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Asexual Reproduction and its Potential Influence on the Distribution of an Invasive Macrophyte
Rebecca A. Urban and Matthew E. Dwyer

Northeastern Naturalist, Volume 23, Issue 3 (2016): 408–419

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Northeastern Naturalist 408 R.A. Urban and M.E. Dwyer 22001166 NORTHEASTERN NATURALIST 2V3(o3l). :2430,8 N–4o1. 93 Asexual Reproduction and its Potential Influence on the Distribution of an Invasive Macrophyte Rebecca A. Urban1,* and Matthew E. Dwyer1 Abstract - Utricularia inflata (Swollen Bladderwort) is a submersed macrophyte that is expanding its range in the northeastern US. Although Swollen Bladderwort is a new addition to this region, Utricularia purpurea (Purple Bladderwort) and U. vulgaris (Common Bladderwort) are 2 morphologically similar free-floating species within the invaded waterways. Through a series of greenhouse and field studies, we sought to distinguish traits among these 3 macrophytes. We conducted a greenhouse experiment to compare the vegetative propagation of Swollen Bladderwort and Common Bladderwort in a temperature-controlled tank. In field trials, we examined the displacement of all 3 species by water movement and their distribution across a range of depths at 5 lake sites. Swollen Bladderwort and Common Bladderwort both produced potential propagules, but exhibited differences in their asexual reproduction. New Common Bladderwort branches grew significantly longer, while Swollen Bladderwort fragments exhibited a greater number of new branches. Each new branch has the potential to develop into a new individual as the original stolon decays; this trait may help explain how Swollen Bladderwort is quickly establishing populations in newly colonized lakes. The results of the displacement experiment showed that all species were less likely to remain in the shallows compared to deeper waters. However, displacement of Swollen Bladderwort was greater than Common Bladderwort. Vegetation sampling also indicated that Common Bladderwort and bladderwort species attached to the sediment (U. resupinata [Lavender Bladderwort] and U. intermedia [Flat-leaf Bladderwort]) are found in the shallows, while Swollen Bladderwort and Purple Bladderwort are found at greater depths. These results suggest that Swollen Bladderwort is more susceptible to water movement and may be spread to downstream systems at a faster rate, compared to Common Bladderwort and attached bladderwort species. Introduction Submersed aquatic vegetation shapes the littoral zone in freshwater ecosystems (Carpenter and Lodge 1986). These species mediate trophic interactions (Diehl and Kornijόw 1998), increase sedimentation (Petticrew and Kalff 1992), and influence water-column chemistry (Ondok et al. 1984). Aquatic plant communities are threatened by the introduction of invasive species. Invasive macrophytes jeopardize biodiversity, ecosystem functioning, and the survival of native aquatic plants (Caraco and Cole 2002, Madsen et al. 1991). Utricularia inflata Walt. (Swollen Bladderwort) has expanded its range into the northeastern US, including Pennsylvania (Block and Rhoads 2011), New York (Barringer and Clemants 2003, Mitchell et al. 1994, Titus and Grisé 2009), 1Biology Department, Lebanon Valley College, 101 North College Aveniue, Annville, PA 17003. *Corresponding author - Manuscript Editor: C. Thomas Philbrick Northeastern Naturalist Vol. 23, No. 3 R.A. Urban and M.E. Dwyer 2016 409 Massachusetts (Sorrie 1992), and Rhode Island (K. DeGoosh, RI Department of Environmental Management, Providene, RI; pers. comm.). Previous research has documented how the presence of Swollen Bladderwort may cause the local extirpation of native isoetid species in Adirondack lakes due to an increase in light attenuation (Urban et al. 2009). The decline of the native isoetid Eriocaulon aquaticum (Hill) Druce (Seven-angle Pipewort) results in cascading effects on lake chemistry, including increased sediment porewater pH, carbon dioxide, and ammonium, as well as decreased sediment redox potential (Urban et al. 2006). The effect is first observed in the sediment porewater; increased concentrations of porewater ions in turn facilitate a diffusion of these ions into the water column, eventually causing a positive feedback and accelerating