The present pilot study tested and validated the use of the ‘Invasive Alien Species in Europe’ smartphone app (IASapp) in the Lower Danube Region. The study was conducted in three stages: (1) Testing the effectiveness of the app in Bulgaria, Romania and Serbia by organizing information and field testing campaigns; (2) Improving and extending the functionality of the app and developing a list of the invasive alien species (IAS) of concern to the Danube River Basin (DRB); and (3) Testing the updated app during the Joint Danube Survey 4 campaign. Overall, 13 campaigns were carried out, during which seven aquatic IAS of EU concern and nine IAS relevant to the DRB were recorded. Currently, the developed new functionality of IASapp allows the recording of 64 IAS of DRB concern. The updated IASapp has an important role in increasing public awareness on IAS issues, as well as in enlarging the participation and output of citizen science in the IAS early detection, monitoring and reporting in the DRB. Therefore, the constant communication with existing stakeholder groups and establishment of new citizen science groups is crucial for increasing the effectiveness of using the IASapp in the Danube Region and tackling the IAS issues.

The aim of this study was to assess the status of aquatic invasive alien species (IAS) in the shoreline zone of the Bulgarian sector of the Danube River, Danube tributaries, and adjacent standing water bodies in the frame of the Joint Danube Survey 4 (JDS4). Seven benthic macroinvertebrates and seven fish IAS were identified. The crayfish Faxonius limosus was frequently found and abundant in the tributaries. The mussels Corbicula fluminea, Dreissena rostriformis bugensis, and Sinanodonta woodiana dominated in the Danube River and the middle and lower reaches of the tributaries, while the fish Carassius gibelio, Perccottus glenii, and Lepomis gibbosus were most abundant in the standing water bodies. The integrated biocontamination estimated by taxonomic groups (crayfish, molluscs, and fish), sampling methods, and type of water bodies ranged from moderate in the shoreline zone of the Danube River, through moderate to high in the canals and lakes, to severe in the Danube tributaries and the reservoirs. The results demonstrate the importance of IAS in the assessment of the ecological status/potential of the water bodies in the DRB. The comprehensive evaluation of the aquatic IAS pressure will provide valuable information and support for the implementation of the national and EU IAS and water policies in the DRB.