Local dispersion of nonmotile invasive bivalve species by winddriven lake currents

Asian clam (Corbicula fluminea) is among the most aggressive freshwater invaders worldwide causing major ecological and economic damage. However, the mechanisms leading to the water-borne dispersion of the species within aquatic ecosystems, particularly lakes, is an area where research is at a relatively early stage. A numerical model has been developed to analyze and describe the dispersion that is produced by the actions of waves and currents. The model represents the basic particle processes of release (R), water-borne transport (T), and survival (S). The model has been applied to a large, deep lake—Lake Tahoe. The dispersion model results reveal that (1) under episodic, extreme wind forcing, larvae are carried away from the original areas, along a discrete number of preferred pathways, (2) bays can act as retention zones, with low current velocities and recirculating eddies, and (3) the majority of the larvae released in the infested areas stay within these areas or disperse on a small spatial scale. The spread of aquatic invasive species is one of the major ecological and economic threats to lakes and waterways worldwide (Wilcove et al. 1998; Pimentel et al. 2005). In the United States alone, there are about 50,000 invasive species established that cause economic losses estimated at more than $120 billion per year (Pimentel et al. 2005). Invasive species may cause dramatic changes in the ecosystems through perturbations to the interspecific competition, predator–prey interactions, food web structure, nutrient dynamics, hydrologic cycle, and sedimentation rates. Those changes typically lead to the displacement of native species from their natural habitats. The pressure posed by invasive species on native organisms is of such magnitude that their introduction has been ranked second only to habitat loss in the factors that threaten native biodiversity at the global scale (Wilcove et al. 1998). The development of management guidelines for early detection and eradication appears as the primary tool to maintain the ecological integrity of uninvaded habitats (Vander Zanden and Olden 2008). But these guidelines need to be grounded on the sound understanding of the mechanisms by which invasive species spread and colonize new habitats. Such understanding, however, still remains incomplete due to the complex interactions among nonindigenous and indigenous species, humans, and local environmental conditions (Moles et al. 2008). Several modeling approaches have been proposed in the literature to represent the dispersion of aquatic invasive species. Most approaches, although, have focused on the analysis of dispersion between aquatic ecosystems conceptualized as “islands” isolated by extended areas of nonsuitable terrestrial habitats (Figuerola and Green 2002). Dispersion in this case is largely mediated by human activities (Green and Figuerola 2005). For example, the pattern of recreational boating traffic between inland water bodies has been shown to be a good proxy for the spatial distribution patterns of the aquatic invasive bivalve Dreissena polymorpha (zebra mussel) (Buchan and Padilla 1999). Adult mussels and their larvae tend to attach primarily to macrophytes that entangle on boat trailers (Johnson et al. 2001). Once in a given water body, the local dispersion of the invasive species from colonized to uncolonized areas can be facilitated by natural processes, such as water currents (Prezant and Chalermwat 1984; Forrest et al. 2012). Mixing and dispersion of invasive species in a river and a semienclosed harbor have been investigated through recent tracer studies (Carr et al. 2004; Wells et al. 2011; Sun et al. 2013). Similarly, Hrycik et al. (2013) have taken a modeling and measurement approach to this problem in a strongly, tidally forced coastal system. In all these cases, there is either unidirectional flow (albeit reversing for a tidal system) or an enclosed environment. In lakes, however, the flows can have *Correspondence: [email protected] 1 LIMNOLOGY and OCEANOGRAPHY Limnol. Oceanogr. 00, 2015, 00–00 VC 2015 Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10046 additional complexities, driven as they are by episodic, variable winds, subject to waves, and with the interactions of boundaries. For these reasons, less is known about the role of currents in the local dispersion of invasive species or other planktonic organisms. This work focuses on the local dispersion of the larval form of an invasive species by wind-driven and wave-driven currents in lakes. Specifically, we address the prediction of the local transport and dispersion patterns and spatial evolution of the invasive bivalve Corbicula fluminea (Asian clam) in Lake Tahoe, a large subalpine lake on the crest of the Sierra Nevada mountain range (CA-NV; Fig. 1A). C. fluminea was first observed in Lake Tahoe in 2002 in very low numbers (Hackley et al. 2008). Its population has increased now to a level where it is having apparent environmental impacts. Its current known distribution (area 10 m) is patchy along the southeast and south shore of the lake, with the densest population established in Marla Bay (Fig. 1). This distribution is believed to be changing, although, due to C. fluminea’s rapid growth rate and the presence of abundant suitable habitat existing along the shoreline in Lake Tahoe (Herold et al. 2007). C. fluminea is among the most aggressive freshwater invaders worldwide (McMahon 1999). Its invasion success is based on its rapid population growth, early sexual maturity and short turn over time rather than on its tolerance to environmental fluctuations (McMahon 2002). The species is sensitive to low oxygen conditions and requires sustained water temperatures of 15–16 C or above for reproduction (McMahon 1999; Sousa et al. 2008; Wittmann et al. 2012). C. fluminea generally forms colonies or beds with densities that may exceed 6000 clams m (Aldridge and McMahon 1978; Wittmann et al. 2012), preferably in areas of coarse and sandy sediments (Karatayev et al. 2003). C. fluminea filters out phytoplankton and other suspended particles in the water column which are also food sources for native filter-feeding organisms. It can use its pedal foot to feed on organic matter in the sediment and competes for food resources with native benthic organisms (Hakenkamp et al. 2005). C. fluminea can also affect aquatic ecosystem processes in other ways. Excretion of inorganic nutrients, particularly nitrogen, can stimulate the growth of algae and macrophytes (Sousa et al. 2008). The species is believed to facilitate the introduction of parasites, diseases, and other invasive species (Vaughn and Hakenkamp 2001; Sousa et al. 2008). They have also been shown to facilitate the invasion of zebra or quagga mussel by creating localized high calcium environments, as shells from dead clams leach this potentially limiting element (see Hessen et al. 2000, and references therein). Passive (natural) hydraulic transport by water currents is considered to be the main mechanism for the local dispersal of C. fluminea (McMahon 1999). The local dispersion largely occurs during the larval and juvenile stages of their life when, as a result of their low density (total dry weight of 0.1 mg at 200 lm shell length) (Aldridge and McMahon 1978), larvae may remain suspended in the surface mixed layer even under minimal turbulence (McMahon 1999). Larvae are not motile but can travel long distances drifting with water currents. The contribution of currents in the local dispersion of Asian clam, however, is not known. Laboratory studies on the dispersion of C. fluminea by water currents have focused on the transport of adults (Prezant and Chalermwat 1984; Williams and McMahon 1989). These studies have been conducted with strong, unidirectional and steady currents, more characteristic of rivers than lakes. In lakes, currents are largely forced by winds, waves and convective processes, and are characterized by lower magnitude as well as a higher temporal and spatial variability. Our goal is to characterize the transport pathways of young life stages of C. fluminea and analyze the development of the clam population in a lake environment. Lake Tahoe (CA-NV) is used as a test case, although the modeling approach may be applied in any aquatic system. The approach embodies a number of consecutive steps. These are the determination of the mechanism of larval entrainment (suspension) into the ambient flow; the transport and dispersion characteristics due to the spatially and temporally varying meteorological conditions, lake stratification, and larval density; the resulting preferred migration pathways; and the exposure to environmental stressors (temperature and light) experienced during the journey.