MARCH 2016 RESEARCH Physical Controls on the Distribution of the Submersed Aquatic Weed Egeria densa in the Sacramento–San Joaquin Delta and Implications for Habitat Restoration John Durand1, 2, William Fleenor1, 3, Richard McElreath4, Maria J. Santos5, and Peter Moyle1, 2 water turbidity, and water column depth. We found Volume 14, Issue 1 | Article 4 doi: http://dx.doi.org/10.15447/sfews.2016v14iss1art4 that increasing water column depth strongly limited E. densa occurrence, especially when depth at mean * Corresponding author: [email protected] lower low water (MLLW) exceeds 2 m. The highest 1 Center for Watershed Sciences probability of occurrence occurred at locations with a University of California–Davis, Davis, CA USA water column depth of -1 to 2 m at MLLW. Turbidity 2 Wildlife, Fish and Conservation Biology, University of California–Davis, Davis, CA USA had a reliably negative effect on E. densa occurrence; 3 Department of Civil Engineering, as water clarity has increased in the Delta, it has University of California–Davis, Davis, CA USA likely favored the spread of the plant. Neither mean 4 Department of Human Behavior, Ecology and Culture water velocity nor maximum water velocity had Max Planck Institute for Evolutionary Anthropology a reliable effect on E. densa probability, in spite Leipzig, Germany of scientific and observational evidence that it is Department of Anthropology sensitive to flows. These results suggest potentially University of California, Davis Davis, CA USA serious problems with restoration projects that 5 Department of Innovation, Environmental and Energy Sciences Universiteit Utrecht, Utrecht Netherlands emphasize shallow water habitat in the range favored by E. densa. Without some way to manage spread of the plant—through spraying, sediment loading, or gating—channels in such projects are at risk of being ABSTRACT taken over by E. densa. However, these results should be interpreted in light of the fact that water outflow The invasive aquatic plant Egeria densa (Brazilian in water year 2008 was very low, and that E. densa waterweed) is a submersed aquatic plant that has abundance may be partially controlled by higher expanded its distribution in both its native and water flows than those considered here. introduced range. Because the plant grows so densely, it can become a problem for management of waterways and habitat restoration projects. KEY WORDS It is difficult to remove once established and submersed aquatic vegetation, invasive organisms, mechanical and chemical controls have shown estuaries, turbidity, water quality, hydrodynamics, limited effectiveness. Here we analyze the distribution restoration of E. densa in the Sacramento–San Joaquin Delta (the Delta) of California, USA, using environmental variables that include mean water velocity, mean SAN FRANCISCO ESTUARY & WATERSHED SCIENCE VOLUME 14, ISSUE 1, ARTICLE 4 INTRODUCTION can be effective for short periods, but rapid E. densa growth during warm periods allows it to quickly Among the challenges facing stream and estuarine return after mow-down. Mowing can also promote restoration is development of appropriate geomorphic vegetative dispersal by creating plant segments that and hydrodynamic conditions to favor ecosystems readily disperse and propagate (Oliveira et al. 2005). that support native species. Measures to improve abiotic conditions can sometimes be rendered inef- The ability of Egeria densa to invade and alter aquat- fective by the secondary invasion of alien species ic habitats, combined with its resistance to control, that out-compete or prey upon desired natives. In contributes to the development of restoration-resis- addition, some alien species serve as ecosystem engi- tant novel ecosystems (sensu Hobbs et al. 2009). This neers that fundamentally and irreversibly change the is among the challenges facing restoration projects in physical and biotic habitat, creating, in effect, novel the Sacramento–San Joaquin Delta (the Delta) (Essex ecosystems. The invasive aquatic plant Egeria densa Partnership 2009; Yarrow et al. 2009), where E. densa Planchon from the frog-bit family (Hydrocharitaceae) became established around 1946 by introductions is one such invader. Native to Brazil, Uruguay and from the aquarium trade (Anderson 1990). Concerns Argentina, and distributed internationally by the about the plant as an invasive weed date from the aquarium trade (Cook and Urmi–König 1984; Haynes 1990s, when it rapidly expanded its local range 2000), E. densa has expanded its range so extensively (Jassby and Cloern 2000). As late as 1993, E. densa that it has become a nuisance weed throughout the is mentioned without alarm (Lehman 1996), but by temperate zones of the world (Bini et al. 1999; Bini 1996 it was reported