European Lowland Rivers Research Articles

Contribution of groundwater-borne nutrients to eutrophication potential and the share of benthic algae in a large lowland river

Groundwater inflow can be a significant source of nutrients for riverine ecosystems, which can affect eutrophication i.e., the elevated primary production and the corresponding accumulation of algal biomass. Experimental and modelling work has shown that benthic algae (autotrophic biofilms) in particular benefit, as they have direct access to the inflowing groundwater-borne nutrients. Primarily the supply of phosphorus (P) enhances pelagic algal biomass, as it is the limiting nutrient for primary production in most freshwater systems. In this study, we estimate the effect of groundwater inflow on overall eutrophication of a large, European lowland river and tested its seasonal effect on biofilms in particular. We calculated the effects on overall eutrophication during summer according to the estimated input of groundwater-borne P and the C:P stoichiometry of planktonic algae in the Elbe River. Our model indicated that these diffuse P inputs have the potential to significantly increase eutrophication. Groundwater-P can contribute up to 1.5 t/d PO4 over the investigated 450 km stretch of the Elbe River under low flow conditions. This would result in an additional planktonic load of about 46 t/d of particulate organic carbon, thereby contributing to eutrophication at the regional scale in this river. In contrast, at the local scale, biofilms were collected seasonally from artificial substrata exposed in the river either in hydrogeologically active areas with groundwater inflow, or in areas of varying hydraulic connectivity. Analyses of biofilm macronutrients, structural components and biofilm community composition show distinct effects of season, hydrogeology and groundwater inflow. The dominant predictors were season and the interaction between hydrogeology and groundwater. Benthic eutrophication is most likely to occur in autumn in areas of loose rock with high groundwater inflow. The strong interaction of environmental factors in determining benthic eutrophication highlights the need to assess these factors in combination rather than in isolation.

Is it worth the extra mile? Comparing environmental DNA and RNA metabarcoding for vertebrate and invertebrate biodiversity surveys in a lowland stream.

Environmental DNA (eDNA) metabarcoding has emerged as a promising approach to assess biodiversity and derive ecological status classes from water samples. However, a limitation of eDNA surveys is that detected DNA molecules may originate from other places or even dead organisms, distorting local biodiversity assessments. Environmental RNA (eRNA) metabarcoding has recently been proposed as a complementary tool for more localized assessments of the biological community. In this study, we evaluated the effectiveness of eDNA and eRNA metabarcoding for inferring the richness and species distribution patterns of vertebrates and invertebrates in a Central European lowland river. We collected water samples and analyzed them using a 12S marker for vertebrates and a COI marker for invertebrates. We detected 31 fish, 16 mammal, 10 bird and one lamprey species in the vertebrate dataset. While results were largely consistent, we detected a higher number of species when analysing eRNA (mean = 30.89) than eDNA (mean = 26.16). Also, eRNA detections had a stronger local signature than eDNA detections when compared against species distribution patterns from traditional fish monitoring data. For invertebrates, we detected 109 arthropod, 22 annelid, 12 rotiferan, eight molluscan and four cnidarian species. In contrast to the pattern of vertebrate richness, we detected a higher richness using eDNA (mean = 41.37) compared to eRNA (mean = 22.42). Our findings primarily show that eDNA and eRNA-based detections are comparable for vertebrate and invertebrate taxa. Biological replication was important for both template molecules studied. Signal detections for vertebrates were more localized for eRNA compared to eDNA. Overall, the advantages of the extra steps needed for eRNA analyses depend on the study question but both methods provide important data for biodiversity monitoring and research.

The new concept for the heavy vehicles crossing in the large lowland riverbeds in the North East Strategic Direction

The current method of crossing river channels by heavy vehicles involves engineering equipment such as the Motorized Floating Bridge. It has high efficiency when crossing regulated channels of large European Lowland rivers with stable hydrological conditions. In the case of unregulated rivers of the Polish Lowlands, this concept needs to be modified due to the high variability of water levels and the shape of the river bottom. This paper presents a new approach to crossing the channels of large lowland rivers by heavy vehicles. It is based on hydrological analyses and experience gained during many years of hydrographic, geological, and geomorphological studies conducted at the Department of Hydraulic Engineering and Applied Geology of the Warsaw University of Life Sciences (SGGW).

