The external morphology of native (Bulgarian) and non-native (Slovak) populations of Neogobius melanostomus was examined using standard distance-based measurements and, in addition, comparison of fin ray number between populations was made. Five of the 29 morphological characters measured differed between populations, both for males and females. The non-native population was characterised by a smaller mouth and eye, shorter pre-dorsal distance and greater inter-orbital distance. Three characters, post-orbital distance, head depth and minimal body depth, differed between populations only in females, though the overall trend was the same in both sexes. A lower head depth value, greater post-orbital distance and minimal body depth was recorded for females from the non-native population. The range in fin ray numbers was similar between native and non-native populations. Populations differed, however, in the frequency of pectoral fin rays. Individuals with 17 pectoral fin rays were more common in the...

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.

Spatial, seasonal, and annual bacterioplankton dynamics in recently infested by the speciesDreissena polymorphaOgosta Reservoir were studied for the first time during three year period. Bacterioplankton total number was higher in spring in ecotone zones, than in summer at thermocline. NH₄-N, PO₄-P, turbidity, dissolved oxygen, COD and chlorophyll-a correlate positively, while transparency and Ca²⁺negatively with bacteria. Nematode species composition, included 22 species studied (13 rarely found andRhabditis brevispinanew for Bulgaria) belonging to nine families. TheD. polymorphaimpact is positive on nematodes and phytoplankton, negative on zooplankton and bacterioplankton, but weak positive on larger bacteria, rods and attached bacteria.