the establishment of dreissenid mussels (zebra mussel, dreissena polymorpha and the quagga mussel dreissena rostriformis bugensis) in lake simcoe in 1997 and 2008, respectively, in lake simcoe has drastically altered the benthic invertebrate community in the lake. non-dreissenid benthic invertebrate abundance greatly declined in the year 2009 during the quagga mussel invasion, whereas dreissenid abundance increased greatly in the same time period. pre-invasion abundance of gastropods, amphipoda, oligliochaetes, plecypoda and chironomids followed similar density patterns after zebra mussel establishment. a decline in several taxa to undetectable densities was found post-quagga mussel invasion, except for chironomids. these changes in benthic communities have likely impacted fish populations because fish had to shift to hard shelled food sources possibly causing decreased fish health and decreased individual growth, a direct result of the zebra mussel as they invade shallower depths. the gastropod and chironomid populations were able to slightly increase their populations in 2017, facilitated by an increase in quagga mussels. water quality of lake simcoe has also likely been negatively impacted by dreissenid mussel establishment, given changes in benthic community, oxygen presence, and nutrients available.