Print PageLady Bird Lake and Lake Austin are riverine reservoirs on the Colorado River in Travis County, Texas. Aquatic plants provide habitat, sediment entrainment and other benefits to these ecosystems, but native macrophytes were limited in both reservoirs since their initial impoundments. Macrophyte communities were dominated by Eurasian watermilfoil (Myriophyllum spicatum) until 1999, when hydrilla (Hydrilla verticillata) infested Lake Austin, rapidly covering 320 acres. Its dense, deep water growth negatively impacted public safety, recreation, water conveyance, water supply intakes, and ecosystem health. With limitations on herbicide use, triploid grass carp were periodically stocked beginning in 2003. While the fish have provided significant hydrilla control, potential loss of non-target vegetation in both reservoirs was a concern.
In order to limit this potential and reduce exotic re-infestations, a program was initiated in 2004 to increase diversity and cover of desirable native aquatic vegetation. Due to reservoir size and physical complexity, a “founder colony” approach was used that would establish small islands of source vegetation, generating propagules to spread throughout the reservoirs over time. Twenty sites were installed on each reservoir, using a total of twelve species of submersed, floating-leaved and emergent plants at varying depths with different types of herbivore exclosures.
Results/Conclusions
In 2004, five species were planted both individually in ring cages and grouped in low profile tray cages for herbivore protection. Three species (Vallisneria americana, Sagittaria graminea, and Justicia americana) showed first year survival rates of over 85 % and between 1.4 and 2.8 fold increases in cover outside these exclosures. Additional species were planted in following years, but intense herbivory was the primary factor limiting both survival in and spread outside exclosures. In 2008, 4 m X 8 m pens were constructed around the smaller cages for added protection. Nymphaea odorata and Nymphaea mexicana spread from single plants to fill the pens in one growing season and Pontederia cordata colonies expanded by at least 50 percent each year. Overall diversity also increased within these larger exclosures, with six other species increasing in cover. The most successful species in both reservoirs were Vallisneria americana, with a five-fold increase in coverage outside founder colonies, and Sagittaria graminea, with a doubling in spread. These results indicate that larger pen exclosures planted with a variety of species (including V. americana and S. graminea) are critical to overcoming herbivore pressure and establishing founder colonies needed for any successful reservoir revegetation effort.
