AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. (2015) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/aqc.2585 Freshwater mollusc assemblages and habitat associations in the Danube River drainage, Hungary ERIKA BÓDISa,*, BENCE TÓTHa and RONALDO SOUSAb,c aMTA Centre for Ecological Research, Danube Research Institute, Karolina, Budapest, Hungary bInterdisciplinary Centre of Marine and Environmental Research (CIIMAR/CIMAR), University of Porto, Porto, Portugal cCBMA – Centre of Molecular and Environmental Biology, Department of Biology, University of Minho, Braga, Portugal ABSTRACT 1. Native freshwater mollusc diversity has been declining over the past decades. For developing efficient conservation management plans for molluscs, it is especially important to identify areas with high native biodiversity. 2. The River Danube is one of the most important freshwater ecosystems in Europe and should receive special attention. The main objective of this work was to characterize the composition, diversity and conservation status of freshwater molluscs occurring in the Hungarian Danube River Drainage in five river habitat types (main channel of the Rivers Danube and Tisza, side channels of River Danube, tributaries of Rivers Danube and Tisza). 3. In total, 53 mollusc species were identified including 10 invasive species and 12 species of conservation concern. The main determining factors of mollusc composition were sediment characteristics, current velocity and oxygen content. Nitrate–nitrogen, ammonium–nitrogen, and calcium content also play a key role. 4. Overall, density, species richness and diversity showed significant differences between habitat types. The highest diversity values were detected in the side channels, but the highest proportion of threatened species was observed in the tributaries. The highest number and proportion of invasive species was detected in the main channel of the Danube. Most of the recorded invasive species were previously documented in side channels and the adjacent wetlands, but did not invade the tributaries. 5. Tributaries may provide refuges for native molluscs and could be considered as ‘source’ populations, which may be critically important in the regional maintenance of threatened species. Tributaries are still less disturbed and less invaded; and should be given priority in future conservation management plans. Copyright # 2015 John Wiley & Sons, Ltd. Received 10 November 2014; Revised 10 May 2015; Accepted 22 July 2015 KEY WORDS: biodiversity; conservation evaluation; endangered and invasive species; Bivalvia; Gastropoda; River Danube *Correspondence to: Erika Bódis, MTA Centre for Ecological Research, Danube Research Institute, Karolina út 29–31, 1113, Budapest, Hungary. Email: [email protected] Copyright # 2015 John Wiley & Sons, Ltd. E. BÓDIS ET AL. INTRODUCTION effects that may alter the whole ecosystem. Despite their central ecological role, studies dealing with Global freshwater biodiversity is experiencing far their distribution and diversity patterns in large greater declines than terrestrial or marine ecosystems river systems are insufficient. (McAllister et al., 1997; Dudgeon et al., 2006; Although many freshwater ecosystems have Strayer and Dudgeon, 2010). For example, future undergone declines in native diversity they have also extinction rates of freshwater faunal species in North experienced the introduction of many invasive America are predicted to be five times higher than species (Sousa et al., 2014). The River Danube, one for terrestrial fauna (Ricciardi and Rasmussen, 1999). of the most important shipping routes in Europe, is a Freshwater ecosystems are extremely threatened good example (Bódis et al., 2012a). Some invasive and their conservation is very difficult since they mollusc species deserve special attention since they have high levels of endemism owing to natural can dominate the benthic community in density and isolation, catchment division and presence of biomass triggering significant changes in the saltwater barriers which impair re-colonization. In structure and functioning of freshwater ecosystems addition, they have a special landscape position as (Strayer et al., 1999; Sousa et al., 2009, 2011, 2014). receivers being affected by all the changes and In the Danube River Basin the Asian clam, disturbances from adjacent terrestrial areas Corbicula fluminea andtheChinesepondmussel, (Carpenter et al., 2011). Moreover, rivers are open, Sinanodonta woodiana became dominant in a short directional systems, and many species migrate period of time and may impair the conservation of longitudinally or laterally by using different native molluscs (Bódis et al., 2011, 2012a, b, 2014a, b). habitats during their life