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

Longitudinal dynamics of Odonata assemblages in an anthropogenically impacted lotic system

European lowland rivers are extensively impacted by hydromorphological pressures, and the relationship between individual benthic macroinvertebrate groups and these degradations are insufficiently investigated. Therefore, we studied distribution and ecological traits of Odonata inhabiting a lotic system in the Pannonian lowland ecoregion (ER 11) in Croatia affected by degraded water quality and hydromorphological stressors. The study encompassed multihabitat sampling of 20 longitudinally distributed sampling sites, selected for their representativeness of disturbances. Only five species were recorded withPlatycnemis pennipes(Pallas, 1771) andOnychogomphus forcipatus(Linnaeus, 1758) dominating. We found woody debris samples contained a disproportionately higher number of Odonata, especially Zygoptera, compared to all other sampled microhabitats. The downstream longitudinal increase in Odonata abundance was not followed by an expected increase in species richness. Only five (oxygen saturation, pH, ammonium, water temperature and total nitrogen) of the sixteen tested water quality parameters were significant variables in determining the variation of Odonata assemblages.Calopteryx virgo(Linnaeus, 1758) and juvenileGomphidaewere associated with sites of somewhat higher ammonium and total nitrogen concentrations while all other taxa showed a negative association to ammonium. Odonata abundances were affected by modification of the river channel where significant decrease in abundance was observed with increased modification. Our results suggest that even species with a broad ecological tolerance are sensitive to hydromorphological pressures and represent an important background for further research and conservation practices of lotic European Odonata.

Environmental requirements and heterogeneity of rheophilic fish nursery habitats in European lowland rivers: Current insights and future challenges

AbstractThe large‐scale degradation of riparian ecotones and of the connectivity between rivers and their floodplains has resulted in a drastic decline of rheophilic fish populations in European temperate lowland rivers. Recent river restoration projects have had variable success in effectively restoring these fish populations. Knowledge on nursery habitat requirements is considered essential for effective population restoration. However, a detailed understanding of the role of habitat heterogeneity in young‐of‐the‐year (YOY) fish population development is limited. Therefore, we carried out a synthesis of the available knowledge on nursery habitat requirements of rheophilic fish species found in European temperate lowland rivers (<200 m elevation). From a total of 603 papers, 77 studies with primary information were selected, containing 390 associations between habitat features and YOY fish. As expected, most studies focused on static components of physical riparian habitat. Generally, YOY fish require habitats of shallow depth (<0.5 m), with slow‐flowing water (<0.2 m/s), gentle bank slope (<20°), variety in substratum types (fine sand to gravel), relatively warm water and high food availability. Surprisingly, no clear ontogenetic habitat shifts between larvae and juveniles were found, which may be explained by the limited spatial–temporal resolution of most studies. Since 2011, studies on habitat heterogeneity have increased, but few have explicitly assessed its role in relation to movement patterns of YOY fish for nursery success. Therefore, we recommend that future research focuses on fish movement patterns between habitat patches in heterogeneous (river‐floodplain) environments, to increase the knowledge base for effective recovery of rheophilic fish populations.

Sediment Transport and Water Flow Resistance in Alluvial River Channels: Modified Model of Transport of Non-Uniform Grain-Size Sediments

The paper presents recommendations for using the results obtained in sediment transport simulation and modeling of channel deformations in rivers. This work relates to the issues of empirical modeling of the water flow characteristics in natural riverbeds with a movable bottom (alluvial channels) which are extremely complex. The study shows that in the simulation of sediment transport and calculation of channel deformations in the rivers, it is expedient to use the calculation dependences of Chézy’s coefficient for assessing the roughness of the bottom sediment mixture, or the dependences of the form based on the field investigation data. Three models are most commonly used and based on the original formulas of Meyer-Peter and Müller (1948), Einstein (1950) and van Rijn (1984). This work deals with assessing the hydraulic resistance of the channel and improving the river sediment transport model in a simulation of riverbed transformation on the basis of previous research to verify it based on 296 field measurements on the Central-East European lowland rivers. The performed test calculations show that the modified van Rijn formula gives the best results from all the considered variants.

