Lowland River Research Articles

Diet composition and trophic niche differentiation of Neogobius melanostomus along an invasion gradient in a large lowland river

The Ponto-Caspian goby Neogobius melanostomus (Pallas 1814) is a widespread non-native fish species in European rivers. It occurs in high abundance and can potentially alter the food webs of invaded water bodies profoundly. However, the invasion process usually goes unnoticed, and changes in food webs during the early stages of invasions are rarely described. Here, we studied populations of N. melanostomus along an invasion gradient in the Elbe River (Germany) and tested the effects of time since invasion on the diets and the associated trophic dynamics in two dominant habitats, i.e., rip rap and sand. Results showed that the abundance and biomass of N. melanostomus was lowest at the most recently invaded sites. The trophic niche space quantified by stable isotopes (δ ¹³ C and δ ¹⁵ N) showed that populations at earlier invaded sites exhibited a wider trophic niche than those at the most recently invaded sites. Diptera and Crustacea were the most abundant taxa in N. melanostomus guts at both habitat structures. At rip rap habitats, N. melanostomus showed a significant increasing preference for Crustacea and a decreasing one for Gastropoda along the invasion gradient. At sand habitats no significant relationships between the invasion gradient and electivity index for N. melanostomus were detected. We demonstrated that the invasion stage and habitat in which N. melanostomus occurs affect the feeding preference of this invasive fish and that these factors are therefore important for tracing changes in the macroinvertebrate communities of affected river systems.

Natural and anthropogenic factors drive large-scale freshwater fish invasions

We analyzed the large-scale drivers of biological invasions using freshwater fish in a Mediterranean country as a test case, and considering the contribution of single species to the overall invasion pattern. Using Boosted Regression Tree (BRT) models, variation partitioning and Redundancy Analysis (RDA), we found that human factors (especially eutrophication) and climate (especially temperature) were significant drivers of overall invasion. Geography was also relevant in BRT and RDA analysis, both at the overall invasion and the single species level. Only variation partitioning suggested that land use was the second most significant driver group, with considerable overlap between different invasion drivers and only land use and human factors standing out for single effects. There was general accordance both between different analyses, and between invasion outcomes at the overall and the species level, as most invasive species share similar ecological traits and prefer lowland river stretches. Human-mediated eutrophication was the most relevant invasion driver, but the role of geography and climate was at least equally important in explaining freshwater fish invasions. Overall, human factors were less prominent than natural factors in driving the spread and prevalence of invasion, and the species spearheading it.

Influence of invasive crayfish on fine sediment transport, ingress and bed storage in lowland rivers

AbstractHistorically it has been assumed that abiotic forces dominate fluvial sediment dynamics. However, a growing body of work indicates that biological energy can also exert a significant control over sediment dynamics. The role that invasive species may play in altering fine sediment dynamics is particularly pertinent given that any influence may disrupt the natural equilibrium of the ecosystem. Here we investigated the effect of invasive signal crayfish (Pacisfastcus leniusculus) on the transport and storage of fine sediment in a densely populated river compared with a nearby control river without crayfish, over an 18‐week period. We observed clear evidence of diurnal fluctuations in turbidity associated with crayfish presence including periodograms with power peaks at a period of 1 day. Fine sediment fluxes indicated that crayfish contributed on average 18.5% extra to baseflow loads than would be likely under abiotic forcing alone. Temporal variations in suspended sediment concentrations were also observed at the control site but these were different in character and exhibited no clear temporal pattern or consistency as confirmed by periodogram analysis. Crayfish did not have an effect on sediment ingress rates relative to the control site and, at the crayfish site, the reach scale sediment budget was in net equilibrium during the sampling period. Our results provide further evidence that biological energy alters riverine fine sediment dynamics and warrants consideration in sediment dynamic models.