that E. densa stands were har- and Thomaz 2005). Among the problems it creates boring invasive sunfish (Centrarchidae), including the are waterway blockages, reservoir flow interruption, piscivorous largemouth bass (Micropterus salmoides) water quality alteration, native vegetation displace- (Grimaldo and Hymanson 1999). A 1999 study sug- ment, and fish habitat degradation (Anderson 1990; gested that E. densa could dominate subtidal restora- Yarrow et al. 2009). The plant roots in the substrate tion habitats in the Delta (Simenstad et al. 1999). of slow-moving rivers, lakes, and estuaries, develop- Dense stands of E. densa in the Delta facilitate fish ing elongated shoots that form a thick canopy at the species that do well in warm, clear, slow-moving water surface. Dense vertical stands reduce water water with vertical physical structure (Nobriga et al. velocity, which increases both sediment deposition 2005; Ferrari et al. 2014), particularly Centrarchidae and thermal stratification (Santos et al. 2009; Yarrow introduced from the southeastern United States that et al. 2009). The resulting increases in water clarity are adapted to such conditions in their native habitat. and temperature can promote the further growth and These include bluegill sunfish (Lepomis macrochi- spread of E. densa itself, while facilitating invasion rus), green sunfish (L. cyanellus), redear sunfish (L. by other alien species, particularly fishes (Grimaldo microlophus), warmouth (L. gulosus) and largemouth and Hymanson 1999; Brown 2003; Nobriga et al. bass (Brown 2003; Nobriga et al. 2005). These fishes 2005). use the stems and canopy of E. densa as structure Egeria densa is difficult to control once established upon which to carry out feeding and predation: (Curt et al. 2010; Cal-IPC 2013). Attempts at bio- sunfish navigate inside stands, seeking invertebrate control have been limited, in part because of the prey; while larger bass wait at the edges, preying dearth of herbivores feeding exclusively on the plant upon invertebrates and fish that move along the edge (Yarrow et al. 2009). Control using triploid grass carp (Nobriga and Feyrer 2007). The sunfish are suspected (Ctenopharyngodon idella) has had limited success, to be direct and aggressive competitors of some and poses some dangers to native organisms (Bain native fishes such as Sacramento perch (Archoplites 1993). Control using aquatic insects and fungi has interruptus), which were largely extirpated from the been largely ineffective (Mitchell 1980; ARS 2012; Delta by the 1960s (Marchetti 1999; Moyle 2002); Walsh et al. 2013). Chemical control is commonly while largemouth bass may compete with and prey used, but requires repeated applications of herbi- upon native and introduced fishes (Nobriga and cide to be effective, and is potentially toxic to other Feyrer 2007; Ferrari et al. 2014). organisms (Yarrow et al. 2009). Mechanical control 2 MARCH 2016 Anecdotal evidence suggests that E. densa facilitates pumped into a system of canals for agricultural and predators’ ability to capture fishes, especially species urban use in the central and southern parts of the that are poorly adapted to such habitat (Brown 2003). state (Lund et al. 2007). However, the modern Delta Largemouth bass are known to feed upon splittail environment results from changes to the estuary (Pogonichthys macrolepidotus), Mississippi silversides that began over 150 years ago, when Delta reclama- (Menidia audens), and sunfishes (Nobriga and Feyrer tion efforts responded to erosional and depositional 2007). Native minnows (Cyprinidae) may be particu- processes set in motion by the California Gold Rush larly vulnerable under these conditions because their of the 1850s (Thompson 1957; Lund et al. 2007; streamlined, fusiform shape is best adapted to take Whipple et al. 2012). These alterations created an advantage of open water or water moving along an inverted topography typical of many of the world’s edge. This may make them vulnerable to predation by anthropogenically influenced estuaries. Steep dikes largemouth bass while being unable to capitalize on border agricultural islands that have subsided below refuges within the stands of submersed aquatic veg- sea level from decades of plowing, burning and oxi- etation (SAV) (Ferrari et al. 2014). Likewise, regions dation (Thompson 1957; Mount and Twiss 2005). that are altered from open water to structurally com- These subsided islands are vulnerable to dike failure plex habitat are generally undesirable to the largely (Lund et al. 2007; Suddeth et al. 2010). Once flooded, pelagic delta smelt (Hypomesus transpacificus) and they create deep lake-like environments that can be longfin smelt (Spirinchus thaleichthys). In fact, such uneconomical or impractical to reclaim,