A framework to diagnose the causes of river ecosystem deterioration using biological symptoms

Abstract River assessments are predominantly based on biological metrics and indices selected or designed to integrate the impact of multiple causes of deterioration (stressors) operating at various spatial scales. Yet, the integrative nature of many bioassessment systems does not allow for tracing back individual stressors and their influence on the overall assessment result. Thus, river managers often fail to link bioassessment with programmes of management measures, to improve ecological quality. Here, we present a novel diagnostic approach that allows to estimate the probability of individual stressors being causal for biological degradation at the scale of individual riverine ecosystems. Similar to medical diagnosis, we use various symptoms (macroinvertebrate metrics) and probabilistically link them to various potential causes of ecological status degradation (stressors). Symptoms and causes are informed by a training dataset of 157 samples (stressors, taxa lists) from central European lowland rivers and are linked through a Bayesian network (BN). Three separate BNs addressing three different spatial scales (catchment, reach and site) are presented. Water quality‐related causes are most influential at the catchment scale, while hydromorphological causes prevail at finer scales. Causes indicating riparian degradation are most influential at the reach scale. Many symptoms show strong linkages to causes and reveal ecologically meaningful relationships, thus pointing at the potential diagnostic utility of the symptoms selected. BNs are validated using an independent dataset of 47 samples. Overall, model accuracies range 53%–58% for the three BNs, while for individual nodes (causes and symptoms) up to 100% concordance of predicted and actual node states in the validation data is achieved. The BNs are implemented as interactive online diagnostic tools to allow end users an easy application. Synthesis and applications. Bayesian inference can greatly assist the diagnosis of potential causes of ecosystem deterioration based on a selection of diagnostic biological metrics. If integrated into a Bayesian network, symptoms and potential causes can be linked and inform management decisions on appropriate measures, to improve biological and ecological status. Diagnostic Bayesian networks thus support end users bridge the gap between biological monitoring and appropriate programmes of management measures.

Microplastics in freshwater fish from Central European lowland river (Widawa R., SW Poland)

In contrast to marine organisms, the presence of microplastics (MPs) in freshwater animals remains insufficiently studied. The aim of this study was to identify the occurrence of MPs in the digestive tracts of two fish species from a small lowland river (Widawa R., SW Poland). In total, 202 gudgeons and 187 roaches were collected, of which 54.5% and 53.9% had ingested MP-like particles, respectively. Feeding type and behaviour, sex and capture site (above or below the dam reservoir) did not affect the number of fish with MP-like particles.

Fish diversity assessment in the headwaters of the Volga River using environmental DNA metabarcoding

Abstract The headwaters of the Volga River exhibit large reaches with near‐pristine conditions, and therefore long‐term biodiversity monitoring of this catchment can provide rare and valuable information on a European lowland river. More specifically, freshwater fish species assemblages are a good indicator of ecosystem status, as they are particularly sensitive to environmental changes and hydromorphological alterations. Historical records show that the fish fauna of the Upper Volga has changed over time, both in species composition and in abundance. The construction of the Volga–Kama cascade (a series of large dams) has specifically affected the migration of diadromous species. Environmental DNA metabarcoding offers a non‐invasive approach to determine the number of species in an aquatic ecosystem, as well as their identity and distribution. This approach is especially useful for fish fauna surveys along large rivers and long‐term biomonitoring, with the advantage of having no impact on the species and their habitats. To infer the current fish species diversity and the spatial distribution of each species in the free‐flowing section of the Upper Volga River, as well as in selected tributaries, an environmental DNA metabarcoding approach was applied, using three mitochondrial DNA markers. This method allowed the positive identification of 23 fish species and their respective distributions in the headwaters of the Volga. This assessment provides a valuable example of the application of environmental DNA metabarcoding in a large river system, and constitutes a starting point for future investigations and long‐term biomonitoring in the Upper Volga system. In addition, the results can also serve as a reference for fish diversity assessments of other large European lowland rivers, and can guide future conservation and management measures in the headwaters of the Volga.

