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A Survey of the New River Aquatic Plant Community in Response to Recent Triploid Grass Carp Introductions into Claytor Lake, Virginia
Matthew A. Weberg, Brian R. Murphy, Andrew L. Rypel, and John R. Copeland

Southeastern Naturalist, Volume 14, Issue 2 (2015): 308–318

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Southeastern Naturalist M.A. Weberg, B.R. Murphy, A.L. Rypel, and J.R. Copeland 2015 Vol. 14, No. 2 308 2015 SOUTHEASTERN NATURALIST 14(2):308–318 A Survey of the New River Aquatic Plant Community in Response to Recent Triploid Grass Carp Introductions into Claytor Lake, Virginia. Matthew A. Weberg1,*, Brian R. Murphy1, Andrew L. Rypel1, 2, and John R. Copeland3 Abstract - Aquatic plant communities play critical roles in the form and function of stream ecosystems. In this study, we surveyed the aquatic-plant community along a 39-km reach of the New River, VA, in response to triploid Ctenopharyngodon idella (Grass Carp) stockings to control Hydrilla verticillata (Hydrilla) in Claytor Lake. We utilized drift-net sampling methods and visual observations to document the current plant community in this reach. Nine of 12 aquatic plant species identified in our survey have been documented as preferred forage for Grass Carp. These findings may indicate that migrating Grass Carp could alter the plant community in this reach. We recommend continued monitoring of this system to characterize any future effects of Grass Carp herbivory. Introduction Aquatic plants are vital to the overall structure and function of lotic ecosystems (Minshall 1978). In mid-sized rivers, aquatic plants often comprise a significant fraction of primary production (Hill and Webster 1983, Minshall 1978, Rodgers et al. 1983, Vannote et al. 1980), and are thus especially important in these environments. For example, diverse aquatic-plant communities provide complex and heterogeneous habitat for a large variety of aquatic species, as well as refuge from predators (Allen and Castillo 2007, Grenouillet et al. 2002). Furthermore, aquatic plants in lotic habitats are known to play important roles in nutrient dynamics and sediment transport (Clarke and Wharton 2001, Madsen et al. 2001). Therefore, changes to the diversity and abundance of aquatic plants have the capacity to severely alter river ecosystems (Holmes et al. 1998), including the recreational and industrial benefits these environments provide to humans (Strang e et al. 1999). Invasive species are one of the foremost threats to the integrity of aquatic ecosystems at multiple scales. Pimentel et al. (2005) estimated the monetary cost of invasive species management for 6 developed nations at >$US335 billion per year and growing. Additionally, the economic effects of invasive species can be highly localized and severe. For example, property values in Wisconsin lakes invaded by Myriophyllum spicatum L. (Eurasian Water Milfoil) on average experienced a 13% decline following invasion (Horsch and Lewis 2009). Similarly, Hydrilla 1Department of Fisheries and Wildlife Conservation, Virginia Tech, Blacksburg, VA 24061. 2Current address - Wisconsin Department of Natural Resources, Bureau of Science Services, Madison, WI 53707. 3Virginia Department of Game and Inland Fisheries, Blacksburg, VA 24060. *Corresponding author - Manuscript Editor: Julia Cherry Southeastern Naturalist 309 M.A. Weberg, B.R. Murphy, A.L. Rypel, and J.R. Copeland 2015 Vol. 14, No. 2 verticillata (L.f.) Royle (Hydrilla) infestations can block irrigation canals, hasten sedimentation in reservoirs, interfere with water supplies, impede boat navigation, and reduce fisheries productivity (Langeland 1996). Hydrilla was first documented in 2003 in Claytor Lake, Pulaski County, VA, by Virginia Department of Game and Inland Fisheries (VDGIF) biologists (J.R. Copeland, VDGIF, Blacksburg, VA, pers. comm.). Claytor Lake is an impoundment of the upper New River located in the Valley and Ridge physiographic province. In 2011, triploid (reproductively sterile) Ctenopharyngodon idella (Valenciennes in Cuvier and Valenciennes) (Grass Carp) were stocked into the reservoir to manage the expanding Hydrilla infestation using an incremental stocking approach. This strategy aimed to gradually reduce Hydrilla abundance over several years through periodic low-level Grass Carp stockings (Bain 1993, Chilton and Magnelia 2008). However, relatively long