Testate amoebae and δ13C of Sphagnum as surface-moisture proxies in Alaskan peatlands

Abstract

Testate amoebae and stable isotopes have been used as surface-moisture proxies in peatlands. However, adequate modern calibration is critical to successful application and interpretation. Testate amoebae, δ¹³C of Sphagnum, and environmental conditions were examined at 126 sites within 12 peatlands of south-central and central Alaska to assess the potential of testate amoebae and δ ¹³C as surface-moisture proxies. Results indicate that water-table depth and pH were both correlated with testate amoeba community composition. However, the relative importance of these two variables varied, with pH more important in wetter habitats and water-table depth more important in drier habitats. Cross-validation of transfer functions indicated that water-table depth and pH can be inferred from testate amoeba communities with mean errors of ~8 cm and ~0.4 pH units, respectively, an improvement over previous calibration work in the region. However, application of these transfer functions should consider potential temporal variability in the relative importance of pH and water-table depth, and we applied our calibration data set to a subfossil testate amoeba record to highlight how knowledge of changes in the relative importance of these environmental variables can inform interpretation. In contrast with testate amoebae, δ¹³C of Sphagnum was found to be a relatively weak indicator of water-table depth. Variable carbon sources for Sphagnum photosynthesis, such as CO₂ released by methanotrophic bacteria, likely complicate the relationship between δ ¹³C and moisture. Although more work is needed before δ¹³C of Sphagnum can be used as a proxy for water-table depth, testate amoebae should be useful in paleoenvironmental studies of peatlands in Alaska.