The Danube River Basin

Nineteen countries share the Danube catchment area, making it the world’s most international river basin. Given the number of the countries and the diversity of social, political and economic conditions, the transboundary river basin management is of supreme importance in the Danube River Basin. The Danube River Protection Convention signed in 1994 is the legal instrument for cooperation and transboundary water management, and it led into establishing the International Commission for the Protection of the Danube River (ICPDR). In reaction to the requirements of the EU Water Framework Directive and of the EU Floods Directive, the Contracting Parties of the ICPDR committed themselves to use the ICPDR as a platform for implementing these directives in the Danube River Basin District and for coping on a basin-wide level with the key pressures related to organic pollution, pollution by nutrients and hazardous substances, hydromorphological alterations, flood protection, navigation, hydropower, sediment management and groundwater management. The ICPDR established the Transnational Monitoring Network which regularly monitors water quality in the Danube River Basin as well as the Danube Accident Emergency Warning System which alerts the Danube countries in case of transboundary pollution accidents. The first Danube River Basin Management Plan was published in 2009, and it set the programme of measures with the view of reducing the pressures on the surface and groundwater. At present the first Danube Flood Risk Management Plan is under finalization focusing on flood prevention, protection and preparedness taking into account the environmental objectives of the EU Water Framework Directive. Of high importance is also the cooperation with the other sectors such as navigation and hydropower aiming at sustainable economic development while avoiding the adverse effects on the water status. Using a synergy between implementing the Convention and the current EU legislation, a significant progress has been achieved in ensuring the protection and improving water quality in the Danube River Basin.

The assessment of pollution by nutrients in the Danube River has a long-term history at the basin-wide level, especially for estimating the influx of these substances to the Black Sea. The main aim of this chapter is to provide a general overview of the nutrient levels in the Danube Basin based on the data collected in the frame of long-term Trans-National Monitoring Network (TNMN) of the International Commission for the Protection of the Danube River (ICPDR) during 2001–2009. For selected monitoring sections, the dependence of the nutrient concentrations on corresponding daily discharges is also investigated. A comparative view of the surveillance TNMN data and investigative data obtained during the two monitoring programs known as Joint Danube Surveys 1 and 2 (JDS1 and JDS2) is presented. In order to get a general overview of the nutrient levels over the studied period, the temporal trends were analyzed using nonparametric Spearman correlation coefficient before and after removing the impact of the daily discharge on the measured concentration.

Heavy metals were identified as relevant pollutants of the Danube River some decades ago. This chapter reviews and evaluates the concentrations of heavy metals measured in the Danube and its tributaries by the monitoring activities of the International Commission for the Protection of the Danube River (ICPDR) – the TransNational Monitoring Network and Joint Danube Surveys 1 and 2.

Following the requirements of the European Water Framework Directive (WFD), a process of selecting pollutants relevant at the river basin scale started in 2001. In the Danube river basin, the process was aided by two Joint Danube Surveys (JDS1 and JDS2) organised by the International Commission for the Protection of the Danube River (ICPDR) in 2001 and 2007, respectively. This study was retrospectively analysing all data on organic substances identified in the water samples collected within the two surveys and comparing them to the latest Environmental Quality Standards (EQSs) as well as ecotoxicological threshold values (Predicted No Effect Concentrations; PNECs) that were not available at the time of writing the JDS1/2 Final Scientific Reports. The results showed that 26 out of 89 substances detected in the samples exceeded the EQS/PNEC values in at least one sampling site and 53 substances were found above their limit of quantification (LOQ) at more than five sampling sites within the basin. The above-mentioned 26 substances deserve closer attention as candidates for the list of Danube River Basin Specific Pollutants (DRBSPs).

A novel approach of ranking gas chromatography–mass spectrometry (GC–MS) nontarget screening data, based on the assessment of (1) available literature PNEC values (19 substances), (2) derived provisional PNEC (P-PNEC) values (160 substances) and (3) estimated concentrations of tentatively identified substances, has been applied too. Sixty-five out of a total of 179 compounds identified in the JDS samples exceeded the ecotoxicological threshold value in at least one sampling site, which makes them potential candidates for inclusion into future investigative monitoring schemes.

