Great Lakes fens, such as those along the eastern edge of Lake Ontario, provide important habitat for a diverse assemblage of animals and plants, including a disproportionately large number of rare and endangered species. These wetlands, many of which have become dominated by low-diversity Typha stands in the last 50 years, are considered to be especially sensitive indicators of anthropogenic impacts such as water level regulation or eutrophication. Nonetheless, little is known about the long-term development and disturbance regimes of Great Lakes fens or the role of Typha in these fens prior to European settlement. Peat cores from several Lake Ontario fens were analyzed for plant macrofossils to determine the long term history of disturbance and plant community composition in several eastern Lake Ontario fens and to test the hypothesis that modern dominance by dense Typha stands is a novel state for these wetlands. AMS radiocarbon dates were obtained to determine chronology of peat deposition and loss on ignition was used to characterize depositional environment. In addition, peat nutrient analyses were performed to explore the relationship between eutrophication and Typha expansion.

Results/Conclusions

Wetlands along the eastern shore of Lake Ontario were determined to be 3000-4,000 years old, originating around the time of the coalescence of the upper and lower Great Lakes drainages. Although Typha seeds in the peat record suggest the long-term presence of Typha in these communities, macrofossil analysis reveals that these wetlands have been persistently dominated by graminoid and ericoid fen species prior to the impacts associated with European settlement. Some time after European settlement, a combination of cultural eutrophication, water level regulation, and introduction of the hybrid cattail Typha x glauca appears to have promoted the dominance of Typha in these wetlands, leading to reduced plant diversity.  Additionally, charcoal layers provide evidence of recurring periods of fire in these wetlands prior to European settlement, possibly ignited by native Americans, which may have played a part in maintaining open, graminoid-dominated communities.