migrations (up to 500 km) by Grass Carp have been observed in large-river environments in both their native range and the US (Gorbach and Krykhtin 1988). Such occurrences could bring stocked Grass Carp into contact with macrophyte communities in river reaches adjacent to reservoirs. The New River upstream of Claytor Lake is an important aquatic resource for the region and supports a highly valued sport fishery (Copeland 2014). Therefore, this river reach could be negatively affected if upstream migrations by Grass Carp lead to reductions in native vegetation abundance. In 2012, we documented low levels of Grass Carp migration into this reach of the New River through a concurrent telemetry study (Weberg 2013). Thus, it is important to understand the current aquatic-plant community present within this river reach as a baseline for assessing potential future ecological alterations due to Grass Carp herbivory. Despite the documentation of Hydrilla within the watershed and the recent introduction of Grass Carp into Claytor Lake, no studies have examined the New River aquatic-plant community since the late 1970s (Hill and Webster 1983, Rodgers et al. 1983). We conducted a drift survey of the aquatic-plant communities at 8 sites along a 39-km reach of the New River directly upstream of Claytor Lake. The objectives of the survey were to: (1) determine if Hydrilla had become established within this reach and (2) document the relative abundances of submersed and emergent macrophytes present within this reach to compare with identified plant preferences of Grass Carp and assess the potential for future herbivory effects should significant Grass Carp migrations occur. Methods Study site The New River originates in the Appalachian highlands of North Carolina and flows northwest through Virginia and West Virginia before joining the Ohio River (Hill and Webster 1982). Within southwest Virginia, the New River is characterized by a steep gradient, narrow floodplain, and primarily bedrock channel. Our study focused on the 39-km river reach between Buck Dam and the head of Claytor Lake (generally marked by a set of riffles located near Allisonia, VA; Fig 1.). Southeastern Naturalist M.A. Weberg, B.R. Murphy, A.L. Rypel, and J.R. Copeland 2015 Vol. 14, No. 2 310 Assessment of aquatic plant community upstream of Claytor Lake During July 2012, we surveyed the aquatic-plant community by canoe starting at Buck Dam and concluding at the Allisonia rapids at the head of Claytor Lake. We visually surveyed for aquatic plant species along this reach; in deeper pool sections, we randomly threw a double-sided rake attached to a rope and slowly retrieved it to check for plant presence. We recorded all aquatic-plant species as we encountered them, maintained a running list, and placed voucher specimens of each species on ice for verification by taxonomic experts at the Massey Herbarium at Virginia Tech, Blacksburg, VA. To gauge the occurrence and abundance of aquatic-plant species along this reach, we also collected a single 5-minute drift-net sample using a 7.6- m beach seine approximately every 5 river-km using the methodology outlined by Owens et al. (2001). We collected drift samples by wading into the river at each sampling site and stretching the seine net perpendicular to the flow of the river. We removed from the net all aquatic plant fragments collected during each drift sample and stored them on ice. At the conclusion of the survey, we separated the samples by species, and blotted dry and weighed (g fresh weight [FW]) them. Results We identified 13 macrophyte species, of which 9 have been identified as readily or moderately consumed by Grass Carp (Table 1; Opuszynski and Shireman 1995). Figure 1. Surveyed section of the Upper New River including locations of drift-net sampling sites between Buck Dam near Ivanhoe, VA, and the start of Claytor Lake near Allisonia, VA. Southeastern Naturalist 311 M.A. Weberg, B.R. Murphy, A.L. Rypel, and J.R. Copeland 2015 Vol. 14, No. 2 Four of the 7 species sampled in the drift-net survey occurred in relatively low abundance (less than 21% of the plant-fragment sample per site [g FW]); however, we detected Elodea canadensis (Water Weed) and Potamogeton crispus (Curly Leaf Pondweed) at all sites (Table 2). While absent from the site-6 drift sample, we also observed Podostemum ceratophyllum (Riverweed) throughout the entirety of the survey, especially within shallow