Since its publication in the year 2000, the EU Water Framework Directive (WFD) became the most important legal act for water protection not only within the European Union but also in the Danube River Basin. In its strategy against water pollution, the WFD identifies priority substances (PS). PS are hazardous chemical compounds forming a special threat to the quality of surface waters. The goal is to reduce concentrations of all PS at least below substance-specific environmental quality standards (EQS). EQS are concentration limit values derived on the basis of ecotoxicological substance data and additional information. In the Danube River Basin, the level of contamination of the Danube and its tributaries by PS was investigated within the monitoring activities of the International Commission for the Protection of the Danube River Basin (ICPDR). Especially the results of ICPDR’s research expeditions in 2001 and 2007, the Joint Danube Surveys, revealed the exposure situation for PS in different aquatic matrices. For the subgroup of organic PS, widespread pollution problems with partial exceedance of the respective water EQS were found for nonylphenol, a decomposition product of surfactants, the plasticizer di(2-ethylhexyl) phthalate, and tributyltin compounds, formerly used in antifouling paints for ships. The mostly banned pesticide atrazine could also be found in many water samples. For all other PS, only local problems were identified or they have not been detected at all. The results for suspended particulate matter, sediment, and biota support the findings above.

During the second joint Danube survey (JDS 2) in autumn 2007, water, sediment, suspended particulate matter and mussel samples were collected from 23 sites covering the River Danube and important tributaries from Germany until the Black Sea. The compound classes investigated were polychlorinated dibenzodioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), organochlorine pesticides (OCPs) and polyaromatic hydrocarbons (PAHs).

The results revealed no exceeding of the environmental quality standards (EQS) according to the Directive 2008/105/EC for all investigated compounds except the ∑benzo(g,h,i)perylene and indeno(1,2,3-cd)pyrene, where the concentrations at most sites were close to the EQS of 2 ng/L. In five sites the EQS were slightly exceeded, with a maximum concentration 3.1 ng/L close to Bratislava.

OCP concentrations in water were orders of magnitude below the EQS except for HCH that reached levels up to 25% of the EQS in the lower Danube. Maximum PBDE concentration in water was at 20% of the EQS.

The longitudinal concentration profiles in water and sediment suggest DDT, HCH and to a lower extent chlordane and heptachlor releases into the lower Danube originating from left bank sources and tributaries especially Arges, Siret and Prut. PBDEs showed a maximum in the middle Danube stretch impacted from releases from the right bank tributaries such as Drava, Sava and Velika Morava.

Concentrations in the investigated compartments were generally at the lower end of the concentration ranges typically found in European freshwaters.

The occurrence of alkylphenolic compounds along the Danube River revealed a ubiquitous fingerprint of wastewater impact, recorded in various extents and being the most prominent in the main tributaries and side arms, as well as in vicinity of industrial areas and some Danubian capitals.

As revealed by the Joint Danube Survey 2 (JDS2) in 2007, there was a significant decrease in nonylphenol and octylphenol levels in both sediments and suspended particulate matter (SPM) compared to the findings of the Joint Danube Survey 1 (JDS1) in 2001, validating the effects of the EU regulations.

Nevertheless, the JDS2 results showed that the inputs of untreated or insufficiently treated wastewater mostly from metropolitan and industrial areas are still large enough to (occasionally) cause nonylphenol concentrations above environmental quality standards (EQS) for freshwater sediments.

Nonylphenol mono- and diethoxylates (NP1EO and NP2EO) often coexist with nonylphenol in sediments and SPM in comparable concentrations, which may induce additive mixture effects on Danube biota.

Given that there are no EQS for alkylphenolic compounds in SPM, it is difficult to estimate potential risks that SPM-linked contamination may exert on Danube biota. Slight nonylphenol accumulation in mussels was evident at the sites where nonylphenol levels in SPM were continuously high.

Based on the JDS2 findings, octylphenol and its lower ethoxylates rarely occur and in low concentrations, thus appear to be of no concern for the Danube environment.

Nonylphenol and nonylphenoxyacetic acid (NPE1C) were frequently found in water during the JDS2, exceeding the valid (or proposed) EQS for freshwater in some tributaries. Which possible additive or synergic effects these two compounds may have on aquatic organisms remains however unclear.

The results of the Danube surveys highlighted the necessity of reduction of untreated wastewater discharges, especially in areas where alkylphenolic compounds exceeded EQS, in order to protect quality and environmental conditions of the Danube River.

