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.