ecosystem change (Urban et al. 2013). Although Swollen Bladderwort is a new addition to Adirondack mountain lakes, there are a number of closely related bladderwort species already present in the invaded systems. All bladderwort species are rootless and have carnivorous traps (Taylor 1989). Some of these plants have stems that penetrate the sediment and thereby anchor themselves in place, such as U. resupinata B.D. Greene (Lavender Bladderwort) and U. intermedia Hayne (Flat-Leaf Bladderwort). Utricularia purpurea Walt. (Purple Bladderwort) and U. vulgaris L. (Common Bladderwort) are also common in Adirondack lakes, but are more similar to Swollen Baldderwort— they are free-floating just above the sediment and daughter shoots (hereafter referred to as branches) may emerge along the stem and form new individuals when they separate from the original plant. The rate of vegetative propagation in bladderworts has been studied by counting the number of branches per shoot (Adamec and Kovářová 2006); bladderworts grown in favorable conditions have a higher branching rate (Adamec 2011). Vegetative reproduction helps macrophytes spread in aquatic environments (Sculthorpe 1967). Fragmentation can facilitate the dispersal of invasive macrophytes (Ceccherelli and Piazzi 2001), and in the case of seagrasses, fragments can travel thousands of km farther than seeds (Berković et al. 2014). Vegetative propagules are also more likely to colonize bare sections of streams compared to seeds (Sand-Jensen et al. 1999). The exact mechanism for Swollen Bladderwort’s northern spread is uncertain, but it is likely fragments were carried on boats, boat trailers, or waterfowl (Figuerola and Green 2002, Johnstone et al. 1985). Water movement and successful asexual reproduction have facilitated Swollen Bladderwort’s downstream spread into Adirondack lakes connected by the Raquette River and the Middle Branch of the Moose River (T itus and Urban 2013). We conducted a series of greenhouse and field studies that compared features of the invasive Swollen Bladderwort to those of native species. Our first objective was to conduct a fragmentation experiment to determine differences in asexual reproduction between Swollen Bladderwort and Common Bladderwort. A previous greenhouse experiment showed that fragments just 1.0 cm in length had a 100% survival rate (Titus and Urban 2013). We hypothesized that Swollen Bladderwort reaches high frequencies in invaded lakes due to greater asexual reproduction than Common Bladderwort. Northeastern Naturalist 410 R.A. Urban and M.E. Dwyer 2016 Vol. 23, No. 3 Our second objective was to determine how the local distribution of Swollen Bladderwort compared to those of other bladderworts. We conducted a field experiment to investigate how water movement displaced unattached bladderwort species. We sampled vegetation with SCUBA to determine bladderwort distribution across a range of depths at 5 lake sites. We hypothesized that Swollen Bladderwort would have a greater dispersal ability, resulting in fewer individuals at waveexposed shallows. Field-site Description Our field sites were located within 4 lakes of the circumneutral Fulton Chain of Lakes in the southwestern Adirondack Mountains of New York. We conducted vegetation sampling at a single site in First (43.7088°N, 74.9322°W), Second (43.7248°N, 74.9107°W), and Third Lakes (43.7251°N, 74.9036°W), as well as 2 sites in Seventh Lake (site A: 43.7519°N, 74.7254°W; site B: 43.7394°N, 74.7434°W). We selected sites with abundant macrophytes from a range of relatively exposed and sheltered locations. We carried out a displacement experiment at 1 site in Seventh Lake (43.7511°N, 74.7315°W). The substrate at all sites was sandy in the shallows, and got increasingly silty as depth increased. We use the term shallows to refer to depths <2 m. In addition to Utricularia species, common macrophytes included Eleocharis robbinsii Oakes (Triangle Spike-rush), Eleocharis acicularis (L.) R. & S. (Needle Rush), Isoetes sp. (quillwort), Juncus pelocarpus Mey. (Brown-fruit Rush), Myriophyllum heterophyllum Michx. (Variable-leaf Watermilfoil), Najas flexilis (Willd.) Rostk. & Schmidt (Nodding Water Nymph), Najas gracillima (A. Br.) Magnus (Slender Water Nymph), Nitella spp. (green algae), Potamogeton robbinsii Oakes (Robbins’ Pondweed), Potamogeton epihydrus