The invasion of dreissenid mussels to Lake Simcoe coincided with notable changes in the ecosystem, including a period of lower phytoplankton biovolume, particularly at shallower sites. Dreissenid grazing effects are typically most prominent during the summer season in the nearshore areas of lakes. Grazing effects in the winter are small because dreissenid filtration rates, especially those of zebra mussels (Dreissena polymorpha), decrease in cold temperatures. Phytoplankton dynamics in the winter season are poorly characterized, particularly in lakes that experience ice cover. The purpose of this study was to examine the effects of dreissenids on the seasonality of phytoplankton dynamics in the nearshore waters of the lake using long-term monitoring data from unchlorinated water treatment plant (WTP) intake pipes. The long-term dataset from the WTPs showed significant and sustained declines in phytoplankton biovolumes and chlorophyll a (Chl-a) concentrations in the 12-year period following the invasion of dreissenid mussels to Lake Simcoe. The decline was smallest during the ice-covered winter months (Jan-Mar). The fall phytoplankton peak observed in the pre-dreissenid years shifted to a smaller peak during the winter in post-dreissenid years. We hypothesized that reduced dreissenid grazing pressure during the winter, and possible improved under-ice light conditions that we attribute to climate change, may be contributing to this shift. Phytoplankton biovolume data collection continues to be important in Lake Simcoe as a measure of phytoplankton biomass. Our results also indicate that winter phytoplankton biovolumes should be considered when managing lakes experiencing both climate change and dreissenid mussel effects.