LATE HOLOCENE PALEOENVIRONMENTAL RECONSTRUCTION OF A GREAT LAKES COASTAL WETLAND

Peat forming ecosystems provide an archive from which it is possible to study the processes driving change in wetlands. From an ecological standpoint, peatlands are useful records since they respond to external factors such as temperature and precipitation but they also reflect processes internal to the plant communities that generate them. A transect of peat-rich sediment cores was collected across an 1100-ha wetland complex on the north shore of Lake Erie, southwestern Ontario, Canada. The paleoecological history of this site was investigated using C-14 dating, pollen, diatom and sediment analyses. My objectives were to determine the factors driving long-term plant community change and to evaluate the contributions of allogenic and autogenic processes. Basal radiocarbon dates on several cores of about 4200 C-14 years BP indicate the establishment of a peat forming wetland in lagoonal conditions associated with the adjacent development of a cuspate foreland. The major allogenic disturbance affecting this site since the mid-Holocene is water level fluctuation, notably the Nipissing Highstand. During the Nipissing high water event, this wetland was an extensive diatom-rich, graminoid marsh. As lake levels dropped, this community was replaced by a shrub swamp dominated by Cephalanthus occidentalis and Salix spp., in which the water table was too low for diatoms to persist. The onset of peat development in shallow sloughs on the sand spit at 750 C-14 years BP coincides with the Little Ice Age, documented elsewhere for southern Ontario. The record shows significant ecological changes associated with the arrival of Euro-Canadian settlement in the area including substantial increases in the rate of sediment accumulation, the spread of Typha angustifolia and T. x glauca and the decline of other wetland species. During periods in which these major allogenic factors were not operating, the paleoecological record shows that some plant community changes still occurred, suggesting that autogenic successional processes also have a role in explaining long-term plant dynamics in coastal wetlands.