Great Lakes coastal wetland communities are threatened by the impacts of invasive plants on ecosystem function and biodiversity. What allows invasive plants to become dominant in invaded communities can be hard to define and context-dependent. Traits associated with invasion success in wetland systems – rapid vegetative growth, competitive superiority in resource acquisition, and tolerance for high nutrient levels – are shared by two co-occurring invasive plants, hybrid cattail (Typha × glauca) and European frogbit (Hydrocharis morsus-ranae). European frogbit is a free-floating weed causing substantial negative impacts to native ecosystems in the Great Lakes region. It is thought to be facilitated by the presence of emergent plants like hybrid cattail, but the nature of this relationship has not been empirically demonstrated or utilized in management strategies. The purpose of this thesis was to advance understanding of the phenology, ecology, and control of European frogbit within an invaded hybrid cattail marsh along the St. Marys River, a connecting channel between Lakes Huron and Superior. This marsh was a valuable site both for investigating the relationship between hybrid cattail and European frogbit and for assessing the role of deep water in the development and control of European frogbit. In an observational study, measures of the phenological
development of European frogbit were accompanied by measures of environmental variables and estimates of plant community abundances to explore associations between European frogbit development and environmental conditions during a high-water period in the Great Lakes. European frogbit reached its highest abundance and density in 65 to 80 cm of water, along the shallowest of six transects in the study site. Water depth was found to be strongly negatively correlated with hybrid cattail abundance and weakly positively correlated with European frogbit abundance. An experimental study designed and implemented by Loyola University’s Institute for Environmental Sustainability Restoration Ecology research group tested the effectiveness of combinations of European frogbit control methods with the harvest and herbicide of hybrid cattail. Univariate analyses of this study did not detect a significant effect of treatments on European frogbit abundance (p = 0.085), but the combination of below-water harvest of hybrid cattail and hand removal of European frogbit was estimated to decrease European frogbit. Multivariate analyses demonstrated that the plant community post-treatment was predominantly structured by the removal of plants from below-water harvest treatments. Results of both studies suggest that the presence of hybrid cattail increases the favorability of habitat for European frogbit, even in deep water conditions. The long-term impacts of managing hybrid cattail simultaneously with European frogbit should be evaluated.