Ecology and fluvial dynamics of an Early Holocene medium‐sized European lowland river valley (Upper Scheldt, northern Belgium)

The fluvial environment of Early Holocene small‐ to middle‐sized lowland rivers in northwest Europe is mostly unstudied due to a lack of preserved and accessible deposits. A rescue excavation in the Scheldt valley in northern Belgium offered the opportunity to study a Boreal alluvial succession in detail. The results of palaeoecological and sedimentological analyses (diatoms, pollen, botanical macro‐remains, molluscs, grain size) characterize the biotic and physical environment in the middle reach of this medium‐sized river system. Although the Early Holocene in the Scheldt Basin has often been portrayed as a period of fluvial stability with marshy conditions and diffuse discharge, this study showed evidence of point bar formation by a small, low‐energy meandering river between ~9.5 and ~8.8 cal. ka BP. The point bar was at least temporarily vegetated and shows a shift from herbaceous riparian vegetation to an open willow‐dominated alluvial forest. This evidence points to a more open vegetation and a more energetic environment than traditionally described for rivers of this size and age. A link to the 9.3 ka BP cooling event is suggested and possible reasons for the scarcity of records of this type of deposits are discussed.

Dam reservoirs as an efficient trap for historical pollution: the passage of Hg and Pb through the Ohře River, Czech Republic

The concentration of Hg and Pb pollutants was examined in sediments of a central European lowland river, the Ohře River. The pollution originated from several sources that were active mainly between the thirteenth and twentieth centuries. Its passage through the river system has been altered by two dam reservoirs built in the 1960s. The pre-dam pollution levels were evaluated from historical floodplain and dam reservoir sediments. We evaluated influence of damming on pollution status of sediments using the concept of ambient background concentrations based on the examination of polymodality in the distribution of concentrations and identification of breaks between the concentration modes. The Skalka Dam Reservoir has suppressed the transport of Hg pollution from a chemical factory in Marktredwitz, Germany (nineteenth and twentieth centuries): the dam has decreased the Hg concentrations in the downstream channel sediments by an order of magnitude compared to pre-dam conditions. The concentration of Hg in suspended particulate matter in the outflow from the Skalka Reservoir varies between 10 and 60% of the inflow concentrations depending on the discharge. The Nechranice Dam Reservoir has suppressed the passage of Pb pollution originating from historical metal mining, which has led Pb concentrations in sediments downstream to decrease to the levels of average topsoil pollution in the sub-catchments without ore mining. Both reservoirs act as sediment traps and thus play a beneficial role for the quality of the Ohře River downstream environment.

The Volga: Management issues in the largest river basin in Europe

AbstractThe Volga is the longest river in Europe and 16th longest in the world. The riverine landscape of the Volga is of exceptional scientific and economic importance to Russia; the basin contains approximately 40% of the Russian population and relates to 45% of the country's industrial and agricultural produce. The Volga River drains an area of 1.4 million km2, covering various biomes from taiga to semidesert. Anthropogenic impacts in the 20th century include pollution as well as hydropower production and navigation purposes, incurring a cost for its historically important migratory fish (e.g., sturgeons) and related fisheries. River basin management in Russia, since 2006, is based on the water code that determines federal competencies in water management. Extensive water quality monitoring programmes provide feedback to regional managers. Monitoring of biological parameters is spatially limited and should be extended in order to provide sufficient data for informed management. Some initiatives have been implemented in recent decades in order to restore the ecological health of the river and manage fisheries resources (e.g., restocking programmes and the definition of total allowable catches). As recreational fishing is popular but presently unregulated in Russia, we suggest additional monitoring. Finally, the headwaters and lower river floodplain of the Volga have remained as free‐flowing and relatively undisturbed systems. Because reference conditions with low levels of anthropogenic disturbance cannot be found in Central European lowland rivers, both the headwaters and lower Volga floodplains below Volgograd are of great importance on European level.

