understanding species interactions is critical to predicting the ecosystem-level impacts of anthropogenically-caused species extirpations. in particular, the loss of keystone species - those that have exceptionally large effects on their environment relative to their abundance - would be expected to have dramatic and cascading effects throughout an ecosystem. following the experimental acidification of lake 223 at the iisd-ela in the early 1980’s, all species that had been extirpated due to the experiment have since returned to the lake with the notable exception of the freshwater shrimp, mysis diluviana. mysis have had dramatic impacts when introduced into non-native habitats, but the role mysis in structuring ecosystems in their native habitat is unclear. through this unique opportunity to study and compare two different time periods where the only difference was the presence and absence of mysis in the same ecosystem, i evaluated the impacts of mysis on the community structure of lake 223. my results suggest that mysis may serve as a keystone species in their native environment, having significant impacts on the biomass of both fish and zooplankton species; in the absence of mysis, zooplankton species composition shifted towards dominance of chaoborus and large cladocerans. stable isotope analysis suggests that chaoborus, the largest predatory zooplankton species in the absence of mysis, represent an energetic bottleneck that has resulted in lake trout having slower growth rates, lower total biomass, and lower recruitment compared to when mysis were present in the lake. this work demonstrates the importance of mysis diluviana in structuring aquatic food webs in which they are native and provides support for considering re-establishment of mysis into atmospherically acid-damaged lakes as a potential recovery strategy.