Peatland type influences the response of testate amoeba taxonomic composition and functional traits to decadal-scale drying

Testate amoeba community analysis is a widely used proxy for inferring past water table fluctuations in peatlands, while their functional traits offer a novel complement to the traditional taxonomy-based approach. Changes in peatland hydrology is a crucial factor, as widespread drying has been documented overs the last ca. 150yrs and further drying due to increasing evapotranspiration has been projected for northern regions. Peatland drying stimulates the growth of shrubs and trees, causing changes in abiotic conditions and possibly turning peatlands into net C sources. However, the responses of testate amoeba taxa and associated functional traits to the interaction of decadal-scale drying per se and the associated secondary changes are still empirically poorly constrained.  

In this study, we aim to deepen the understanding of testate amoeba autecology and enhance their use as a palaeoecological tool. We use a 20-year-long field experiment to assess the effects of decadal-scale water level drawdown (WLD) and associated changes in habitat characteristics on testate amoeba community composition and functional traits. The experiment includes three peatland types: a rich fen, a poor fen, and a bog. Notably, WLD has caused significant vegetation changes in the fen sites.

Our results revealed that both taxonomic composition and functional traits were most strongly affected by WLD in the rich fen and least affected in the bog. This pattern aligns with previous observations in vegetation changes, and it is likely related to the establishment of woody vegetation in the fen sites. For example, the mixotrophic species Archerella flavum did not respond to WLD in the bog, while in the fen sites it almost completely disappeared due to increased shading. Despite this drastic vegetation shift in the fen sites, several previously known indicator species maintained their significance. In the two fen sites, WLD resulted in smaller test and aperture sizes, a reduced proportion of mixotrophic taxa, and fewer taxa with axial apertures or proteinaceous/xenosomic tests. However, in the bog, individual traits did not significantly respond to WLD, suggesting that the value of functional traits as bioindicators is stronger in minerotrophic conditions.

Overall, our findings suggest that the establishment of tree stands in the fen sites and the associated abiotic changes, in addition to the water level drawdown itself, have significantly influenced testate amoeba communities.