run and riffle habitats. The highest amount of plant fragments collected in our drift-net samples was at site 5 (365 g; Fig. 2). We did not detect Hydrilla on the surveyed river reach. Overall, aquatic plant fragments collected in our drift-net samples were dominated by either Riverweed or Water Weed (Table 2, Fig. 2). In total, Water Weed comprised more than 62% of the total plant-fragment sample (g FW) during all driftnet surveys while Riverweed accounted for of approximately 23%. Interestingly, Riverweed dominated fragment samples at the 4 most-upstream sites, but Water Table 2. Percent by weight of total sampled plant fragments for each species from drift-net samples taken approximately every 5 river-km during an aquatic plant survey of the New River between Buck Dam and Allisonia, VA, in July 2012. Site Common name 1 2 3 4 5 6 7 8 Water Weed 29.0 46.9 9.6 9.5 87.5 58.3 69.2 20.2 Curly Leaf Pondweed 0.4 5.4 1.1 0.3 10.8 29.7 12.9 42.0 Longleaf Pondweed 4.6 6.1 1.5 - 0.7 - - - Leafy Pondweed - - 0.1 0.3 0.4 1.2 12.9 20.5 Wild Celery 0.4 12.2 0.2 0.6 0.3 10.7 - - Riverweed 65.7 29.3 87.5 89.3 0.3 - 4.8 17.3 Musk-grass - - - - - - 0.1 - Table 1. List of aquatic plant species documented during a float survey of the New River between Buck Dam and Allisonia, VA, in July 2012. Determinations of prior species documentations were based on survey results from Hill and Webster (1984). *Indicates plants identified as readily or moderately consumed by Grass Carp (Opuszynski and Shireman 1995). Prior Common name Scientific name Classification documentation Water Weed* Elodea canadensis (Michx.) Britton Submersed Yes Curly Leaf Pondweed* Potamogeton crispus L. Submersed Yes Longleaf Pondweed* Potamogeton nodosus Poir. Floating-leaved No Leafy Pondweed* Potamogeton foliosus Raf. Submersed No Wild Celery* Vallisneria americana Michx. Submersed Yes Riverweed Podostemum ceratophyllum Michx. Submersed Yes Musk-grass* Chara L. Algae No American Water-willow Justicia americana (L.) Vahl Emergent Yes Giant Duckweed* Spirodela polyrhiza (L.) Schleid. Floating-leaved No Arrowhead* Sagittaria sp. Emergent No Common Cattail* Typha latifolia L. Emergent Yes American Bulrush Schoenoplectus pungens (Vahl) Palla Emergent No Grassleaf Mudplantain Heteranthera dubia(Jacq.) MacMill Submersed No Southeastern Naturalist M.A. Weberg, B.R. Murphy, A.L. Rypel, and J.R. Copeland 2015 Vol. 14, No. 2 312 Weed dominated the samples obtained at 3 of the 4 lower-most sites. We also detected Curly Leaf Pondweed in low abundance in drift-samples at the 4 upstream sites, but its abundance increased substantially at the 4 downstream sites. The final downstream site was in fact dominated by Curly Leaf Pondweed and also had more equal fractions of Riverweed and Water Weed in sampled drift fragments. Longleaf Pondweed and Vallisneria americana (Wild Celery) were relatively uncommon species and appeared to be confined to upstream river reaches. Discussion Aquatic plant community of the New River upstream of Claytor Lake Understanding aquatic-plant communities in mid-sized rivers can provide important insight into ecosystem structure and stability (Gregg and Rose 1982, Minshall 1978). However, comparatively few studies have addressed riverine aquatic-plant communities in the US, especially in the Southeast (Franklin et al. 2008). Our study identified a more-diverse aquatic-plant community in this stretch of the New River than was found during prior investigations (Hill and Webster 1984). In both terrestrial and aquatic-plant communities, greater occurrence and abundance of native species is believed to provide resiliency against the establishment of introduced species (Capers et al. 2007, Dukes 2001, Larson et al. 2013), which could explain the apparent absence of Hydrilla within this reach. However, a lack of Hydrilla may also be a function of early detection within Claytor Lake and the possibility that this section of the New River may have been sampled prior to a Figure 2. Plant fragments (g FW) collected for the 3 most-abundant species in drift-net samples taken in July 2012 at 8 sites in the New River between Buck Dam and Allisonia. The vertical dashed line (grey) indicates the furthest location upstream of Claytor Lake at which we documented Grass Carp during a concurrent telemetry study (Weberg 2013). Southeastern Naturalist 313 M.A. Weberg, B.R. Murphy, A.L. Rypel, and J.R. Copeland 2015 Vol. 14, No. 2 future “invasion wave” (Neubert and Caswell 2000, Skarpaas and Shea 2007). For example, there are increasing reports of established Hydrilla beds within the New River downstream of Claytor Dam (J.R. Copeland, pers. comm.), a reach that was not sampled in this study. Suitable habitat for aquatic plants in riverine environments is often limited by flow conditions (Butcher 1933, Sand-Je nsen and Madsen 1992, Sprenkle et al. 2004) as well as through variations in dispersal (Bunn and Arthington 2002, Riis and Sand-Jensen 2005, Santamaria 2002), often leading to patchy distributions on the landscape. Similarly, the amount of plant fragments collected in our drift-net samples varied greatly among sites, which could be attributed to the high gradient and primarily bedrock channel of the upper New River. The most lush stands of more-abundant species such as Wild Celery, Potamogeton foliosus (Leafy Pondweed), and Water Weed appeared to be highly localized at depositional zones within the river. Therefore, these depositional areas may be of significant ecological importance for aquatic biota within this reach. Prior to our study, Hill and Webster (1984) identified Riverweed as the most abundant plant species within this reach of the New River while Water Weed accounted for just 0.03% of macrophyte coverage. Conversely, the results from our drift-net survey indicate Water Weed may be the most abundant macrophyte, possibly suggesting a temporal shift in community structure. Riverweed was also abundant in our driftnet survey, although due to its epilithic nature, our sampling method may have underestimated its true abundance in this reach of the New River. Additionally, Hill and Webster (1984) used aerial photography combined with ground-truthing to determine overall coverage and abundance of plant species, which could further explain the observed differences in results. We collected no emergent plant species during our drift-net survey; however, we observed patchily distributed stands of Justicia americana (American Water-willow) throughout the survey. Hill (1981) identified American Water-willow as the most productive macrophyte within the upper New River, although he speculated that its localized distribution limited the species’ overall contribution to the stream’s energy budget. Although our study provides a much-needed description of the current aquatic-plant community of the New River upstream of Claytor Lake, future monitoring may also be important to identify potential alterations of plant abundance or community structure due to Grass Carp herbivory. Evidence and implications of Grass Carp herbivory on Riverweed Riverweed can be the dominant source of autotrophic production in Appalachian Rivers (Hill and Webster 1983) and may promote increased macroinvertebrate production (Hutchens et al. 2004) and stream-fish abundances (Argentina et al. 2010). If Grass Carp herbivory on Riverweed were to increase substantially, it could have major ecological repercussions. Currently, no studies have identified Riverweed as preferred forage for Grass Carp; however, we incidentally observed Riverweed within the alimentary tract of numerous Grass Carp collected near the Allisonia rapids during a concurrent study of Grass Carp growth in fall 2012 (Weberg 2013). The presence of Hydrilla in nearby shoal areas of Claytor Lake at the time of our Southeastern Naturalist M.A. Weberg, B.R. Murphy, A.L. Rypel, and J.R. Copeland 2015 Vol. 14, No. 2 314 Grass Carp sampling efforts offers additional circumstantial evidence of Grass Carp herbivory on Riverweed. Riverweed has been noted as a preferred macrophyte for other herbivorous taxa such as Branta canadensis L. (Canadian Geese) and Procambarus spiculifer (LeConte) (White-tubercled Crayfish) (Parker et al. 2007); however, the voracious feeding pattern of Grass Carp on preferred plant species (up to 100% of body weight per day; Osborne and Riddle 1999) is of particular concern. For example, prior to 2012 Riverweed was abundant on the substrate at the Allisonia rapids, whereas in fall 2012 the substrate in this area was apparently devoid of Riverweed presumably due to Grass Carp herbivory (J.R. Copeland, pers. comm.). Prior studies have noted overall declines of Riverweed density within Appalachian streams (Argentina et al. 2010, Munch 1993). If this trend has already begun in the New River, it could be compounded by Grass Carp herbivory in this reach. Implications