dramatic area increase under riverine classes has led to huge landmass loss along the northern and eastern stretch. Though the BR and the local flood pulse are essential source of nutrient replenishment in the park, they also act as primary drivers of land cover alteration. Hydro‐geomorphic processes like bank erosion, high‐flood events, and siltation in association with vegetation succession and invasive species control the inter‐conversion of land cover classes in the KNP. The findings of this work provide an insight into the natural factors that instigate the land cover modification on large floodplains in general, and particularly in KNP.

Characterization of European lampreys and fishes by their longitudinal and lateral distribution traits

This study aims to complement existing fish-based assessment approaches by classifying European lampreys and fish species according to their probability of occurrence in six different longitudinal river regions and five types of floodplain water bodies under unimpaired conditions. The main objective was to provide for the first time harmonized occurrence traits for European lampreys and fishes in both longitudinal and lateral dimensions of floodplain river systems in a Fish Region Index (FRI) and Floodplain Fish Index (FFI), respectively.Altogether 163 lamprey and fish species established in European rivers have been scored according to their longitudinal and lateral occurrence probabilities. The species-specific Fish Region and Floodplain Fish indices and their variances inform about species’ occurrence probabilities, dispersal traits and potential species inventories of different river regions and floodplain waters. The final scores allow identifying characteristic fish assemblages, i.e. a set of type-specific species for the different longitudinal river regions and lateral floodplain water bodies.The species-specific index values and variances serve to calculate summary metrics for the total fish assemblage (FRItotal and FFItotal) as macro-scale indicators for the deviation of the total fish assemblage from a reference state. The resulting index values indicate degradation as well as rehabilitation success at the level of fish assemblages, i.e. at an ecologically relevant macro-scale. Both species-specific and total assemblage indices are directly comparable among all biogeographic regions of Europe independent of the local species pool. As such, the indices serve as assemblage indicators for the fish-based assessment of the ecological status of water body types and river regions in floodplain river systems, which are required in particular for the assessment of large floodplain rivers.

Operational Impacts of a Water Management Structure on the Surrounding Fish Assemblages in a Restored Backwater and a Large Floodplain River

Water management structures (WMS) are used to regulate water levels between large floodplain river backwaters and their adjacent river systems offering a balance between maintaining quality backwater habitat and providing benefits to the river systems. The design and operation of these WMS is dependent on unique management goals, and their impacts on the surrounding ecosystem are understudied. From 2016–2018 we quantified the response of the surrounding fish assemblages (i.e., backwater and river abundance and composition) to the WMS operation at the Emiquon Preserve, a 2723 ha restored floodplain lake located adjacent to the main channel of the Illinois River, U.S.A. Environmental conditions were recorded, and fish were collected using boat electrofishing on both sides of the WMS under three operational levels: closed and no flow, gravity flowing water, and mechanically pumping water to the river. Multivariate analyses determined that there was no difference in fish community composition within the Emiquon Preserve among operation levels. Within the river there were differences in community composition when the WMS was closed and mechanically pumping water. Environmental parameters varied seasonally and WMS operation level varied but indicated an increase in chlorophyll-a in the river when the WMS was mechanically pumping water, although the chlorophyll-a decreased within 200 m downstream of the WMS in the Illinois River. This study suggests that managed connections between restored backwater and habitats operated like this WMS have variable, spatially isolated impacts on fish communities despite supplying chlorophyll-a to large river ecosystems.