Management challenges related to long‐term ecological impacts, complex stressor interactions, and different assessment approaches in the Danube River Basin

For centuries, rivers have experienced massive changes of their hydromorphic structures due to human activities. The Danube River, the second largest river in Europe, is a case in point for long‐term societal imprint. Resulting human‐induced pressures are a key issue for river management, aiming to improve the ecological conditions and guarantee the provision of ecosystem services. As the most international river basin in the world, the management of the Danube is particularly challenging and needs a well‐organized cooperation of 19 nations. The recent river basin management plan has identified pollution and hydromorphological alterations as most pressing problems, but it has also acknowledged newly emerging issues. In this article, we present 3 specific examples of highly relevant issues for the future river basin management of the Danube: (a) long‐term impacts in the catchment such as changes in flood patterns and potential ecological consequences; (b) complex feedback loops linking the spread of neozoa with intertwined stressor responses due to river engineering for different purposes; and (c) linkages between different assessment approaches based on European legal frameworks to analyse the specific pressures at different spatial scales. These examples highlight the need for a more integrated approach in future Danube River Basin management schemes. Furthermore, large‐scale effects such as climate change and interactions of multiple pressures need to be addressed in future management to increase resilience of the river system and to allow a sustainable ecosystem‐based management of rivers.

urbanization, hydropower generation, and navigation date back to medieval times or at least to the early industrialization period. During the 20th century, human activities and civil engineering operations have caused river ecosystem fragmentation and habitat destruction and have modified dynamic system properties, such as the flooding regime; the longitudinal, lateral, and vertical connectivity; biogeochemical cycles; and biodiversity patterns (Gergel, Carpenter, & Stanley, 2005). Consequently, riverine ecosystems are among the most endangered ecosystems worldwide (Fryirs & Brierley, 2016).
The Danube River is such an example of a multiple-stressed, highly vulnerable riverine system, which still shows a high ecological potential despite its long-term exposure to socio-economic usage. Ongoing, partly conflicting demands within and among the different neighbouring countries, inconsistencies in legislation, and drivers of change aggravate the problem of a joint, sustainable management further. This article presents both changes of key ecosystem properties of the Danube River and management strategies for the Danube River Basin (DRB) to underline challenges for future ecosystem-based management. In particular, we focus on long-term changes in the hydrograph and the discharge regime of the Danube River as well as on the historic development of human activities, such as hydropower generation and navigation, and related changes in the dispersal of alien species. Furthermore, we highlight the interplay of different pressure assessment methods in relation to protected areas along the navigable stretch of the Danube River. The presented examples can be of valuable support in the sustainable management of river basins in Europe and around the world.

| CURRENT SITUATION IN THE DANUBE RIVER BASIN
The DRB is the most international river basin in the world, shared by 19 nations and draining a catchment of 807,827 km 2 in Central and South-Eastern Europe ; Figure 1). The Danube River flows from the Black Forest Mountains in Germany to the Black Sea and has a total length of 2,857 km, which is divided into three sections: the Upper (1,066 km), Middle (860 km), and Lower Danube (931 km; Figure 1; Sommerwerk et al., 2009). Exceptionally diverse ecological and socio-economic properties characterize the DRB (Sommerwerk et al., 2010). Its unique biodiversity and high ecological potential make the DRB one of the Earth's 200 most valuable ecoregions (Olson & Dinerstein, 1998). At the same time, the DRB is listed among the world's top 10 rivers at risk, mainly due to river engineering, navigation, pollution, and invasive species (Wong, Williams, Pittock, Collier, & Schelle, 2007 Figure 1). This has affected the sediment regime of the Danube considerably, leading to significant reductions of sediment transport in upstream sections and riverbed incision in downstream reaches . Point and diffuse pollution as well as the effects of land use changes have aggravated the ecological impacts of barriers, river channelization, and navigation. According to the ICPDR, less than 19% of the floodplains existing in the 19th century remain in the entire basin, equivalent to a loss of approximately 33,800 km 2 of floodplain area (ICPDR, 2009). Before main river regulations were conducted, the active floodplain width amounted to >10 km in the Upper and >30 km in the Middle and Lower Danube, respectively ( Figure 1). In the Upper Danube, most floodplains and fringing wetlands have been converted into agricultural and urban areas or have been isolated by dams and artificial levees and are, thus, hydrologically and functionally decoupled. However, along the Middle and the Lower Danube, large, near-natural floodplain areas still exist .

