AN ASSESSMENT OF ECOLOGICAL CHANGES DURING THE RECENT HISTORY OF LAKE ONTARIO BASED ON SILICEOUS ALGAL MICROFOSSILS PRESERVED IN THE SEDIMENTS1,2

Quantitative analysis of siliceous algal microfossils (diatoms and chrysophytes) in a radiometrically dated core of Lake Ontario sediments shows striking modifications of the flora produced and preserved, apparently as a result of anthropogenic modification of nutrient loadings to the lake, Qualitatively similar microfossil assemblages were deposited during the period ca. 1700–1815. A single major event is recorded in assemblages deposited ca. 1769. Between this event horizon and ca. 1815 there was an apparent increase in algal production, but minimal change in species composition. A major disturbance occurred at the level of the Ambrosia horizon, ca. 1831–1847. Assemblages deposited between ca. 1855 and 1900 indicate increasing eutrophication. We infer that silica limitation became important by about 1860 and that the system reached its greatest effective production ca. 1900. Major modification of the flora occurred in the period ca. 1900–1945, with most local extinctions of major indigenous populations occurring ca. 1920–1945. Assemblages deposited in the period ca. 1945–1975 are composed of some stenothermal populations also present in pre-disturbance assemblages, a large component of benthic and nearshore planktonic species, and a number of apparently planktonic populations rarely recorded from large temperate lakes, particularly species of the genus Nitzschia. Our results indicate that the major factor driving species succession in Lake Ontario was phosphorus enrichment and consequent depletion of available silica supplies. Our results also show the importance of a species' physical niche in determining its survival under these conditions.