Responses of Aquatic Plant Communities to Stream and Riparian Restoration and Management

Submersed macrophytes (plants that grow underwater) are important components of freshwater ecosystems, providing food and habitat for a variety of invertebrates, fish, and other wildlife, and influencing key chemical and physical processes. In moderate abundance, native submersed macrophytes contribute positively to overall stream health, but excessive proliferation of submersed macrophytes, particularly non-native invasive species, can cause ecological harm and create management difficulties. My dissertation examines how management actions can affect submersed macrophyte communities, and thus, the ecological integrity of stream ecosystems. This work incorporates and contributes to ecological theory from the disciplines of restoration ecology, invasion ecology, and plant community ecology, but also addresses sustainable management of streams and riparian areas.;My first chapter asks how riparian restoration, through the alteration of canopy shading, might influence the ability of a native and a non-native invasive submersed macrophyte species---elodea (Elodea nuttallii ) and Eurasian watermilfoil (Myriophyllum spicatum), respectively--- to establish and grow. I conducted separate experiments in artificial stream channels in two locations in California, USA, using shade cloth as a proxy for four different levels of overstory canopy shading. I found that increasing shade decreased the growth rates of both species but had no effect on establishment: both species had high survival in 0- 90% shade.;For my second chapter, I built on the work from chapter one and conducted a similar experiment in an artificial stream channel system in Davis, CA. I examined how shade levels could affect competitive dynamics between Eurasian watermilfoil and elodea. I also asked whether giving the native elodea a temporal priority (i.e., planting it several weeks earlier than the non-native watermilfoil) could reduce the growth rates or survival of the non-native, and whether this priority effect would interact with shade level. Similar to my earlier findings, the growth rates of both species decreased with greater shade, but I found no significant effect of priority on the growth rates of the non-native species in any shade level.;For my third chapter, I examined the abiotic factors influencing the prolific submersed macrophyte growth in the Interdam Reach of Putah Creek in CA. In this stream reach, huge quantities of aquatic vegetation pose significant management challenges for the water resource agency in charge of regulating water flow and delivery in this area. I surveyed macrophyte cover and a suite of environmental factors throughout the Interdam Reach and, with a collaborator, created boosted regression tree models to identify the most important factors related to macrophyte cover. I found that factors associate with light availability and water velocity had the greatest influence on nuisance submersed macrophyte abundance in the models.