Landscape development history of the Zabolotsky peat bog in the context of initial settlement of the Dubna River lowland (Upper Volga basin)

Zabolotsky peat bog is a unique biospheric and cultural-historical archive located in the north of the Moscow Region on the territory of the Dubna River lowland. Despite the advances in studying the Zabolotsky region, the question of reconstruction of the primitive population habitat remains unresolved. Until recently, it has been be-lieved that in the Late Valdai period, the Dubna River lowland was covered by the waters of an extensive glacier-dammed Tver paleolake, drained only at the turn of the Pleistocene and Holocene. It was assumed that the lake's existence prevented the settlement of the territory, whereas after its drainage, the shallow residual water pools were actively exploited in the economic activities of the primitive population. However, paleogeographic and ar-chaeological materials have been accumulated during the last two decades that questioned the existence of large dammed lakes in the Upper Volga basin in the Late Valdai time. This paper presents the results of three years (2018–2020) of research, allowing revision of the ideas about the Quaternary geology and development of the geomorphic conditions of this area. A program of research, comprising topographic and geodetic surveys, drilling using a portable boring rig, lithologic description of the core, radiocarbon (AMS) dating, paleo-soil studies, biologi-cal analysis of organic macrofossils, and ground-penetrating radar, has been carried out aimed at reconstruction of the paleogeographic setting and landscape development. Drilling data were used to build the profile across the left bank of the Dubna River floodplain with extension to the low terrace. The lithofacial analysis of samples and AMS dating allowed identifying three generations of ancient riverbeds, the deepest of which (with the bottom at 12 m below the water edge) is more than 30 thousand years old. The biological residues from the dark-coloured loams directly below the peat bottom belong almost exclusively to higher plants, both arboraceous and wetland, which may have been brought in by the floodwaters. The ground-penetrating radar profiles clearly show the boundaries of three electromagnetically homogeneous sedimental layers — the peat, silted peat, and loam. The paleo-geographic data, in conjunction with the geophysical profiling data, indicate the existence of a copious waterway in the lowland (the ancient Dubna River) no later than 15,000–16,000 years ago which formed a floodplain with large features of fluvial paleorelief available for settlement. These data agree well with the new serial AMS-dates for the resin from the grooves of the bone and horn artifacts, which permit extension of the time of the initial de-velopment of the Zabolotsky peat bog by the bearers of the Resseta Culture to 15,500 years ago. The conclu-sions drawn have major significance for the development of an evidence-based chronology of the events and dynamics of the settlement strategy of the population during the transition from the Pleistocene to the Holocene. The new data not only are consistent with the system of global paleoecologic events and history of the develop-ment of the outwash plain zone in Eastern Europe, but also provide the basis for refinement, and, possibly, revi-sion of a range of current concepts.

Environmental Gradients Shaping the Freshwater Bryophyte Communities of Croatia (Western Balkans)

A comprehensive field survey of 527 sites on 293 watercourses across Croatia revealed 76 sites (14.42%) in which bryophytes were the dominant part of the macrophyte vegetation. Using classification and ordination analyses, we obtained five community types segregated across the gradients of several climatic, physiographic and water chemistry parameters. The Didymodon tophaceus–Apopellia endiviifolia and the Berula erecta-Cratoneuron filicinum communities were mostly confined to the clean and basic karstic rivers of the Dinaric Ecoregion under the influence of the Mediterranean climate, with the Didymodon tophaceus–Apopellia endiviifolia community being a tufa-forming community associated with the seasonally dry watercourses of small catchment areas and cascades along the larger karstic rivers, while the Berula erecta–Cratoneuton filicinum community was mostly associated with rivers with larger catchment areas and permanent flow. On the other hand, the Oxyrrhynchium hians–Chiloscyphus pallescens community and the Fissidens pusillus–Veronica beccabunga community were associated with eutrophic water restricted to small rivers of the Pannonian Ecoregion under the influence of the temperate climate and flowing over silicate bedrock. The most represented and widespread in Croatia was the Cinclidotus community, displaying the widest ecological range in the study. It was mostly associated with the relatively clean karstic rivers of large catchment areas belonging to the Dinaric Ecoregion, with the majority of the sites under the influence of a temperate climate with higher precipitation during the warm period of the year. The geographical patterns of the freshwater bryophyte communities showed that the relatively clean, fast and cold karstic rivers belonging to the Dinaric Ecoregion provide habitats that harbour a greater diversity of bryophyte communities than the watercourses of the Pannonian Ecoregion, where bryophyte-dominated communities are restricted to a small number of small lowland and semi-montane rivers and predominantly occupy periodically flooded microhabitats such as river margins.

