As members of the Presidio Trust stand on a fingernail of beach on the southern shore of Mountain Lake, an excited San Francisco resident stops by to comment on the organization’s lake restoration efforts. “This looks a lot better,” he yells, looking out over the only natural lake in the Presidio of San Francisco, a former U.S. Army base that became a part of Golden Gate National Recreation Area in 1994. Since 2001, the Presidio Trust, a federal agency created to preserve and transform the Presidio, has been working to make this little urban oasis healthy and functional again. Mountain Lake is adjacent to the West Coast’s high-traffic Highway 1, a busy golf course, and the Richmond neighborhood of San Francisco. These lively urban areas have contributed to degradation of the lake’s water clarity. In addition, the ecosystem’s biological diversity has been affected by non-native plants and animals that have pushed out indigenous species. Terri Thomas, the Presidio Trust’s director of conservation, stewardship and research, points to the lake and a section of shoreline that used to be ringed by non-native eucalyptus trees. “After we took the eucalyptus out, the lake got clearer,” she says. The removal of the eucalyptus trees was part of the first phase of restoration work at Mountain Lake. The initial phase also included weeds being removed and 14,000 native plants being planted. Restoration volunteers include Presidio Park Stewards, local students, corporate groups, and Friends of Mountain Lake Park. Thomas then points to traffic zooming by on Highway 1, perched right above the body of water. “We have spent the last 15 years trying to remediate the bottom of the lake from lead and contaminants from the highway,” she says. In addition to polluting the water, the highway runoff contributed to changing Mountain Lake’s depth from 9 meters (30 feet) to just 3 meters (10 feet). The lake has also shrunk from the size when the Anza expedition camped beside the body of water in 1776. The Anza expedition was an exploration of California led by Mexican-Spanish explorer Juan Bautista de Anza. “We think [the lake's size] has been reduced about a third,” Thomas says. Between September 2012 and December 2013, the Mountain Lake was dredged. The effort removed 13,380 cubic meters (17,500 cubic yards) of contaminated sediment, which was relocated to a nearby landfill. Reintroduction Research While standing on the shore, Jonathan Young, a wildlife ecologist interning for the Presidio Trust, spots a carp moving along the shallows. “That would be so easy to get,” he says, clearly wishing he had a net to nab the non-native. Getting rid of Mountain Lake’s carp is one of the biggest challenges to the Presidio Trust’s restoration efforts. “Our biggest problem is that there are non-native fish that damage the bottom and the vegetation,” Thomas says. Young notes that being a natural space within an urban area means that over the years, San Franciscans have deposited non-native turtles, frogs, and fish into Mountain Lake. “This is a repository for people’s pets,” he says. The Presidio Trust initially used electrofishing in an attempt to eradicate the carp. The method uses electricity to stun fish that are then pulled out of the water with nets. Currently, Young uses gill nets to remove the carp from Mountain Lake. After the carp are removed from the lake, they are transported to Al Wolf at the Sonoma Reptile Rescue Center in nearby Sebastopol. “He adopts them out to private, secluded vineyard ponds, where they won’t cause the same issues up there as they do down here,” Young says. Once these non-native species are removed, the Presidio Trust can begin reintroducing native species back to the lake. Young says the organization has been looking at museum records and other documents to see what native species used to call Mountain Lake home. “We were able to get an idea of plants and animals that used to be in the area,” he says. The Presidio Trust hopes to begin with placing Pacific chorus frogs, western pond turtles, freshwater mussels, Pacific newts, and three-spined stickleback (a small fish) back into Mountain Lake and the surrounding land. Eventually, they hope to welcome the endangered California red-legged frog. They are hoping to start reintroducing the three-spined stickleback in late 2014. The organization still needs to do more work to get Mountain Lake ready for the fish. “First, we need to remove all the predatory invasive fish, while at the same time, get the submerged plants [back in the lake], which will provide needed habitat for the sticklebacks,” Young says. The group has already begun putting aquatic plant species back into Mountain Lake. They place the seeds and roots of the plants in mud balls that are then thrown into the lake. Cages are then erected around the mud balls and vegetation so carp cannot de-root them while foraging for invertebrates that live in and around the new plants. The restoration is a complicated process that relies on goals having to be met before the native plants and animals can be reintroduced. “It’s a really dynamic project,” Young says. “There are a lot of layers to it.” Since Mountain Lake is a natural feature surrounded by a city, its successful restoration depends on local residents who use the park. “Ultimately, urban restoration projects like this,” Thomas says, “depend on the community.”