CUSUM phytoplankton and chlorophyll functions illustrate the apparent onset of dreissenid mussel impacts in Lake Ontario

Untreated lake water samples were collected weekly, year-round from the drinking water intakes of five municipal water treatment plants on Lake Ontario and one plant with an intake in the upper St. Lawrence River and analyzed for chlorophyll a and phytoplankton density. CUSUM (cumulative sums of the differences between monthly means and the grand mean of each data set) chlorophyll and phytoplankton functions revealed rapid and dramatic reductions in chlorophyll and phytoplankton at four of the six sampling locations. Break-points in the CUSUM functions were in 1991 for western Lake Ontario, 1993 for the upper St. Lawrence River, 1994 for the Kingston (eastern) outlet basin of the lake, and 1995 for the central, north-shore area of the lake. These dates reinforce the anecdotal information available on the invasion history and spatial distribution of Dreissena spp. (zebra and quagga mussels) in Lake Ontario. Three-year "before and after" comparisons revealed that the highest percentage reductions in phytoplankton (several exceeding 90%) occurred during fall, winter, and spring, despite water temperatures near 0 degreesC (during winter), and were similar to reductions previously reported for the north shore of Lake Erie using similar methods.