Biomechanical properties of aquatic plants: The effect of test conditions

AbstractFour aquatic plants (i.e.,Potamogeton pectinatusL.,Potamogeton crispusL.,Myriophyllum spicatumL., andCeratophyllum demersumL.) that commonly grow in European lowland rivers and lakes and exhibit a variety of morphologies and differences in the internal structures of their stem cross sections were selected to investigate the influence of initial conditions on biomechanical tests. A new method of sample testing in wet conditions is proposed and employed using a bench‐top testing machine. The obtained biomechanical characteristics, such as breaking strain, force, and stress; Young's modulus; flexural strain; flexural rigidity; and flexural modulus are presented and discussed. Even when fresh specimens were kept in water before testing in air, their biomechanical parameters were sensitive to fast drying. If the turgor pressure was not maintained in dry tests, specimen stiffness was not preserved and the measured biomechanical properties deviated from those observed in wet conditions. Approximately 43% of the three‐point bending tests and 20% of the tension tests showed significant differences between dry and wet conditions in all considered plant species. Bending tests for both conditions performed onP. pectinatusL. andP. crispusL. showed the highest differences in flexural rigidity values, reflecting the adaptation of these plants to changing hydraulic conditions. Stem resistance to tension forces in terms of Young's modulus was found to be different for wet and dry conditions only forP. pectinatusL. Overall, it was revealed that plants constantly submerged in water tend to be stiffer than plants exposed to dry conditions.

Spatial distribution of organic pollutants (PAHs and polar pesticides) in the floodplain of the Ohře (Eger) River, Czech Republic

The spatial distributions of organic pollutants, polycyclic aromatic hydrocarbons (PAHs), and polar pesticides were studied in the floodplain of a Middle European lowland river (the Ohře River, Czech Republic) to evaluate their possible use for pollution chemostratigraphy. The floodplain evolution and internal structure were evaluated from historical maps, a digital terrain model, and electric resistivity tomography (ERT) at sampling sites. The approximate age of the sediments was inferred from pollution by Cu, Pb, and Sn (sixteenth century and younger), as well as Hg and U (nineteenth to twentieth centuries), based on the known history of metal mining in the Ohře catchment. Polycyclic aromatic hydrocarbons, as non-polar persistent pollutants, have a stable stratigraphic distribution, with peak concentrations attributed to the local peak in the use of coal under low environmental concern from the middle nineteenth century to the late twentieth century as well as a minor peak due to more ancient charcoal production. Polar pesticides range from partially migrating (e.g. atrazine, simazine, and chloridazon) to very mobile ones with depth profiles that are not related to the sediment stratigraphy (e.g., chloroxuron and acetochlor). The post-depositional pollution maxima of the latter are near the contacts of more permeable channel sands and overlay finer, less permeable deposits near the accumulation of mobile elements (Ca, Mn) and minor secondary maxima for Hg and U pollutants. Polycyclic aromatic hydrocarbons can be used for pollution chemostratigraphy; the use of more mobile polar pesticides is limited. The occurrence of pesticides in the forested channel belt (not used for agriculture) and their seasonal variation document considerable dynamics of polar pollutants in fluvial systems.

Identification and interaction of multiple stressors in central European lowland rivers

Interactions of multiple stressors in lotic systems have received growing interest and have been analysed in a growing number of studies using experiment and survey data. In this study, we present a protocol to identify, display and analyse stressors of rivers and their interactions (additive, synergistic or antagonistic). We used a dataset of 125 samples of central European lowland rivers comprising hydromorphological, physico-chemical and land use stressor and pressure variables as well as benthic macroinvertebrate traits as biological response variables. To identify and visualise multiple stressor combinations jointly operating in the data set, we applied social network analysis. The main co-occurring stressor combination was fine sediment accumulation (hydromorphological stress) and enhanced phosphorus concentration (nutrient stress). Agricultural (cropland) and urban land use were identified as the main large scale environmental pressures. Stressor interactions were analysed using generalised linear regression modelling (GLM) including pairwise interaction terms. Altogether, 14 macroinvertebrate response variables were tested on six stressor combinations and revealed predominantly additive effects (80% of all significant models with absolute standardised effect sizes >0.1). Significant antagonistic and synergistic interactions occurred in almost 20% of the models. Fine sediment stress was more influential and frequent than nutrient stress. The methodology presented here is standardisable and thus could help inform practitioners in aquatic ecosystem monitoring about prominent combinations of multiple stressors and their interactions. Yet, further understanding of the mechanisms behind the biological responses is required to be able to derive appropriate guidance for management. This applies to rather complex stressors and pressures, such as land use, for which more detailed data (e.g. nutrient concentrations, fine sediment entry, pesticide pollution) is often missing.

