PLANKTONIC GREEN-ALGAE IN WESTERN LAKE ERIE - THE IMPORTANCE OF TEMPORAL SCALE IN THE INTERPRETATION OF CHANGE

1. Total densities of planktonic Chlorophyceae collected in weekly sam pling of the Kingsville (Ontario) municipal water intake in western La ke Erie were evaluated for potential effects of the recent zebra musse l (Dreissena polymorpha) invasion and for the longer term effects of t he Lake Erie phosphorus loading control programme. 2. At a relatively small temporal scale of about 10 years, an apparent zebra mussel-relat ed impact was clearly revealed in 1988 as an inflection point on the c umulative sum chlorophyte density curve. However, at a temporal scale of nearly three decades, this inflection point was not distinct. There was a steady decline in total Chlorophyceae throughout the 1970s whic h accelerated during the early 1980s; this corresponds to declining we stern Lake Erie phosphorus loading rates and phosphorus concentrations reported by others over the same period. In the absence of zebra muss els, average annual chlorophyte density decreased by 94% between the e arly 1970s and the mid-1980s. 3. The dramatic long-term decline of pla nktonic chlorophytes in western Lake Erie reveals the success of the p hosphorus control programme, places the recent impact of the zebra mus sels in its proper perspective and underscores the need for and value of long-term limnological data for management of the Laurentian Great Lakes.