Postglacial paleoecological history of a cedar swamp, manitoulin island, Ontario, Canada
Abstract A cedar swamp on Manitoulin Island, Ontario, Canada was chosen for a paleoecological study of pollen, macrofossils and Cladocera. A new elevation of 316.7 m a.s.l. is given for the position of Main Lake Algonquin on northeastern Manitoulin Island based on a wave-cut feature 4.7 m above the swamp, immediately adjacent to the site. Deposits inferred to predate 10,500 yr. B.P. are dominated by Picea, Artemisia , Cyperaceae and other herbs, likely derived from spruce parkland. Climate on the small proto-Manitoulin Island group was cool and possibly dry. The cladoceran assemblage includes Alona lapidicola, Acroperus harpae and Chydorus brevilabris which resembles the modern fauna of wave-swept rocky shores of Georgian Bay. This suggests that the site was an embayment of Main Lake Algonquin. Local lake waters were cold and shallow. Pinus banksiana/resinosa, Populus , Cupressineae, Artemisia, Pteridium and other herbs dominate pollen assemblages from 10,500 to 10,000 yr. B.P. An open, pine-dominated woodland prevailed under a warming and dry climate, and marked progression towards the Holocene climatic optimum on Manitoulin Island. Glacial Lake Algonquin levels fell, and the embayment was transformed into a small alkaline lake. Potamogeton filiformis, Hippuris vugaris and Scirpus plants provided habitat for a diverse littoral and planktonic cladoceran fauna which included Chydorus brevilabris, C. cf. gibbus and Bosmina longirostris . Waters in the swamp basin were rich in nutrients, warm and possibly transparent. From 10,000 to 8000 years B.P., Picea returned as the dominant pollen type and was slightly more abundant than Pinus banksiana/resinosa . Toward the end of this period, Abies, Larix and Betula increased in frequency. Picea forest with P. banksiana/resinova grew on shallow, well-drained soils. Climate became more continental, winters more severe and summers drier and hotter, as the moderating effects of the Great Lakes ceased during the Lake Stanley—Lake Hough stage. At the site, a shallow pond with abundant submersed and emergent aquatic plants and a diverse littoral cladoceran community existed. At 8000 yr. B.P., Pinus strobus replaced Picea and Pinus banksiana/resinosa . Shortly after, Tsuga canadensis and Larix become important. This conifer-dominated pollen assemblage persists to the present and reflects modern forests. The presence of Nyssa pollen and Decodon verticillatus seeds contemporaneous with their range extensions elsewhere in the Great Lakes Region, implies that the Hypsithermal maximum occurred about 6000 yr. B.P. on Manitoulin Island when climate remained warmer than present but became humid after about 8000 yr. B.P. By 5000 yr. B.P. conditions became cooler. Prairie and Cordilleran floristic elements in the modern Manitoulin flora are probably remnants, predating 10,000 yr. B.P. The southern floristic element may represent late Hypsithermal relicts. Fluctuating water levels and influence of mineral-rich groundwater probably were key factors controlling hydroseral succession in cedar swamps on Manitoulin Island.