Recent Changes in Lake Ontario 1981–1987: Microfossil Evidence of Phosphorus Reduction

Quantitative analysis of siliceous microfossils in a finely sectioned (0.5 cm) core of near-surface sediments from Lake Ontario indicates the lake is responding to reductions in phosphorus loading. The magnitude of response, however, is very small. Increased diatom accumulation and modified species composition indicates some relaxation of silica limitation, particularly in early spring. Abundance of some diatom species associated with grossly polluted areas of the Great Lakes has been reduced, but most recently deposited assemblages are composed of species tolerant of eutrophic conditions. No reappearance of species assemblages abundant in Lake Ontario prior to 1935 and presently associated with oligotrophic conditions in the upper Great Lakes were noted. Changes in species composition are consistent with observed recent changes in N:P ratio. © 1991, International Association for Great Lakes Research. All rights reserved.