Essential and non-essential metals in three lowland rivers of temperate South America (Argentina): Distribution and accumulation

BackgroundGeographic distribution of essential metals on earth is largely uneven. Therefore, regional specific approaches to assess metal contents in freshwater ecosystems are mandatory. Cd, Cu, Cr, Pb, Ni, Mn and Zn concentrations were measured in water, sediments and fish tocharacterize the partitioning of these essential and non-essential metals in three river ecosystems of the Pampean Plain==. MethodsWater, sediment and eight fish species were collected at three rivers from the Pampean Plain. After sample-specific pretreatments, Cd, Cr, Cu, Mn, Ni, Pb and Zn concentrations in water, sediment and muscle were determined in triplicate by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Contamination (Hakanson´s index) and bioconcentration factors were calculated to assess the degree of contamination and the relationship between the matrices, while the target hazard quotient was estimated to explore the risk to human health from fish consumption. ResultsWater metal concentration was below detection limit in all sampled rivers. Sediments were mostly enriched by Mn (186–474 mg kg−1) followed by Zn (36.8–40.3 mg kg−1), whereas in fish, the situation was the opposite, suggesting different biogeochemical pathways and uneven bioaccumulation of these elements. The largest concentrations for all explored metals were invariably recorded in fish from the Sauce Grande River, although only Cd, Cu and Ni in sediments were highest at this site. Indeed, the bioconcentration factor showed that Cd and Zn are actively accumulated by almost all species in all sampled rivers. The Targeted Hazard Quotient based on the muscle metal concentration tissue of the edible fish species (Odontesthes bonariensis) was less than 1. ConclusionNone metal seemed to pose a significant risk for humans. In some sites biota-sediment concentration factors of Cd and Zn and muscle Cd, Cr, Ni, Zn and Pb levels were highest in the silurid Pimelodella laticeps, suggesting that it may be used as a sentinel species for biomonitoring programs in Neotropical fish assemblages.

Freshwater fish biodiversity restoration in floodplain rivers requires connectivity and habitat heterogeneity at multiple spatial scales

With a sixth mass extinction looming and freshwater biodiversity declining at unprecedented rates, evaluating ecological efficacy of river restoration efforts is critical in combatting global biodiversity loss. Here, we present a comprehensive study of the functioning for fishes of 46 river restoration projects in the river Rhine, one of the world's most heavily engineered lowland rivers. Floodplains with permanent, either one- or two-sided lateral connectivity to the main channel, favour total fish abundance, and are essential as nursery areas for riverine fishes. Habitat heterogeneity had a strong positive effect on species richness but was negatively related with fish abundances. However, the effects of environmental variables varied between ecological groups and spatial scales. Surprisingly, richness of critical rheophilic fishes declined with large-scale habitat heterogeneity (~1000 m), while it increased at small scales (~100 m), possibly because of the presence of unfavourable habitats for this ecological group at larger scales. Clearly, there is no one-size-fits-all design for river restoration projects. Whether a river section is free-flowing or impounded dictates the scope and efficacy of restoration projects and, within a river section, multiple complementary restoration projects might be key to mitigate freshwater fish biodiversity loss. An essential element for success is that these projects should retain permanent lateral connection to the main channel.

About the longitudinal slopes of flat rivers

The article discusses the methods of determining the longitudinal slopes of rivers, their advantages and disadvantages are noted. Various methods of determining the longitudinal slopes of rivers are analyzed. It is revealed that the weighted average slope adopted by the hydrometeorological service, determined by the graphoanalytic method, does not fully reflect the essence of the influence of slopes on the speed of water movement. It is noted that it is expedient to use the differential equation of uneven motion to determine the slopes of the water surface in the lower reaches of lowland rivers. The criterion ratio of slopes characterizing the stability of the riverbed in the longitudinal direction is revealed. The hydraulics-morphological method of determining the level and longitudinal slope of the free flow surface, as well as quantitative criteria for the movement of river flows, has been continued.

