Establishment of wetlands on the southeastern Atlantic Coastal Plain: Paleolimnological evidence of a mid-Holocene hydrologic threshold from a South Carolina pond

The series of climatic events affecting landscapes on the southeastern Atlantic Coastal Plain over the past several thousand years is poorly understood. This is due, in part, to the paucity of depositional basins in this region containing adequately preserved microfossils for paleoecological analyses. This research reconstructs past hydrologic environments from the microfossil record of Peat Bay, a small, peat-filled pond in south-central South Carolina. We cored the deepest location in the pond and retrieved 120 cm of sediment representing 5200 yr of deposition. Fine clays at the base of the core resembled sediments deposited in abandoned channels of the nearby Savannah River, suggesting the basin may have functioned as an oxbow wetland prone to periodic flooding and drying. Overlying peats containing planktonic diatom assemblages mark the onset of persistent open-water conditions ca. 4600 B.P. These data, in conjunction with a small body of paleoecological evidence from other southeastern wetlands, suggest that a hydrologic 'threshold' was surpassed during the mid-Holocene that resulted in flooding and expansion of low-lying depressions that characterize the regional landscape. This is likely the culmination of long-term hydrologic trends related to glacial retreat, including increased precipitation and a rise in ground water levels corresponding to a global rise in sea level. In Peat Bay, lake-like conditions persisted until about 3800 B.P., after which time aerophilic diatoms and remains of other temporary pond organisms dominated the fossil record. Episodic drying of pond sediments during the late-Holocene may have been a response to decreased precipitation, stabilization of the water table and infilling of the pond by decaying vegetation and terrigenous clastics. The microfossil record indicates that temporary pond conditions persisted until recent years (ca. 1985) when the basin returned to a permanently flooded state due to groundwater connection to a large reservoir constructed nearby.     

Diatoms and pollen as indicators of water quality and land-use change: a case study from the Oak Ridges Moraine,Southern Ontario,Canada

Swan Lake is a small kettle lake located on the Oak Ridges Moraine; a moraine that is recognized as an important source of ground water for the nearby and rapidly expanding Greater Toronto Area. A paleolimnological reconstruction using pollen and diatoms from the lake sediments showed significant changes in biological community composition through the last ∼400 years. Alterations in the diatom and pollen assemblages were most dramatic ca. A.D. 1850, correlating with the highest sediment flux in the lake between the period ca. A.D. 1850 and A.D. 1870. These changes were directly linked to regional deforestation and agricultural activities associated with European settlement. The pollen record from ca. A.D. 1850 to present day indicated that tree species (e.g. Pinus spp., Tsuga canadensis) were declining, while grass (Poaceae) and invasive species (e.g. Ambrosia) were increasing. Around A.D. 1850, the diatom flora changed from an assemblage dominated by large, benthic species (e.g. Sellaphora pupula, Pinnularia cf. maior, and Stauroneis phoenicenteron) to an assemblage characterized by smaller, tychoplanktonic (e.g. Fragilaria tenera, Staurosirella pinnata) and epiphytic (e.g. Achnanthidium minutissimum, Rossithidium linearis) taxa. This diatom community change supports the intermediate disturbance hypothesis which predicts a high level of diversity and richness following an intermediate to intense disturbance of short duration. Phosphorus concentrations in Swan Lake were inferred using a diatom-based regional calibration model, and the results indicated marked changes in lake water chemistry through time (from below detection limits before land clearance and settlement to 19.3 μg l⁻¹ in the current sediments), which were concurrent with episodes of regional deforestation and land-use change. Although the sediment and biological records indicate that the lake ecology has stabilized over the last 30–50 years, paleolimnological records show that the water quality and biology of Swan Lake has changed dramatically and not returned to pre-settlement conditions. Swan Lake presents a detailed record of the impact created by deforestation and urban development with a population of <50 individuals per km². Detailed paleolimnological studies like Swan Lake, in tandem with global human footprint studies, can create realistic estimates of land-use impacts at the global scale.