Caspian invaders vs. Ponto-Caspian locals - range expansion of invasive macroinvertebrates from the Volga Basin results in high biological pollution of the Lower Don River

Until the 2000s, faunal exchange between endemic faunas of the Caspian Sea basin and the Azov-Black Sea (aka Pontic) basin was asymmetrical, with fauna heading towards the Volga Delta and Caspian Sea from or via the Black and Azov Seas and little exchange in the opposite direction. This study is based on a hydrobiological survey of the Don River Basin conducted in August 2011 and reflects a period when three new Caspian invaders (Dikerogammarus caspius, Adacna glabra, and Theodoxus major) were already widely distributed in the study area. At the time of the expedition, of all the invasive species present, these three caused the greatest impact on local species and communities in the Don Basin. The greatest biological contamination was found in areas which had already had relatively favorable conditions for existing resident Ponto-Caspian communities. In contrast, those stations devoid of invaders were those under the most severe anthropogenic pressure or subject to estuarine conditions. This pattern was confirmed by biocontamination indices being significantly correlated with standard water quality indices. The situation in the studied area, involving both the Azov-Caspian exchange via the Volga-Don Canal and the eastward invasion of exotic species, such as Ferrissia californica and Potamopyrgus antipodarum, clearly identifies a single Ponto-Caspian Invasion Corridor that connects all three marine basins of the Ponto-Caspian region. The importance of recognising and accounting for regional differences when considering the monitoring of aquatic invasions is discussed.