Expansion of alien gammarids in the Vistula Lagoon and the Vistula Delta (Poland).

Amphipod crustaceans belong to the most successful invaders of aquatic environments. The work provides information concerning the spatial and temporal scales of expansion of four alien gammarid amphipods (three of them of Ponto-Caspian and one of North American origin): Pontogammarus robustoides (G.O. Sars, 1894), Obesogammarus crassus (G.O. Sars, 1894), Dikerogammarus haemobaphes (Eichwald, 1841), and Gammarus tigrinus Sexton, 1939 in the Vistula Lagoon (VL) and the Vistula Delta (VD) in 2008-2010. The mean abundance of these gammarids in nearshore zones was 382 ind m(-2) in VL and 89 ind m(-2) in VD. Their mean biomasses were likewise greater in VL (0.91 g m(-2)) than in VD (0.49 g m(-2)). G. tigrinus was the most dominant species in both nearshore zones of VL and VD and attained the highest frequency in these areas. The study gives evidence of total extinction of native gammarid species.