Ecology of testate amoebae along an environmental gradient from bogs to calcareous fens in East-Central Europe: development of transfer functions for palaeoenvironmental reconstructions

Testate amoebae play an important role in biomonitoring and the understanding of peatland (palaeo)ecology. However, their application has been mainly limited to Sphagnum-dominated peatlands, especially ombrotrophic bogs. To facilitate wider use of these microorganisms, we explored their ecology along a gradient from mineral-poor acidic bogs to mineral-rich calcareous fens with tufa formation using a new calibration dataset of over 250 samples from East-Central Europe. Specifically, we examined environmental controls on testate amoebae along the complete gradient and separately in the minerotrophic (pH >5.5) and ombrotrophic (pH <5) parts. Based on these findings, we developed and statistically evaluated transfer function models for the dominant factors controlling the community composition of these protists. Multivariate statistical analysis showed that groundwater pH was the main driver of species composition in the full and minerotrophic datasets. In contrast, testate amoeba communities in the ombrotrophic dataset were primarily structured by depth to water table (DWT). Under leave-one-out cross-validation (LOO), the best-performing model for DWT gave a prediction error (RMSEP) of 5.43 cm. The most robust models for groundwater mineral richness yielded RMSEPLOO of 0.45 and 0.27 pH units, for the full and the minerotrophic dataset respectively. This is the first study to present a complete set of testate amoeba-based transfer functions that allow reconstructions of Holocene hydrochemical and hydrological (hydroclimatic) variability in different mire types within one region. The new models open up the possibility for testate amoebae to become a valuable tool in palaeoecological research, although further work is needed to fully assess their performance and usefulness in practice.