| LONG-TERM CHANGES IN THE FLOOD REGIME OF THE DANUBE
Both riverine ecosystems and ecosystem services largely depend on the flood regime. Repeated floods sustain dynamic hydrological conditions in both the river and its riparian zone, supply floodplain habitats with water and nutrients, and enable the migration of organisms from the main river to side arms and backwaters (Baart, Hohensinner, Zsuffa, & Hein, 2013;Bayley, 1991;Weigelhofer, Preiner, Funk, Bondar-Kunze, & Hein, 2015). The repeated drying and rewetting of floodplain habitats often results in increased aquatic productivity compared to stable systems in temperate and tropic regions (Bayley, 1991;Tockner, Malard, & Ward, 2000). However, the rapid decline of flood FIGURE 1 Map of the study area including upper, middle, and lower sections; Natura 2000 sites; and hydropower plants along the Danube River are presented. Countries are presented by the respective country codes peaks can cause stranding and subsequent dehydration of fish eggs and larvae in shallow spawning grounds, thus severely compromising the natural recruitment of fish stocks (Pintér, 1992;Welcomme & Halls, 2002). Fast water level changes, combined with high flood amplitudes, also expose semiaquatic and aquatic plants to submergence and desiccation stress to which they may be unable to adapt (Brock, van der Velde, & van de Steeg, 1987;Welcomme & Halls, 2002). This may easily affect diverse water plant communities in riparian areas (Brock et al., 1987;Zsuffa, 2001).
The natural river hydrograph of the Danube is characterized by two major flooding periods, one in early spring (March and April), caused by snowmelt in mountainous areas, and one in early summer (May and June), which is associated with prolonged rainfalls in the Upper Danube catchment (precipitation maximum). Low water levels usually occur in late summer and early autumn due to low precipitation and in winter when the precipitation is bound in the form of snow and ice. Flood pulses during spring and early summer are especially important for river biota as they enable the spawning of typical Danubian fish species, such as pike, carp, or other cyprinids, which depend on vegetation-rich habitats in floodplains (Keresztessy & Farkas, 2005;Pataki, Zsuffa, & Hunyady, 2013;Pintér, 1992).
In recent years, floods from snowmelt have increasingly coincided with early-summer precipitation (rain-on-snow phenomena), thus altering the magnitude and duration of floods especially along the Upper Danube (Lóczy, 2010). River regulation, catchment deforestation, and channelization have additionally affected the hydrograph of the Danube (Zsuffa sr, 1999). To quantify the magnitude of these The results show a decrease in mean, minimum, and maximum water levels of the Upper Danube. At Bratislava, for example, these decreases amount to 340, 300, and 140 cm, respectively, since 1878.
These changes are responsible for the severe loss of aquatic and semiaquatic habitats along the river (Zsuffa, 2001). In addition, the rate of the water level decline after flood peaks has increased during the past two centuries by a factor of more than two in the Upper Danube today. Likewise, differences between the highest and the lowest water levels have increased ( Figure 3). The increased variability of the hydrological regime has contributed to the decline of fish stocks and water plants (Pintér, 1992;Zsuffa, 2001).
The main reason for the overall decrease in water levels is riverbed incision, which is a general phenomenon in the Danube . Incision was triggered by increasing water velocities caused by meander shortcuts, side channel closures, and main channel trainings conducted in the 19th and 20th centuries . A chain of HPPs built along the German and Austrian Danube, especially in the second half of the 20th century, additionally contributed to channel incision, as the dams reduced bed load transport Reckendorfer et al., 2005). River straightening, reductions of floodplains, and decoupling of the remaining floodplains from Danube floods have severely intensified the dynamics of flood pulses in the Danube (Zsuffa, 2001;Zsuffa sr, 1999). Although current restoration efforts try to compensate some of the effects of former river regulation (Schmutz et al., 2014), the above-mentioned changes in both, the overall water supply and the flood dynamics, may be further aggravated by the ongoing climate change and soil sealing in the catchment (Beniston & Stoffel, 2014;Lóczy, 2010).   Means and standard deviations of the differences between highest and lowest water levels for the four periods Hein et al., 2016). The Danube River is the main southern migration route of the aquatic Ponto-Caspian fauna (Bij de Vaate, Jazdezewski, Ketelaars, Gollach, & Van der Velde, 2002). In addition, Danube, Main, and Rhine are connected to the North Sea through artificial navigation channels since the early 1990s. The severe ecological impacts of alien species dispersal in the Danube River was already elucidated by the Joint Danube Survey 2 (ICPDR, 2008), as among the 10 most frequent macroinvertebrate species, nine are considered as alien (Graf et al., 2008). The Joint Danube Survey 3 (ICPDR, 2015) confirmed these findings . Benthic assemblages of the Danube River are nowadays dominated by nonindigenous, invasive, or cosmopolitan species (Graf, Csányi, et al., 2015;Graf, Leitner, & Pletterbauer, 2015). The replacement of native-and in some cases-vulnerable taxa by rapidly spreading nonnative taxa will lead to spatially more homogenized assemblages with much lower diversity.