Microplastic loads within riverine fishes and macroinvertebrates are not predictable from ecological or morphological characteristics

Microplastics are a relatively new but important form of freshwater contamination that can be ingested by a range of different species, with particle counts thought to be predictable from species ecology and morphology. Here, we report levels of microplastics in a 26 μm-5 mm size range within the macroinvertebrate and fish community of a lowland river (Dorset Stour, SW England), and test the hypothesis that counts are predictable from characteristics such as feeding guild, body length and trophic position. Macroinvertebrates (n = 257, 12 taxa) and fish (n = 418, 9 species) were collected from distinct river reaches by kick sampling and rod and line angling, respectively. Batches of whole macroinvertebrates and individual fish gastrointestinal tracts were digested with 30% hydrogen peroxide before microplastic screening and FTIR polymer confirmation on a particle subset. Particles were found in 40% of pooled macroinvertebrate batches (taxa incidences: 14–75%) and 39% of fishes (species incidences: 29–47%). Dominant particle feature categories were ≤100 μm, blue/green, fragments and fibres identified as various polyolefins. Although particle counts in macroinvertebrates were highest in Ephemeroptera (mean of 0.74 particles per individual), the relationships between particle loads, batch number and guild were all non-significant. In fishes, particle counts were not significantly related to species, stomach structure, feeding guild or body length, with spatial differences also not apparent across the catchment. Individual fish particle counts were similarly not significantly associated with their trophic positions (calculated from bulk δ15N values for a subset of fishes) and parasite load of Pomphorhynchus tereticollis. Correlations between fish and macroinvertebrate particle counts within specific river reaches were also not significant. In entirety, these results indicated although loadings of microplastic particles were relatively consistent within the two communities, they were not predictable from any of their ecological or morphological characteristics.

Mosaic Evolution of Craniofacial Morphologies in Ghost Electric Fishes (Gymnotiformes: Apteronotidae)

Ghost electric knifefishes (Gymnotiformes: Apteronotidae) are a dominant component of the species diversity and biomass of large lowland rivers in Greater Amazonia, including 77 species that exhibit diverse craniofacial morphologies associated with trophic and secondary sexual traits. Here we use open-source computed micro-tomography (µCT) scans to generate 3D reconstructions for a majority of apteronotid species and almost all valid genera, and geometric morphometric and phylogenetic analyses to explore patterns of skull evolution. As with most vertebrates, principal component 1 (PC1) summarizes variance from brachycephalic to dolichocephalic morphologies, previously described as heterocephaly, and PC2 summarizes variance from recurved (upturned) to decurved (downturned) snout morphologies, described here as heterorhynchy. Phylomorphospace and traitogram analyses found instances of both convergent and divergent evolution along both of the first two PC axes, as well as a preponderance of clades characterized by morphological stasis or phylogenetic conservatism. Certain phenotypic combinations predominate among species and clades, including elongated-downturned snouts and foreshortened-upturned snouts, while other phenotypic combinations are not observed, including elongated-upturned snouts and foreshortened-downturned snouts. These results highlight the power of 3D geometric morphometrics in the study of craniofacial evolution and indicate developmental or functional constraints in the evolution of craniofacial phenotypes in an ecologically dominant clade of riverine Amazonian fishes.

ANALIZA WYBRANYCH CHARAKTERYSTYK HYDROLOGICZNYCH DWÓCH MAŁYCH ZLEWNI NIZINNYCH

Aim of the study: i. Comparison of characteristic discharges (mean and mean of annual minimum, i.e. mean low discharge) of small lowland river between two periods (1963-1990 and 1991-2020) estimated on the base of field measurements. ii. Comparison of specific discharges of two small catchments of various size in the period 1991-2020. iii. Evaluation of local indirect methods for estimation of mean and mean low discharge applied for the two small catchments. Material and methods: Since 1962, hydrological investigations have been conducted in a small lowland catchment of the Zagożdżonka River. The catchment area is characterized by agro-forested land use, and is located in the south part of the Masovia region, Poland. Continued monitoring of water stages and discharge measurement at the river gauge Płachty Stare were conducted for estimating daily and characteristic discharges for two periods: 1963-1990 and 1991-2020. Other methods were also used for both of the hydrological characteristics. They were based on maps of specific discharges from the Hydrological Atlas of Poland. Results and conclusions: Mean discharge of the Zagożdżonka River at the gauge Płachty Stare between the two periods decreased by 24%. Mean low flow decreased by 43%. Higher evapotranspiration, caused by climate change, i.e. temperature increase and higher ratio of forested area in the catchment, is indicated as the main reason of the changes. Mean specific discharge from the smaller catchment (at Wygoda gauge) was 36% less than from the larger catchment. The application of the indirect methods for estimation of mean and mean low discharges for the larger catchment seems to give satisfactory results.