Several analytical methods are used to measure petroleum hydrocarbons contamination in the environment. Each method provides different, specific information about the characteristics of the contamination. Only the results obtained with a particular analytical method can be used for a comparative study or a pollution trend analysis. The polluting aromatic hydrocarbons can be characterized in terms of fluorescence patterns; the contamination level/concentration can be calculated from the fluorescence intensity at specified excitation/emission wavelengths.

Interpretation of the fluorescence fingerprint of cyclohexane extracts of water, SPM, and bottom sediment samples, collected during the Joint Danube Surveys, as well as the results of the PAH analysis provided the following findings: (1) petroleum hydrocarbons in water were characterized by the fluorescence of gasoline; the concentrations varied in the range of 2–300 μg/L; (2) the level of oil contamination was similar in the SPM and the bottom sediment, characterized with the fluorescence of crude oil, and the concentrations varied between 5 and 500 mg/kg; (3) PAH determined in water, SPM, bottom sediment, and biota (mussels) showed similar trends in contamination as observed in the case of petroleum hydrocarbons. However, even the highest concentrations were usually below the EQS values according to the Directive 2013/39/EU, or the PELs in the Canadian Sediment Quality Guidelines.

The implementation of the EU Water Framework Directive and the EU Groundwater Directive and the reporting there under give a very good overview of those hazardous substances which are of considerable concern in the Danube River Basin. Thirty-two hazardous substances could be identified of definitely causing considerable pollution of groundwater in the Danube River Basin as they are causing poor chemical status of at least one groundwater body.

The establishment of groundwater threshold values for 72 hazardous substances also indicates that these substances are either already causing significant pollution or are reasonably suspicious of bearing potential to significant pollution. As threshold values are established on a risk-based approach at national, river basin or groundwater body level, considerable variations are evident within the Danube River Basin District. Additionally, national legislations identifying those substances which have to be prevented from entering groundwater according to Article 6 of the EU Groundwater Directive give strong indication of further hazardous substances being relevant.

This article gives an overview on hazardous and emerging substances in several European streams and compares and discusses actual findings from the Danube. Concentrations of priority pollutants, pesticides, pharmaceuticals, industrial chemicals, and artificial sweeteners are mostly lower in the Danube and its tributaries than in the Rhine River. However, tributaries with low discharge and a high portion of wastewater or industrial emissions may strongly contribute to the overall pollution of the Danube and finally the Black Sea. Direct use of surface water without advanced treatment or indirect use of bank-filtrated water with short retention times during subsurface passage is common in parts of the Danube catchment to prepare drinking water. However, due to the comparatively low concentrations of pollutants, drinking water production at the Danube is currently not endangered.

In this chapter, a general review of radioecological research and exemplarily results of radioactivity measurements carried out in the Danube freshwater ecosystem in the last 30 years are presented. Sample collection techniques and sample preparation and radiometric measurement methods, developed and applied in radioecological studies of the Danube River, are shown comprehensively. Results of radiometric analysis of bottom sediment samples, collected continuously by sediment traps and additionally by grab sampling during Danube research cruises, are given and discussed. The main goal of the radioecological research studies is the protection of the environment to manage sustainable use and conservation of the Danube freshwater resource against harmful radioactive exposure.

This article gives a rough overview on the occurrence and distribution of selected benthic invertebrates along the Danube River. The description of the benthic community within typological units of the Danube is based on the results from the Joint Danube Surveys. Species richness and abundance illustrate the structure and dominant groups of the benthic community. Furthermore the role of environmental impacts like hydromorphological changes, pollution, navigation as well as neozoa is shortly addressed and highlighted. In this context a conceptual framework of the multi-stressor complex of large rivers is introduced and discussed. Finally the biodiversity losses of selected species are reflected on a European scale.

Benthic algal flora of the River Danube is presented with implications for ecological status assessment. Structure of algal biofilms, species diversity, algal abundance, and biomass are described and discussed based on most recent algal investigations supplemented by methodological insight to community structure evaluation. Comparisons of literature data are provided. Seasonal and longitudinal changes of benthic algal assemblages are evaluated in terms of species abundance and biomass as well as community structure. In contrast to previous studies of Danubian periphyton that detected prevailing diatom abundance in the biofilms, recent research has found that cyanobacteria and green algae dominated almost along the whole Danube stretch. Ecological status of the entire Danube stretch is evaluated by means of the diatom-based “Indice de Polluosensibilité Specifique” (IPS), which showed distinct differences between the upper and middle section of the River Danube indicating longitudinal increase of general degradation of aquatic environment and increasing nutrient concentrations. The overall indication of ecological status varied between good and moderate.