of potential Grass Carp migrations The majority of macrophyte species observed in our examination have been documented as preferred forage for Grass Carp that could migrate into that area. These findings, combined with the localization of the most-abundant plant species identified during our survey, indicate that the New River plant community could be vulnerable to Grass Carp herbivory. Beyond our observations during 2012, the overall migration rates of Claytor Lake Grass Carp are unknown. However, additional evidence indicates migration rates could increase as Hydrilla abundance declines in Claytor Lake, and as Grass Carp grow in size and approach sexual maturity. A telemetry study of juvenile Grass Carp stocked into Claytor Lake found that just 2 of 75 radio-tagged fish migrated into the New River over the 2-y study, although the instances of migration occurred in 2012 after Hydrilla abundance in Claytor Lake was significantly reduced (Weberg 2013). Thus, migration rates could increase as a result of Grass Carp searching for food if vegetation resources remain limited within Claytor Lake. Additionally, Grass Carp life stage is believed to influence movement patterns (Gorbach and Krykhtin 1988). For example, mature Grass Carp (600–730 mm total length [TL], 4.0–6.0 kg) stocked in Lake Guntersville, AL showed significantly higher rates of movement than juveniles, and completed migrations as far as 71 km upstream (Bain et al. 1990). Accordingly, 32 Grass Carp (mean TL = 716 mm) were sampled within the New River upstream of Claytor Lake in the spring and early summer 2013 during electrofishing assessments (J.R. Copeland, unpubl. data). During 2011–2012, the first 2 years following the initial stocking of Grass Carp in Claytor Lake, only 4 Grass Carp had been sampled in this reach. The New River was subject to high flows throughout the spring and early summer of 2013, and 27 of the Grass Carp collected in 2013 were captured within close proximity of Allisonia. Therefore, it is possible that the increase in Grass Carp collections may be a result of high flows allowing access to more habitats. Research implications Based on our examination of the aquatic-plant community in the New River upstream of Claytor Lake, it appears that greater monitoring is needed to fully Southeastern Naturalist 315 M.A. Weberg, B.R. Murphy, A.L. Rypel, and J.R. Copeland 2015 Vol. 14, No. 2 understand the effects of Grass Carp in lotic ecosystems. We suggest that annual surveys of water quality and vegetation, fish, and invertebrate abundance, combined with continued monitoring of Grass Carp migration rates, could provide an important case study for resource managers. Grass Carp have been documented in numerous medium–large rivers throughout the US (Elder and Murphy 1997, Guillory and Gasaway 1978, Pflieger 1978), yet examinations of the effects Grass Carp have on the form and function of aquatic ecosystems has been limited to lakes and reservoirs. In Lake Conroe, TX, the complete removal of macrophytes by Grass Carp resulted in a major biomass shift to more-pelagic fish species (Bettoli et al. 1993), increased nutrient levels, and decreased water clarity due to higher algal biomass (Maceina et al. 1992). However, river systems differ greatly in structure and function compared to lentic environments, thus limiting comparability in the assessment of potential Grass Carp effects. Hydrilla continues to pose major threats to aquatic ecosystems at all scales, including to the integrity of riverine aquaticmacrophyte communities. Grass Carp will likely remain a major management tool for addressing invasive Hydrilla infestations and are also likely to spread outside of their introduced range as an invasive species. Future work on the effects of Grass Carp on the macrophyte communities of the New River could contribute to an important case study of the feasibility of Grass Carp as a management tool for Hydrilla balanced against the conservation needs of upstream ecological communities. Acknowledgments We thank B. Dickinson and P. Chrisman for assisting with field research. Funding for this project was provided by the Virginia Department of Game and Inland Fisheries through a Federal Aid in Sportfish Restoration grant from the US Fish and Wildlife Service, and also was supported by the USDA National Institute of Food and Agriculture, Hatch Project 230537. 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