| DISPERSAL OF ALIEN SPECIES IN RELATION TO THE HISTORIC DEVELOPMENT OF HYDROPOWER GENERATION AND NAVIGATION
Alien species represent a biological response to human interventions in the river and act as stressors for native species. Alien species compete with the indigenous fauna for habitats and resources and, thus, may severely impair the entire functioning of aquatic ecosystems (Statzner, Bonada, & Dolédec, 2008). Due to their usually high densities, alien species may dominate the benthic community, act as bioengineers, and can intervene in the nutrient cycle (Nakano & Strayer, 2014). Hence, the establishment of alien species represents an irreversible shift in the ecosystem state. Restoration and rehabilitation measures may enhance habitat diversity and enable a coexistence of native and nonnative species. However, the extirpation of alien species seems unmanageable in the long term (Orendt, Schmitt, Liefferinge, Wolfram, & Deckere, 2009).
Vessels are considered as the main vectors of alien species, which are transported by ballast water and on vessel hulls. Regarding fish, fishery (including aquaculture) and animal trade were responsible for early invasions, whereas waterways are the main pathway for recent invaders, such as gobies . Changes of habitat conditions additionally favour the establishment of invasive species that may fill up available environmental niches after outcompeting impaired native populations. In the Upper Danube, for example, river regulation and damming for hydropower generation have severely impaired habitat conditions through morphological degradation and homogenization, hydrological alterations, and increased sedimentation upstream of dams (Banning, 1998;Reid, 2004). Damming technically complements navigation as it stabilizes flows, ensures controlled water levels, and supports a wide navigation line. In summary, these influences reduce habitat diversity, disturb sediment equilibrium, and induce adverse and short-term hydrological disturbances due to vesselinduced waves. The resulting irregular, high shear stress at the river banks is particularly negative for the sensitive land-water interface (Liedermann et al., 2014), which is essential for hemilimnic organisms during metamorphosis. Native taxa prefer organic and rather fine substrates, whereas alien FIGURE 4 Temporal development of the number of alien species (dark grey dots) including molluscs, peracarid and eucarid crustaceans, fishes, and reptiles in the middle Danube in Hungary (according to Bódis et al., 2012), the amount of transported goods by inland navigation in Austria (framed triangles), and the number of hydropower plants (HPPs) along the whole Danube River (grey rectangles) species favour larger, stabile substrates, especially rip-rap-dominated habitats (Borza, Huber, Leitner, Remund, & Graf, 2017;Graf, Csányi, et al., 2015). Riverbank restoration can create more lentic areas, such as shallow riverbanks with fine sediments and large woody debris accumulations, which support the distribution of the indigenous fauna (Graf, Hartmann, & Leitner, 2011).
Species distributions oscillated in the past and will show fluctuations and shifts in the future, but human activities support the dispersal of alien species and reduce dynamic processes that are inherent to natural ecosystems. The increasing occurrence of invasive species in combination with the loss of indigenous fauna in large rivers is a pressing problem, which needs further consideration at European-wide scale (e.g., Graf et al., 2008;Moog et al., 2008).

