Living in a hardwater lake – The Late Weichselian and Holocene history of the small kettle hole on the moraine plateau in Ostrowite (N Poland)

Local lithology has a strong influence on lake biocenoses, including macrophytes and invertebrate communities. Calcium carbonate (CaCO₃) content may drive ecosystem components from plankton (Cladocera), through various trophic groups of macrobenthos (Chironomidae) to molluscs settled on plants and bottom of the littoral zone. This study reconstructs the impact of carbonates on the biota in a small palaeolake during the Late Weichselian and Holocene in the recently glaciated area in Northern Poland. The main litho-geochemical component of the studied lacustrine deposits is calcium carbonate (CaCO₃), ranging from 0 to 98.7 % of the sediment content. The variability of the CaCO₃ content reflects changes in hydrological and geomorphological conditions within the palaeolake catchment area. The multi-proxy analyses enabled the reconstruction of a complex environmental history of the studied kettle hole lake. Oligo-/mesotrophic conditions with charophyte meadows during the Late Weichselian supported diverse chironomid communities. Cladocera became more abundant in the Early Holocene, reflecting hydroclimatic changes. The transition into macrophytic meso-/eutrophic conditions in the lake was reflected in the SiO_2bio and TOC increase. This phase ended with a period of the highest CaCO₃ concentration in the lake’s history, which supported mollusc assemblages. The final terrestrialisation followed the maximum decalcification in the catchment. Some Chironomidae appeared to be tolerant to relatively high carbonate concentrations, but this group feeds mainly on the bottom FPOM and most of the taxa declined when CaCO₃ exceeded 70 %. The mollusc abundance and richness were correlated with the carbonate content in the lake. Hardwater lakes defy simple interpretation in palaeolimnology. The classification of invertebrates according to their tolerance for carbonate content presented here helps in reconstructing palaeocenoses reaction to litho-chemical changes in lake basins.