Abrupt changes in Bosmina (Cladocera, Crustacea) assemblages during the history of the Ostrowite Lake (northern Poland)

During the 10 000-year history of the Ostrowite Lake, there have been several episodes of change in dominance amongst species of the genus Bosmina. The dominants were alternately B. longirostris and Eubosmina spp. Amongst the subgenus Eubosmina, two species prevailed in different periods: Bosmina coregoni and Bosmina reflexa. The first species, Bosmina coregoni, is characterized by long antennae and short carapace mucrones, while the second one, Bosmina reflexa, by short antennae and very long mucrones. Bosmina reflexa was dominant at the beginning of the lake's history (Preboreal, 10 000-9000 BP) and during the early Subboreal Period (5000-3500 BP). Only a small number of remains of Bosmina longispina (Eubosmina spp.) were present along the entire profile. Such strong changes in the domination of Bosmina species were observed for the first time in the sediments of Polish lakes. A similar transition from one species to another has been described in a few German lakes and was linked to climate changes. However, the changes in the Ostrowite Lake do not correlate with climate changes, but most probably, with changes in the lake's trophic level. This is suggested by the simultaneous increase of Bosmina reflexa and a good indicator of eutrophy, Bosmina longirostris.