Recording and assessment of aquatic macrophytes was a request for the Joint Danube Survey 2 (JDS2). New insight regarding occurrence, abundance and specific distribution of macrophytes was based on methodological adaptations better adjusted to the size of this large European river and permitted more appropriate statistical interpretation. Regarding the ecological status of sampling stretches, an intentional, preliminary way of interpretation is provided, respecting trendsetting new international literature. Due to longer river stretches recorded, a higher number of species was detected in JDS2. Each of the ten official river sections showed an individual character of the macrophyte vegetation. Results of JDS2 macrophyte survey are put in relation with international literature and side effects are discussed, which are of relevance when assessing macrophytes in large rivers for purposes of science, European Water Framework Directive or regarding conservation issues.

The Danube is a river with the highest fish species richness (102 species ever reported) in Europe. Nevertheless, it is also a river that faces various human pressures with serious negative impacts on its ecosystems, including fish communities. In this chapter, data from both the Joint Danube Survey 2 (2007) and the Gabčíkovo Hydroelectric Scheme Monitoring (1991–2011) are reanalysed briefly (data from JDS3 - 2013 are not included). A total of 69 species of fishes were recorded within the recent surveys of the Danube, a number that still suggest a high diversity of the Danubian fish community. However, as many as 12 of these species were not native in the Danube, at least not in its whole course, and a total of 18 non-native species have been ever recorded in the Danube. Concerning native species, cyprinids, especially bleak, highly predominated along the whole course of the Danube, though invasive species, such as gobies in the Upper and Middle Danube and gibel in the Lower Danube, were found to be extremely abundant. Biological invasions not only indicate deterioration of environments but also may result in an overall decline in biodiversity. Therefore, a predictive risk assessments and management strategies for introductions and invasions of non-native fishes should be developed for the Danube and applied subsequently at an international level. Human impacts on fish communities of the Danube are also briefly illustrated, with the Gabčíkovo Hydroelectric Scheme used as an example.

Invasive alien species (IAS) have been recognized as one of the major threats to native biodiversity in the Danube Basin. The aim of this paper is to present the state of the art regarding IAS along the Danube River and its main tributaries. The work is mainly based on the results of the Danube research expeditions, Joint Danube Survey 2 (2007), Joint Danube Survey 1 (2001) and AquaTerra Danube Survey (2004), but other recent data on IAS were taken into consideration, as well. The complexity of the problem with IAS could be illustrated by the fact that six species of neophytes, 19 alien macroinvertebrates and 15 non-native fish species were recorded during JDS2. The total number of alien species recorded, as well as their frequency and abundance along the Danube, indicates high level of biological contamination. Despite the fact that IAS have been in the focus of the research in the Danube Basin for the last 15 years, we still do not have enough data on their exact distribution and migration. A lot of additional work concerning detection, monitoring, assessment of their impacts and management is necessary in order to deal with the IAS problem properly in river basin management planning.

Investigations on river phytoplankton in the Danube are summarised and placed into a historic perspective. Phytoplankton species composition always has been dominated by diatoms, particularly centric taxa. Longitudinal, seasonal and long-term dynamics are described and their implications are discussed. Factors responsible for the wax and wane of phytoplankton growth in the middle section of the river Danube are analysed and discussed. Survival, growth and production of phytoplankton in the Danube and in large rivers in general are then incorporated and integrated into the existing fundamental concepts of riverine ecosystems.

The EU Water Framework Directive demands the good ecological status in all surface waters within the time frame 2015–2027. The status is monitored by the member states using national sampling and assessment methods designed after the requirements of the WFD and adjusted in an international intercalibration process. In the implementation process, still gaps and uncertainties exist. For solving the open issues, more data and research is necessary; often countries could use approaches developed by other EU member states.

Fecal microbial pollution is a major problem throughout the Danube River Basin, posing a threat to various types of water use, including drinking water production from river bank filtrates, water supply for agricultural and industrial use, and the role of the river as a recreational space. Fecal microbial pollution is introduced into the river by point sources, such as discharges of treated or untreated sewage from human sources or livestock, and by nonpoint sources, such as urban and agricultural runoff. In addition, fecal input from wildlife may be of importance in specific regions. Despite huge efforts to improve wastewater management in the past decade, in many sections, the river and its tributaries exhibit very high levels of fecal microbial pollution. To assess microbiological water quality, indicators of fecal pollution are used as surrogates for the potential presence of intestinal pathogens. However, the standard indicators cannot provide any reliable information regarding the origin of fecal pollution, nor can their concentration levels be directly related to human health risks for many types of exposure and situations.