Invasive Ponto-Caspian gobies in the diet of piscivorous fish in a European lowland river

Between 2012 and 2013, we estimated predation of non-native Ponto-Caspian gobies by native piscivores in a European mid-sized lowland river by analysing 268 digestive tracts from six native species: perch Perca fluviatilis, burbot Lota iota, European catfish Silurus glanis, pike Esox lucius, Volga zander Sander volgensis and European zander S lucioperca. Species closely associated with the rip-rap bank had a higher proportion of gobies in their diet (burbot 66 % weight proportion [%m], >= 1+ perch 53 %m, = 1+ perch showed positive selection for tubenose goby Proterorhinus semilunaris, while burbot and pike showed a preference for round goby Neogobius melanostonius, though the values may have been affected by the relatively low sample sizes. We estimate that predators consumed approximately 52 % of goby biomass over our study stretch each year (burbot 41%, <= 2+ European catfish 5 %, pike 3 %, all other predatory species <2 % each), confirming that gobies rapidly become an important component in the aquatic food web of invaded rivers, both by preying on aquatic invertebrates and as prey to native predators (particularly burbot). On the other hand, our data suggest that the long-term impact on European goby populations, at least in our study area, has been limited, as indicated by their ongoing spread.

Groundwater vulnerability based on four different assessment methods and their quantitative comparison in a typical North European Lowland river catchment (the Pliszka River catchment, western Poland)

The assessment of groundwater vulnerability is a crucial part of planning and water management because it can identify areas where aquifers are more susceptible to contamination. Depending on the vulnerability assessment method, the results can differ significantly. Consequently, different methods can provide ambiguous information that could further influence decision-making processes in planning or water management. For the Pliszka River catchment, the intrinsic groundwater vulnerability of the uppermost aquifer was estimated using four different methods: DRASTIC, GOD, and two methods that are based on empirical formulas of water residence time in an unsaturated zone. The input data include a series of thematic maps supplemented by 1,322 shallow borehole profiles and laboratory tests of samples collected in the course of fieldwork. The collected data were processed in GIS software, and the results of each method were mapped in high resolution. The resulting maps of groundwater vulnerability were then quantitatively compared to validate their applicability for the assessment of groundwater vulnerability in a typical North European Lowland river catchment. The maps generated by the DRASTIC and GOD methods are dominated by areas with moderately high (54.6 and 48.4%, respectively) and moderate groundwater vulnerability (32.7 and 32.3%, respectively). No areas of high groundwater vulnerability are present. One of the water residence time methods provides results similar to the previous methods at the catchment scale, and one method yields high groundwater vulnerability values for the majority of area.

Recovery of fish fauna in the upper Warta River, Poland: Long-term (1963–2012) monitoring study

The Warta River (795km long) is the largest, right side tributary of the Odra (Oder) River. The study presents results from one of the best-documented long-term monitoring projects in Poland based on four terms of electrofishing: 1963–66, 1986–88, 1996–98 and 2011–12, conducted in the upper Warta. The Warta River underwent human-induced modifications typical for most European lowland rivers (damming, regulation, water pollution), of which the most destructive for fish was point-source water pollution. In the late 1980s, pollution reached its highest level and stopped increasing as the former political system collapsed and many industrial plants went bankrupt. Surprisingly, recovered fish assemblages were not recorded during the sampling in 1996–98, but in 2011–12. This is why we believe that in large degraded rivers, it takes about 10–20years before a considerable improvement in fish fauna can be observed. Ichthyofauna recovered to a good status, but was qualitatively different compared to the good status observed in the 1960s. On the one hand, in 2011–12, high species richness and high assemblage diversity were observed, and many species, including seven rheophils, were more common than earlier. On the other hand, the populations of catadromous eel and anadromous vimba have not recovered, and these species were absent in the 2011–12 samples. Because water quality has improved, the most important factor seems to be the impact of the Jeziorsko dam reservoir which is located downstream of the study area and has no fish pass.The above patterns were recognised in this paper with a Kohonen artificial neural network, which is a tool methodically correct for analysing complex non-linear relations. Additionally, indicator species analysis allowed for the identification of significant associations of taxa with specific environmental states and was helpful in determination of ecological statuses of the river stretches. We therefore recommend the combined use of Kohonen artificial neural networks and indicator species analysis in long-term monitoring analyses.