Raf. (Leafy Pondweed), Sagittaria sp. (arrowhead), and Vallisneria americana Michx. (Wild Celery). Species Description Free-floating Swollen Bladderwort, Purple Bladderwort, and Common Bladderwort have a linear stolon with dissected side-branches that are sometimes referred to as leaves (Taylor 1989). The whorled arrangement of Purple Bladderwort’s leaflike side branches easily distinguishes it from the other species (Crow and Hellquist 2000a), while the reddish color and alternate leaves of Swollen Bladderwort can be used to differentiate this invader from Common Bladderwort (Block and Rhoads 2011). Lavender Bladderwort plants are firmly attached to the substrate by an underground stolon with photosynthetic branches, up to 5 cm long, growing into the water column and bearing traps (Crow and Hellquist 2000a, Taylor 1989). Unlike the other species, Flat-Leaf Bladderwort has 2 distinct types of branches: photosynthetic branches that grow erect in the water column but lack traps, and branches that often penetrate the substrate that have traps, but lack chlorophyll (Crow and Hellquist 2000a). Northeastern Naturalist Vol. 23, No. 3 R.A. Urban and M.E. Dwyer 2016 411 Methods Fragmentation experiment On 11 July 2013, we collected Common Bladderwort and Swollen Bladderwort from First Lake in the Adirondack Mountains (Hamilton County; 43.7088°N, 74.9322°W). We cut 8 plants of both species behind the apical meristem at 1.0 cm and 10.0 cm, spread the 9.0-cm fragments in a plastic pot (9 cm high, 8.5-cm-top diameter) over ~300 cm3 of sediment collected from Big Moose Lake (Herkimer County; 43.8401°N, 74.8306°W), and randomly placed the pots in a 550-L cattle tank that was filled to a depth of 40 cm with reverse osmosis water in Lebanon Valley College’s west greenhouse. Water temperature was maintained at 23 °C with a Remcor CFF-500 refrigerated circulator, and plants received ambient sunlight. After 23 days, we recorded the number and length of newly initiated branches, blotted the plants to remove adherent water, and recorded fresh biomass. We used a Welch’s t-test to analyze our results in R software (R Foundation for Statistical Computing, Vienna, Austria). We also examined the fragmentation ability of Purple Bladderwort, but we did not include these data in our analyses because our greenhouse study demonstrated that non-fragmented Purple Bladderwort were unable to thrive in our greenhouse conditions. Displacement trials We collected Common Bladderwort, Purple Bladderwort, and Swollen Bladderwort from Seventh Lake, trimmed the pieces to 15-cm-long terminal apices, kept the plants in water, and labeled each with flagging tape (a 1.5 cm x 1 cm label with a 5 cm x 0.3 cm “tail” used to tie the tag to the plant). In Seventh Lake, we laid a transect line perpendicular to shore, reaching a depth of 3.5 m. At every 0.5-m depth increment, starting at 0.5 m, a SCUBA diver carefully spread 10 individuals of each species in 1.0 m x 1.0 m quadrats. SCUBA divers minimized their influence on sediment and plant movement by using cavediving fin-kick techniques. We conducted field trials 3 times: plants were laid out on 21 June 2007, 21 July 2007, and 8 July 2013. Upon checking the quadrats 4 or 5 days after laying out the plants, we recorded the number of tagged plants remaining. We analyzed the results using a two-way ANOVA in R software. Vegetation sampling Vegetation sampling occurred at 5 sites where Swollen Bladderwort, Purple Bladderwort, and Common Bladderwort grew within the Fulton Chain of Lakes in the Adirondack Mountains. At each site, we established 50-m transect lines following depth contours at 0.5 m, 1.0 m, 1.5 m, 2.0 m, and 2.5 m. A SCUBA diver (R.A. Urban) identified all species present in the 50 contiguous 0.2 m x 1.0 m quadrats along each transect line. For identifying plants, we followed Crow and Hellquist (2000a, b). We documented attached Flat-leaf Bladderwort and Lavender Bladderwort at 4 lake sites and they were included in our calculations. These data were also used previously to compare the distribution of Swollen Bladderwort and isoetids. Urban and Titus (2010) found that the distribution of the former varied with site exposure. Northeastern Naturalist 412 R.A. Urban and M.E. Dwyer 2016 Vol. 23, No. 3 We calculated frequency by determining the number of observations for an individual species