Direct and indirect ecological effects of Dreissenid mussels (the zebra mussel Dreissena polymorpha and the quagga mussel D. bugensis) on submerged macrophytes in North American lakes

Dreissenid mussels (the zebra mussel, Dreissena polymorpha and the quagga mussel, D. bugensis ) are ecosystem engineers that increase water clarity and contribute to oligotrophication and benthification in North American lakes. Several researchers have observed changes in submerged macrophyte communities associated with the dreissenid mussel invasion, and attributed these changes to increased water clarity. In contrast, phosphorus reduction following the implementation of the Great Lakes Water Quality Agreement in 1972 appears not to have had significant impacts on macrophytes. I conducted both observational and experimental studies to investigate both direct and indirect ecological effects of dreissenid mussels on submerged macrophytes in North American lakes. The thesis consists of four chapters: (1) an observational study of the indirect impact of dreissenid mussels on submerged macrophytes through mediating water clarity in Oneida Lake, NY, (2) an observational study of the indirect impact in two bays of Lake Ontario, (3) an experimental study of the indirect impact in a pond, and (4) an observational and experimental study of direct light and nutrient effects of dreissenid mussels on submerged macrophyte growth. Results from the first three chapters showed water clarity increased in lakes after the dreissenid invasion and macrophyte species diversity, their frequency of occurrence, the maximum depth of colonization, and their coverage increased too. In addition, macrophyte species composition changed from low-light-tolerant species to those tolerating a wide range of light conditions. In the fourth chapter, the study of direct impacts of dreissenid mussels on submerged macrophytes revealed species-specific impacts of the shell attachment and no effect of nutrient relocation. However, the nutrient impacts in lakes need further investigations. Together, these results showed light is the overriding factor that determines the distribution and diversity of submerged macrophytes and the indirect effect of dreissenid mussels by mediating water clarity and light penetration is the most important. Other direct and indirect effects of dreissenid mussels are probably not important although more investigations may be needed. Therefore the consequence of dreissenid mussels for ecosystems is indeed indirect and appropriately called ecosystem engineering, which contributes to benthification in North American lakes