PALEOLIMNOLOGIC EVIDENCE OF RAPID RECENT CHANGE IN LAKE-ERIE TROPHIC STATUS

Analysis of diatom assemblages preserved in a Pb-210-dated core from L ake Erie's central basin indicates rapid recent ecological change in t he system. Sediments deposited after ca. 1972 contain increasing numbe rs of diatom populations associated with oligotrophic conditions, part icularly Cyclotella species, which grow in the summer deep chlorophyll maximum in the Great Lakes. The nearest analogs of assemblages deposi ted after ca. 1986 are assemblages deposited around ca. 1900. However, the analogy is far from perfect. Many conspicuous indigenous species have not reestablished and may be extinct. Many species have increased recently that were not abundant in the Great Lakes nora prior to 1950 . Several of them appear to be associated with relatively high nitrate concentrations. Most recently deposited diatom assemblages in Lake Er ie are thus substantially different from any deposited during the past 200 years. The direction and timing of changes observed lead us to co nclude that the most important factors forcing the changes observed ar e (i) institution;of effective phosphorus loading controls in the earl y 1970s and (ii) invasion of zebra mussels (Dreissena polymorpha) in t he mid-1980s.