Long-Term and Seasonal Dynamics of the Population of Cercopagis pengoi (Ostroumov 1891) (Cladocera, Onychopoda) in the Eastern Part of the Gulf of Finland, Baltic Sea

Long-term seasonal studies of the population of Cercopagis pengoi (Ostroumov 1891) (Cladocera, Onychopoda) in the eastern Gulf of Finland were carried out for the first time. Material was collected every two weeks from mid-May to mid-October in the years 2009-2019, and only one species of the genus Cercopagis Sars 1897 (C. pengoi) was registered. Individuals of the first generation were found only during the first month of population development, from mid-May to mid-June. The second and subsequent generations developed from mid-June to mid-October. The appearance of the first generation in plankton in the spring occurred a month later than that observed in the Ponto-Caspian basin. In autumn, the disappearance of C. pengoi occurred at the same time as in the native range, in mid-October. The fecundity of parthenogenetic females was higher at the beginning of the growing season. In some years, individuals of the gamogenetic generation were not observed at all in the population. Compared to the populations inhabiting the Ponto-Caspian region, the duration of the growth season of the C. pengoi population was shorter, but the population density of this species was higher in the eastern part of the Gulf of Finland.