Distribution and seasonal dynamics of the Ponto-Caspian invader Cercopagis pengoi (Crustacea, Cladocera) in the Neva Estuary (Gulf of Finland)

The Ponto-Caspian predatory cladoceran Cercopagis pengoi was first recorded in the Baltic Sea in 1992 (Gulf of Riga). In 1995 it reached high densities in the eastern Gulf of Finland. Seasonal abundance and distribution of Cercopagis in the Neva Estuary were studied in June-October 1996. Until 21 July densities fluctuated between 0.0 and 4.2 ind m(-3). Maximum densities (up to 305 ind. m(3)(-) were observed in August-September, the period of maximum water temperature. On 15 October (surface water temperature 8 degrees C), Cercopagis was still present in plankton at low densities. C. pengoi was present at almost all stations both in the central gulf and in inshore areas along southern and northern coasts. Although Cercopagis was present throughout the water column, its abundance in the upper 10-m was usually higher than deeper down.