Testate Amoebae as Indicators of the Magnitude of Seasonal Moisture Variability in Sphagnum Peatlands
Numerous proxies have been developed over the past few decades to reconstruct the paleohydrology of peatland systems, including testate amoebae, a group of moisture-sensitive protozoa. Testate amoebae have been successfully used to infer patterns of multi-decadal moisture variability; however, variability at shorter timescales may also influence community composition. The objectives of this study were to determine if the composition of testate amoeba communities is influenced by seasonal moisture variability and, if so, develop models to infer the magnitude of seasonal moisture variability from fossil assemblages.
Testate amoebae and surface moisture conditions, including hourly measurements of relative humidity within the upper few centimeters of Sphagnum, were examined throughout the growing season at 78 sites within eleven peatlands of Pennsylvania and Wisconsin. Cluster analysis and non-metric multidimensional scaling (NMS) were used to describe patterns in community composition, and these patterns were compared to environmental variables to assess controls on testate amoebae. Results suggest that testate amoebae can not only be used as indicators of mean water-table depth, as in previous studies, but also seasonal variability, with particular species and communities characteristic of highly variable environments. The calibration data from this study was used to identify time periods of exceptionally high moisture variability in late Holocene testate amoeba records from the Great Lakes region, using a series of modern analogue based models. These reconstructions suggest that this new tool, when used in tandem with mean water-table depth reconstructions, can provide insights into the fine-scale structure of late Holocene moisture changes.
Mutlidecadal Hydroclimatic Fluctuations Trigger the Initiation and Episodic Expansion of a Floating Kettlehole Peatland
Floating Sphagnum-dominated kettle-hole peatlands are common features of glaciated landscapes. Ecologists have long been interested in these systems as archives of post-glacial environmental change and have sought to understand the processes governing floating peatland initiation and expansion. The widely accepted, though seldom tested, developmental model invokes climate-independent succession of pond-marginal plant communities and lateral expansion of floating vegetation over open water. In contrast, we hypothesize that floating mat expansion is episodic and driven by multidecadal hydroclimatic fluctuations. Under this model, drought exposes organic sediments to colonization by sedges and/or wetland shrubs, subsequent increases in water level cause floatation of peat, and the resulting hydrologic stability is conducive to Sphagnum colonization. We have conducted a preliminary test of this hypothesis by reconstructing the developmental history of a floating kettle-hole peatland in Erie County, Pennsylvania (Titus Bog) using multi-core, multi-proxy paleoecological analyses and comparing spatiotemporal patterns of peatland initiation and expansion to independent records of past hydroclimate. Results indicate that Titus Bog transitioned from a pond to a floating peatland at the western margin around 870 50 yr BP, coincident with a period of major hydroclimatic fluctuations during the Medieval Climate Anomaly. The floating peatland rapidly expanded throughout the rest of the basin several hundred years later (390 75 yr BP) during another interval of prolonged drought recorded in tree ring records from much of North America, including western Pennsylvania. Our results suggest that transient high-magnitude droughts may force kettle-hole ecosystems over critical thresholds, permanently transforming their structure and function.