| EUROPEAN STRESSOR ASSESSMENT APPROACHES AFFECTING THE DRB
Currently, there are two main legal frameworks for assessing the ecological integrity of riverine ecosystems in Europe: the European Commission (EC) Water Framework Directive (WFD), which aims at restoring and maintaining a good ecological state of all surface and subsurface water bodies in the EC (EC, 2000), and the EC Habitat and Birds Directives (HBD), which focus on the protection and conservation of a wide range of natural habitats and endangered species to maintain biodiversity (EC Birds Directive, 2009a;EC Habitats Directive, 1992). Both approaches can be used for the assessment of key stressors in the DRB. However, their different focus and administrative spatial scale may yield divergent results in terms of stressor relevance.
Although in the WFD, human pressures on aquatic ecosystems are assessed at national level (ICPDR, 2016), the management of protected Natura 2000 sites according to the HBD is organized at local or regional level (Ostermann, 1998). In addition, the WFD assessment is this corresponds to about 70% of the navigable section of the Danube.
In general, the two different approaches, WFD and HBD, as well as the land use data showed high concordance, as land cover within the Natura 2000 sites changes along the course of the Danube. From Upper to Lower Danube, the proportion of cropland increases, whereas woodland and forest shares decrease (land use data; Figure 6). In accordance, agriculturally used areas (impacted land cover, WFD approach) and agricultural area expansion (HBD approach) within the riparian zones are higher in the Lower Danube than in the  1910-1950 1950-1980 1980-2010   There is also a high congruence of local (HBD) and catchment (WFD) approaches regarding hydromorphological alterations ( Figure 6).  Sustainable Use of the Danube River" from 1994, known as the "Danube River Protection Convention". This convention aims at achieving sustainable and equitable water management, including the ecological integrity, conservation, improvement, and sustainable use of surface and ground waters in the DRB (Chapman et al., 2016).
The main management targets are specified in the "Danube River For the conservation and protection of biodiversity, the integration of the EC Habitat Directive will be an important next step to avoid conflicting views in the future (ICPDR, 2016;Janauer, Albrecht, & Stratmann, 2015). In addition, strategic planning tools should be included in future integrated river basin management to help decision makers governing the implementation of the various EC directives (Seliger et al., 2015). Likewise, recent scientific findings have to be considered, which address the problems of reestablishing reference conditions (Dufour & Piégay, 2009) and restoring human modified ecosystems, including the notion of novel ecosystems or no-analogue communities (Hobbs et al., 2006;Williams & Jackson, 2007).
As highlighted by our three examples, long-term impacts in the catchment, complex feedback loops and interactions within and among different ecosystem components, and linkages between different assessment approaches require more attention in an integrated management of the DRB (Figure 7). Various stressors interacting over several decades to centuries have modified key ecosystem features of the Danube irreversibly. Thus, a sustainable management approach can no longer rely on historic reference conditions but requires new or modified targets for key ecosystem characteristics (Dufour & Piégay, 2009). Human uses of the DRB have not only intensified, as our examples of hydropower generation and navigation show, but also diversified over the past centuries. Previous interventions have caused long-term environmental legacies, and their adverse effects on biota may become visible with delays (Dullinger et al., 2013;Haidvogl, Hoffmann, Pont, Jungwirth, & Winiwarter, 2015). Modern management concepts can no longer exclusively focus on the mitigation of single stressors, as they often did in the past, but have to envisage stressor interactions and develop strategies to counteract multiple stressor effects (Nõges et al., 2016;Schinegger, Palt, Segurado, & Schmutz, 2016). At the same time, management schemes need to consider complex interactions and feedback loops and cannot limit their measures to single ecosystem components or species groups (Figure 7). For international rivers, such as the Danube, an additional challenge is the need for harmonized approaches at catchment scale, whereas both human uses and ecosystem management often act at regional or local scale and vary among nations (Seliger et al., 2015). Global climate change adds to the problem of sustainable river basin management as it requires urgent global actions, thus enlarging the spatial and temporal scale further (Pont, Logez, Carrel, Rogers, & Haidvogl, 2015).
Collaborative biophysical assessments and economic valuations could boost awareness and inclusion of the interdependence of nature and people for a sustainable management of water resources (Grizzetti, Lanzanova, Liquete, Reynaud, & Cardoso, 2016). Thus, for the future, explicit and well-defined ecosystem-based targets need to be formulated and adequate measures need to be defined to achieve more resilient ecosystems, guarantee the provision of a broad range of ecosystem services, and increase the resilience against emerging stressors and large-scale changes such as climate change in river basins.