High‐resolution spatiotemporal analysis of hydrologic connectivity in the historical floodplain of straightened lowland agricultural streams

AbstractIn agricultural watersheds, human interventions such as channel straightening have disrupted the hydrologic connectivity between headwater streams and their riparian environment and have thus undermined the ecological services provided by these small streams. Knowledge of the hydrologic connectivity between these streams and their immediate environment (shallow riparian groundwater in the historical floodplain and on adjacent hillslopes) in human‐impacted settings is critical for understanding and restoring these hydrological systems but remains largely incomplete. The objective of this research is to investigate the hydrogeomorphological conditions controlling hydrologic connectivity in the historical floodplain of straightened lowland streams. Detailed measurements on the spatiotemporal variability of groundwater‐surface water interactions between straightened reaches, historical floodplain including abandoned meanders, and the adjacent hillslopes were obtained using a dense network of piezometers at two sites in the St. Lawrence Lowlands (Quebec, Canada). Results show that the complex mechanisms controlling hydrologic connectivity in naturally meandering lowland rivers also operate in highly disturbed straightened reaches, despite backfilling and agricultural practices. The pre‐straightening hydrogeomorphological configuration of the floodplain partly explains the complex patterns of piezometric fluctuations observed at the sites. The apex of the abandoned meanders stands out as a focal area of hydrologic connectivity as water levels indicate pressure transfer that may reflect flows from the stream, the hillslopes, and the surrounding historical floodplain. These unique field observations suggest that abandoned meanders should be promoted as key elements of restoration strategies in lowland agricultural straightened headwater streams.

Riparian shading mitigates warming but cannot revert thermal alteration by impoundments in lowland rivers

AbstractGiven the actual trajectory of greenhouse gases emissions, in situ actions are needed to limit rivers water warming. Riparian tree planting has been shown as a successful mitigation strategy, but most studies have been conducted on upland forested rivers. The hydraulic, geomorphic and thermal conditions of lowland rivers, subject to straightening and fragmentation, are very different from upland systems. In this study, the role of riparian cover relative to the presence of weirs was investigated in lowland slow‐flowing rivers within the agricultural context of the Bresse region, France. Using airborne thermal infrared (TIR) imaging, nine reaches of 20 to 30 km each were mapped to quantify water temperature, riparian vegetation and assess weir location. Campaigns were carried out during the hot summer 2018. Results have comforted that riparian vegetation is capable of limiting increase in temperature during warm events by 2 to 3°C, but with varying success depending on the rivers' thermal regimes. Rivers with a ‘colder’ regime (i.e., ground/karstwater‐fed) are thermally less influenced by riparian vegetation but also more resilient to the presence of weirs. Rivers with the highest degree of thermal alteration showed the highest positive response to riparian shading. Riparian vegetation prevents stagnant water from extreme warming but does not prevent alteration of the thermal regime, for example thermal stratification, likely to occur on a daily basis and to large extents (> 80%) during warm and low‐flow events in impounded rivers. Thermal conditions were sub‐optimal for most fish species that could be expected in these streams. Although the direct relation with thermal alteration is not proved, hydrobiological scores of flow alteration are correlated with our metric of thermo‐physical alteration. Management strategies to limit temperature increase should carefully consider restoring the thermal regime of rivers through weir removal when it is possible, combined with riparian tree planting as an additional mitigation measure.

A longitudinal snapshot of pioneer plant patterns along lowland temperate rivers

AbstractOverexploitation of water, loss of dynamism and current climate change cause river drought and baseflow events that are globally increasingly intense and prolonged. This strongly affects riverine plant communities, especially those of seasonally exposed sediments. However, few data are available concerning this key component of river biodiversity currently. Furthermore, the role of alien species is expected to be important in riverbeds since fluvial corridors have an extraordinary susceptibility to be invaded and act as primary routes for invader dispersal. This study provides a systematic investigation of the longitudinal patterns of pioneer plants—in terms of richness and cover—in three mid‐size rivers (Ticino, Adda and Oglio rivers) in northern Italy, from 15 different sampling sites. The alien plants dominated the pioneer richness (52 out of 91 species, 57% of the total richness found), whereas plant cover followed less‐defined patterns. Along the upstream–downstream gradient, native plants showed a steep decline (−69% in richness and −52% in cover rates on average), only partially offset by the local (extremely variable) changes in alien taxa (−41% on average, in the range of −2.7 to +4.1 species per plot and with cover rates varying form −10 to +1,018%). The magnitude and sign of the detected trends strongly reflected individual river identity. Further investigations are required to better assess the role of hydromorphology in driving the riverine pioneer plant assemblages.