cycle, which may be If conservation priorities are to be set, and significantly altered by human activities and makes efficient conservation management plans put in the implementation of management and conservation place it is important to monitor the distribution measures especially difficult (Dudgeon et al., 2006). and diversity of freshwater molluscs regularly and Therefore, if freshwater species, declines continue follow up the changes in the range and population at current rates, and the human demands for water size of threatened and invasive species. The River remain constant, the opportunity to conserve Danube is one of the most important freshwater freshwater biodiversity may disappear (Dudgeon ecosystems in Europe and should receive special et al., 2006; Vörösmarty et al., 2010). attention. The main objectives of this work were to All organisms are ultimately affected by human characterize the composition, diversity and perturbations but freshwater molluscs are conservation status of freshwater molluscs in the especially sensitive to anthropogenic and climatic Hungarian Danube River Drainage, in five different disturbances (e.g. habitat loss and degradation, habitat types (main channel of the rivers Danube pollution, introduction of invasive species, floods, and Tisza, side channels of the Danube, tributaries droughts), and species loss has accelerated over the of the rivers Danube and Tisza) and to determine past decades (Régnier et al., 2009). Moreover, the principal environmental variables influencing freshwater bivalves belonging to the family Unionidae the composition of the mollusc assemblages. This are considered one of the most endangered groups information may provide reference data to assess of animals, and many populations are regionally or future changes to the molluscan assemblages and to globally in decline (Bogan, 1993; Lydeard et al., guide the management measures that need to be 2004; Burlakova et al., 2011a, b; Lopes-Lima et al., applied to the conservation of key native species or 2014). Since freshwater molluscs are involved in problematic invasive species. several key ecosystem functions and services (Rosemond et al., 1993; Strayer et al., 1999; MATERIAL AND METHODS Vaughn and Hakenkamp, 2001; Gutiérrez et al., 2003; Spooner and Vaughn, 2008; Spooner et al., Study area 2012; Strayer, 2014; Tolley-Jordan et al., 2015) The Danube is the longest river (2857 km) in their continued decline can result in cascading Central Europe. It has a catchment area of Copyright # 2015 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. (2015) FRESHWATER MOLLUSC IN THE DANUBE RIVER DRAINAGE 817 000 km2 (39 000 km2 in Hungary) and receives Hungary, is located in the wetland of Szigetköz. In all of the running waters of Hungary in a length of general, the Szigetköz has a very high conservation 417 km. Its average annual discharge is value and is usually considered a pristine area. approximately 2350 m3 sÀ1 at Budapest (Tőry, However, hydrological changes caused by the 1952). The River Tisza is one of the main rivers in Gabcikovo hydropower plant have affected the Central Europe, and has the largest catchment and faunal assemblages (Nosek et al. 2007; Bódis et al. length among the Danube tributaries. The Tisza 2008). The Ráckevei-(Soroksári)-Danube has a drains an area of ~ 157 135 km2 (47 000 km2 in length of 57.3 km, and has a regulated water Hungary) and has a length of 966 km. With an discharge through the Kvassay water-gate with average annual discharge of 820m3 sÀ1 at Szeged, anthropogenically disturbed habitats. The side its contribution to the Danube is about 13% channel at Göd is short, and during low water levels (Alföldi and Schweitzer, 2003). The River Rába is a in the Danube (approximately 125 cm at Vác) its tributary of the Danube comprising a total basin northern connection with the main channel is area of 10 113 km2, a length of 283km and an interrupted so that for 45% of the year the water flow average annual discharge of 27 m3 sÀ1 at Győr. The ceases in the side channel. River Ipoly is the largest tributary on the left side of the Hungarian Danube section with a length of Sampling methods 212km, a catchment area of 5108 km2 (143km, 1518 km2 in Hungary) and an average annual Sampling sites (35 in total) were located in the main discharge of 21m3 sÀ1. The River Bodrog is a channel of the River Danube (D1-6); in the side tributary of the River Tisza and has a length of channels of the River Danube: Mosoni-Danube 65km (15km in Slovakia, 50km in Hungary). Its (MD1-3), Göd (SD1), Ráckevei-(Soroksári)-Danube catchment area is 13 579km2 (972km2 in Hungary) (SD2-3); in the tributaries of the River Danube: and its average annual discharge is 115 m3 sÀ1. River Rába (R1-3),