Pollen was analyzed in the surface 1 cm of sediment taken from 90 sample stations collected in 1968 from a 16 km grid on Lake Ontario. Sixty-nine taxa were identified and the more abundant taxa subjected to trend surface analysis. Pollen concentration in 60 samples; ranged up to 240 000 grains per g dry sediment with the higher concentration in the offshore silt and clay sediments of the basins. Pollen percentages of 80 stations were based on 100 or more tree pollen with the average percentage values above 5% for tree pollen being Pinus 33.8, Quercus 23.6, Betula 8.3, Cupressineae 6.3, and Acer saccharum 5.5.The highest values of herb pollen were of weedy taxa, and these are Ambrosia 46.1, Gramineae 20.0, and Chenopodiineae 5.8. Throughout the greater part of the lake the percentages were generally uniform and reflect the vegetation of the drainage basin of the lake. Local variations are related to recycling of pollen from older sediments and to local river sources. Comparison of pollen percentages of Lake Ontario with percentages in the surface sediment of nearby small lakes shows that Lake Ontario has a generalized assemblage derived from a broad source region but that Tsuga, Ulmus, and Fagus are relatively under-represented for poorly understood reasons. The general uniformity of percentages in Lake Ontario suggests that pollen diagrams can be widely correlated within the lake.

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