The Impact of Hydromorphological Alterations on Mayfly Assemblages of a Mid-Sized Lowland River in South-Eastern Europe.

Simple SummaryThe majority of European lowland rivers are under the impact of multiple stressors (water quality, hydromorphological alterations, land-use), however, the consequences of these influences on mayflies have not been sufficiently studied. Therefore, we studied mayfly assemblages and their relationship to environmental factors along a mid-sized lowland river in Croatia. No significant differences in mayfly species richness and their functional traits were observed among the three habitat groups with different levels of hydromorphological alterations (near-natural, moderately altered, and severely altered habitats). This could be related to the river’s connection to the numerous tributaries, and the variety of available microhabitats along the studied system, despite the existing hydromorphological pressures. A stronger relationship was found between mayflies and the water physico-chemical characteristics (water temperature, water velocity, oxygen content, and nutrient (e.g., nitrogen, phosphorous) concentrations) and agricultural and urban land-use. These results can contribute to the planning of management and conservation activities for lowland rivers and their biota according to the requirements of the European Water Framework Directive.Historically, rivers have been anthropogenically modified for different purposes worldwide (e.g., flood control, drinking water abstraction, and land drainage). Although the majority of European lowland rivers are under the impact of multiple stressors (water quality, hydromorphological alterations, land-use), the consequences of these influences on aquatic macroinvertebrates, including mayflies, have not been sufficiently studied. Therefore, with the aim of providing additional data on the response of mayflies to anthropogenic disturbances in riverine habitats, we studied mayfly assemblages and their relationship to environmental factors along a mid-sized lowland river in Croatia. No significant differences in mayfly species richness and their functional traits were observed among the three habitat groups with different levels of hydromorphological alterations (near-natural, moderately altered, and severely altered habitats). This could be related to the river’s connection to the numerous tributaries, and the variety of available microhabitats along the studied system, despite the existing hydromorphological pressures. A stronger relationship was found between mayflies and the physico-chemical water characteristics and land-use. Water temperature, water velocity, oxygen content, and nutrient (e.g., nitrogen, phosphorous) concentrations related to agricultural and urban land-use were found to be the most important factors shaping mayfly assemblages in the studied lotic lowland system. These results can contribute to the planning of management and conservation activities for lowland rivers and their biota according to the requirements of the European Water Framework Directive.

Differences in foraging behaviour of a migrating shorebird at stopover sites on regulated and unregulated sections of a large European lowland river

Differences in foraging behaviour of a migrating shorebird at stopover sites on regulated and unregulated sections of a large European lowland river

Flow Resistance in Lowland Rivers Impacted by Distributed Aquatic Vegetation

This study addressed a research concern that employing a fixed value for the bed roughness coefficient in lowland rivers (mostly sand-bed rivers) is deemed practically questionable in the presence of a mobile bed and time-dependent changes in vegetation patches. Accordingly, we set up 45 cross-sections in four lowland streams to investigate seasonal flow resistance values within a year. The results revealed that the significant sources of boundary resistance in lowland rivers with the lower regime flow were bed forms and aquatic vegetation. The study then used flow discharge as an influential variable reflecting the impacts of the above-mentioned sources of resistance to flow. The studied approach ended up with two new flow resistance predictors which simply connected the dimensionless unit discharge to flow resistance factors, Darcy-Weisbach (f) and Manning (n) coefficients. A comparison of the computed and measured flow resistance values also indicated that 87–89% of the data sets were within ± 20% error bands. The flow resistance predictors were also verified against large independent sets of field and flume data. The obtained predictions using the developed predictors might overestimate flow resistance factors by 40% for other lowland rivers. Based on a different view, according to the findings of this research, seasonal variation of vegetation abundance could show the augmentation in flow resistance values, both f and n, in low summer flows when vegetation covers river bed and side banks. The highest amount of flow resistance was observed during the summer period, during the July–August period.

