Response of the aquatic plants and mollusc communities in Lake Kojle (central Europe) to climatic changes between 250 BCE and 1550 CE

Sediments of Lake Kojle, located in the transition zone between the nemoral and boreal biogeographic zones in NE Poland, were analysed to verify the response of the aquatic ecosystem to major environmental changes. High-resolution plant macrofossil, mollusc and pollen data were derived from two replicate parallel cores and revealed several shifts in the biota composition within the littoral zone between 250 BCE and 1550 CE. The reaction of the aquatic ecosystem to temperature changes was found to be minor, which is ascribed to the mitigating influence of water and the wide ecological tolerance of most of the taxa. Najas marina, considered as an indicator of warmer conditions, was the only species that clearly reacted to the long-term climate shifts. We consider lake level changes that resulted from fluctuations in climate humidity and the filling of the lake with sediments as the most important factors that control the presence of macrophytes species from the genera Nymphaea, Nuphar, Typha, and Chara. Mollusc diversity and abundance increased with the increase in the diversity and biomass of submerged vegetation. However, reduced dissolved oxygen within the water column under the floating leaves of Nymphaea alba and Nuphar sp. resulted in the retreat of Planorbis carinatus, a gastropod species sensitive to oxygen shortages. Major shifts in the developments of fauna and flora were parallel in the two studied cores, but some spatial variability was observed. This might be due to the spatial variability of the macrophyte and mollusc communities within Lake Kojle.