The aim of this book chapter is to summarize the historical developments in microbiological water quality research and to reflect the most recent publicly available data on the fecal microbial pollution status of the Danube River. Moreover, the first results on fecal microbial source tracking by molecular biology methods are presented along with their applicability in river water quality monitoring, including the monitoring of riparian wells and alluvial groundwater resources. Finally, a discussion of the general state of water quality and public health is presented concerning (i) the current situation and potential limitations of the Water Framework Directive regarding the microbiological quality elements, (ii) further improvements regarding sampling and monitoring strategies, and (iii) the recently introduced concept of “integrated framework of fecal pollution monitoring and management” and expected further methodological developments in the context of the Danube watershed. Rapid progress in research and development is currently being made in the area of fecal microbial source tracking, pathogen detection, and health risk assessment, and these innovations are also likely to complement basic fecal pollution monitoring programs for river systems such as the Danube in the near future.

Hydromorphology describes the physical and hydrological characteristics of rivers and its habitats including the underlying processes from which they result. Hydromorphology is a supplementary but mandatory element of WFD ecological assessment, and hydromorphological alterations were recognized as one of the most important river management issues across Europe. Hydromorphological assessments try to integrate and provide information on how far the conditions derive from pristine conditions (so-called hydromorphological reference conditions). The ICPDR Joint Danube Survey (JDS) 2 in 2007 delivered results on hydromorphological alterations for the navigable Danube River (from Kelheim (rkm 2,416) to the Danube Delta) for the very first time. A five-class assessment similar – but not equal – to the WFD ecological status classes was implemented according to European standards and methodological approaches for large rivers using the three main categories (1. channel; 2.banks; 3. floodplains).

Hydromorphological alterations of large rivers are evident and have to be related to multiple anthropogenic pressures. The presented results of an integrated study concerning the actual status of the hydromorphology of the Danube River Basin show that in particular, the sediment regime features a heavily disturbed system at various scales. Combined impacts of flood protection, navigation and hydropower measures applied over a long period of time have been identified based on the river-scaling concept (RSC) for being responsible for these specific alterations (lack of bed load and suspended load in the remaining free-flowing sections). Moreover, long sections of the Danube River have been narrowed, channelized, disconnected from floodplains and morphologically degraded over the last 200 years. This has caused increased bottom shear stresses, increased sediment transport capacities and in addition a lack of lateral self-forming processes and corresponding reduced morphodynamics in the non-impounded sections. As a consequence of both longitudinal and lateral disturbances of the sediment supply and additional impacts of the channelization, the remaining free-flowing sections are subject to various forms of river bed degradation. Such degradation or river bed incision leads to a loss of instream structures in general, with a disappearance of gravel bars at the Upper Danube, and changes of sandbars at the Lower Danube. Hence, for river systems and large river basins, it has to be stated that the preservation and restoration of morphodynamics is one of the most relevant issues for river engineering and ecology. This has to be considered especially for the implementation of legal directives and/or future river basin management plans.

Meeting Danube Basin monitoring and management objectives such as those implied by the EU Water Framework Directive requires a comprehensive understanding about the hydrological and biogeochemical functioning of not only the river system but also the connections between groundwater and surface water across the basin. While hydraulic and geochemical measurements can provide some of this understanding, it is often difficult to obtain knowledge about some of the more critical aspects of basin functioning or it can take decades of intensive monitoring before adequate interpretations can be made. Isotope hydrology approaches can often provide critical insights on surface water/groundwater interactions and biogeochemical cycling with only moderate effort and cost. Such information can help clarify local behaviors as well as overall basin responses. Approaches using “environmental” stable and radioactive isotopes (i.e., isotopes that are already in the environment and not intentionally applied) have been used to understand sources and losses of water in the Danube, the importance of groundwater discharge, basin residence times, tributary mixing, and nitrate cycling using isotope methods. We review existing studies as well as present new isotope data that reveal important spatial and temporal dynamics occurring in the Danube River, tributaries, and across the basin.