Weed cutting in a large river reduces ecosystem metabolic rates in the case of River Gudenå (Denmark)

Problems related to extensive macrophyte growth are widespread both in modified and man-made canals and streams, and in streams with natural morphology and rich vegetation. The weed cutting is a common management practice in order to reduce flood risk and enhance water conveyance. Although the short- and long-term impacts on the stream physical habitats and biota have been extensively studied, only little information exists on the effects of weed cutting on ecosystem metabolism, especially for larger rivers. This study aims to quantify effects of weed cutting on metabolic rates in a large lowland river in Denmark. We measured Gross Primary Production (GPP), Ecosystem Respiration (ER) and physical parameters (water depth, discharge, water velocity and reaeration rate) one week prior and 2–6 weeks after weed cutting in 2014 and 2020. Physical river conditions changed significantly after the removal of approximately 60% of macrophytic volume, and a significant reduction in water depth and increased water velocity was recorded. We found an immediate 38% and 61% reduction in GPP and 28% and 35% reduction in ER after weed cutting in 2014 and 2020 respectively. We also found that the metabolic rates did not recover to pre-weed cutting levels within 2–6 weeks after weed cutting. The higher decline in GPP compared to that in ER indicates that the heterotrophic contribution to ER was higher compared to the autotrophic contribution. Our results display that even in a large macrophyte-rich river, where only one-third of the channel is managed by weed cutting, GPP and ER can be reduced significantly. The cascade effects of metabolic rates alterations on ecosystem structure and functioning need to be considered in the future management plans, where higher plant biomass and increased flow is anticipated due to the ongoing climate change and thus, the demand for weed cutting might be intensified.

High‐resolution water‐quality and ecosystem‐metabolism modeling in lowland rivers

AbstractHigh‐resolution monitoring of water quality and ecosystem functioning over large spatial scales in expansive lowland river catchments is challenging. Therefore, we need modeling tools to predict these processes at locations where observations are absent. Here, we present a new approach to estimate ecosystem metabolism underpinned by a high‐resolution, process‐based model of in‐stream flows and water quality. The model overcomes the current challenges in metabolism modeling by accounting for oxygen transport under varying flows and oxygen transformations due to biogeochemical processes. We implement the model in a 62‐km‐long stretch of the River Thames, England, using observations spanning 2 yr. Model outputs suggest that the river is primarily autotrophic from mid‐spring to mid‐summer due to high biomass during low‐flow periods, and is heterotrophic during the rest of the year. Ecosystem respiration in upstream reaches is driven mainly by biochemical oxygen demand, autotrophic respiration, and nitrification processes, whereas downstream sites also show a control of benthic oxygen demand in addition to the aforementioned processes. Using empirical modeling, we analyze the sensitivity of our estimated metabolism rates to multiple environmental stressors. Results demonstrate that empirical models could be useful for rapid river health assessments, but need improvements to reproduce peak metabolism rates. The process‐based model, although more complex than existing in situ approaches to metabolism quantification, allows inference when gaps in continuous observations are present. The model offers additional benefits for predicting metabolism rates under future scenarios of environmental change incorporating multiple stressor effects.

Transport of Potentially Toxic Elements in Solid Particulate Matter during Flash Flood Events in Upper and Lower Stretch of the Sava River

During extreme hydrological conditions such as flood events, sediments and alluvial soils may become re-suspended. As a consequence, the concentration of solid particulate matter (SPM) in the water column increases. As sediments represent a sink for the contaminants, when such perturbation occurs, the toxic substances may be re-dissolved into the water, causing harmful effects to the freshwater habitat. The purpose of this study was to evaluate the release of potentially toxic elements associated to SPM during flash floods. Two sampling sites on the Sava River were chosen: Litija (Slovenia), where the river has still the characteristics of an alpine river, and in Belgrade (Serbia), where Sava is a lowland river with a flat riverbed, slower flow and bigger quantities of fine-grained sediment. The results of the study showed a good correlation between the SPM mass concentration and water level/discharge. At both sampling sites, elevated concentrations of As, Cd, Cr, Cu, Pb, Ni and Zn were determined in SPM, indicating a moderate level of contamination at Litija and a high level at Belgrade. The results demonstrated that during the two investigated flash flood events limited the dissolution of PTE from SPM-bearing particles.