Late Holocene water-level fluctuations in a shallow forest lake in Central Europe inferred from Cladocera assemblages

Abstract Subfossil cladoceran assemblages from a dated sediment core collected from a small, shallow forest lake in northwestern Poland were used to reconstruct changes in water availability and habitat structure over the past ca. 2000 years. To assess the reliability of cladoceran-based paleohydrological record in a dynamic shallow-lake system, we applied both a qualitative planktonic-to-littoral (P/L) ratio and a quantitative modern analog technique (MAT) approach. Both approaches indicate substantial Late Holocene variability in inferred water availability and habitat structure, with relatively high inferred water availability from ca. 70 BC to AD 200, followed by greater variability and an overall tendency toward lower water availability. Some reconstructed phases broadly overlap with commonly recognized regional climatic intervals, including the Roman Warm Period, Dark Ages Cold Period, Medieval Climate Anomaly, and Little Ice Age, although these relationships should not be interpreted as evidence of precise chronological synchrony or direct climatic causation. Water availability is interpreted as the principal external driver of lake development, whereas trophic state, macrophyte structure, oxygen conditions, and possible fish predation acted as interconnected modifiers of the cladoceran signal. These local ecological processes complicate the interpretation of the hydrological signal and should therefore be considered in paleohydrological reconstructions of shallow lakes. The P/L ratio primarily reflects changes in littoral–open-water habitat structure, whereas the MAT model provides semi-quantitative depth estimates that may be sensitive to other changes in species–environment relationships. Agreement between the two approaches supports broad relative trends but does not constitute independent validation because both are derived from the same cladoceran assemblages. Cladoceran-based reconstructions therefore provide a useful record of relative hydrological variability, although absolute water-depth estimates remain uncertain.