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The long-term dynamics of a lake development and the role of the catchment and climate change in the dystrophication process were studied. Lacustrine sediments from Lake Suchar IV, NE Poland (880 cm long core), were selected for paleolimnological studies. The paleoecological reconstruction was based on pollen, subfossil cladocerans, diatoms, macrofossils and chemical analyses. Sediment chronology was based on the results of C-14 dating and palynological analysis. The obtained data showed that the lacustrine sedimentation at Lake Suchar IV began in the Late Glacial and continues to this day. The plant succession indicates that sedimentation processes started as early as the Oldest Dryas similar to over 15 000 cal yr BP ago. Important changes in the pollen spectrum of aquatic plants were noted from the mid-Atlantic period. At that time, communities of Nymphaea and most species of green algae disappeared from the lake, which indicates a significant ecological transformation in the aquatic environment. This transformation is also manifested in the subfossil Cladocera and diatom communities. The Cladocera community in the sediments of Lake Suchar IV was represented by 37 species, mostly littoral ones. From the transition time (7200-5600 cal yr BP), Cladocera communities were dominated by taxa tolerant of acidification such as Alonella excisa, Acroperus harpae, Alona affinis and Alonella nana. A total of 193 diatom taxa (species and varieties) were identified in the sediment, including 172 belonging to Pennales and 21 to Centrales. In general, the diatom community was dominated by alkaliphilous - on average 45 % of the species composition. In terms of trophic preferences, the largest group of diatoms was represented by oligotraphentic, meso-eutraphentic and eutraphentic taxa. No diatoms were present during about 6000 cal yr BP from the transition stage. Sediment geochemistry and macrofossils also indicate the limnological development and climate trends.

The obtained paleolimnological results indicate that Lake Suchar IV has undergone an ecological transformation from a harmonic to disharmonious ecosystem. During the Late Glacial and the early Holocene, Lake Suchar IV was a typical harmonic lake that transformed into a dystrophic state at the end of the Atlantic period. Sediments of Lake Suchar IV also show a very interesting sequence of trophic changes, from oligotrophic to mesotrophic and to dystrophic conditions.