as a percent of the total number of quadrats along a transect. To evaluate the correlation between plant frequency along each transect versus depth, we performed linear regressions in R software. Results Fragmentation experiment Fragments of both species survived and produced new growth as evidenced by the development of side branches off the original stolon. The average length summation of newly formed branches was 1.4 cm greater for Swollen Bladderwort than Common Bladderwort; however, these results were not statistically significant (t = 0.05, P > 0.05; Fig. 1a). New growth developed differently in Common Bladderwort and Swollen Bladderwort (Fig. 2). Each fragment of Swollen Bladderwort developed 4–12 new branches (average = 8.1), while Common Bladderwort fragments developed 84.6% fewer new branches (t = 7.1, P < 0.001; Fig. 1b). The lengths of new branches were significantly longer for Common Bladderwort than Swollen Bladderwort (t = 4.8, P < 0.001; Fig. 1c). On average, newly developed Common Bladderwort branches were 11.7 cm longer than those of Swollen Bladderwort. While total new growth was similar for Common Bladderwort and Swollen Bladderwort, each species exhibited a different growth pattern; thus, growth was dispersed differently on the plants. Displacement trials Depth significantly affected the percentage of bladderwort plants remaining in quadrats (F6, 43 = 4.1, P < 0.01; Fig. 3). Generally, we recovered a greater number of plants as depth increased, with no plants found at 0.5 m, and no Swollen Bladderwort individuals recovered until depth reached 2.5 m, at which we observed the greatest increase of Common Bladderwort and Swollen Bladderwort recovered. At this depth, the quadrats were adjacent to some sparse Wild Celery plants. Other than a bed of Nitella sp. near 3.5 m, we observed few macrophytes along the rest of the transect line. Common Bladderwort and Purple Bladderwort were less likely to be displaced from their original quadrats compared to Swollen Bladderwort (F2, 43 = 4.1, P less than 0.05). Common Bladderwort plants had a higher recovery rate in shallower water (1.0 m–2.5 m), while at greater depths, Purple Bladderwort had a higher recovery (3.0 m–3.5 m) (Fig. 3). Vegetation sampling Across the 5 sites, frequency of Swollen Bladderwort and Purple Bladderwort were positively correlated with depth (R2 = 0.34, P < 0.01 and R2 = 0.31, P < 0.01, respectively; Table 1). At 3 sites, the frequency values for these species increased as depth increased across the 2.5-m depth profile (Fig. 4c, d, e). For the more sheltered First Lake site and Seventh Lake site A, frequency values increased until 1.5 m or 2.0 m, respectively (Fig. 4a, b). Common Bladderwort followed a similar Northeastern Naturalist Vol. 23, No. 3 R.A. Urban and M.E. Dwyer 2016 413 Figure 1. Average (+ SE) growth measurements of Utricularia vulgaris (Common Bladderwort) and U. inflata (Swollen Bladderwort) at the end of a 23-d greenhouse experiment: (a) total length of new growth (t = 0.5, P > 0.05), (b) number of new branches that developed off the original fragments (t = 7.1, P less than 0.001), and (c) length of each new branch (t = 4.8, P < 0.001). Northeastern Naturalist 414 R.A. Urban and M.E. Dwyer 2016 Vol. 23, No. 3 Figure 2. Photographs of representative (a) Common Bladderwort and (b) Swollen Bladderwort fragments at the end of a 23-d greenhouse experiment. At the beginning of the experiment, the stem fragments were 9.0 cm in length and did not contain a visible apical meristem. Arrows point to where a new branch started to grow from the original fragment. Northeastern Naturalist Vol. 23, No. 3 R.A. Urban and M.E. Dwyer 2016 415 trend of increasing frequency values in the shallows; however, the greatest frequencies for Common Bladderwort occurred at shallower depths compared to the other unattached bladderwort species (Fig. 4). The greatest frequency values of Common Bladderwort were consistently at depths 0.5–2.0 m shallower than Swollen Bladderwort and Purple Bladderwort. In contrast, the relative frequency of the attached Flat-leaf Bladderwort, which was less common overall than the other bladderworts, was negatively correlated to depth (R2 = 0.23, P < 0.05; Table 1). Flat-leaf Bladderwort was absent at depths of 1.5–2.5 m, except for at Seventh Lake site B, where it had a frequency value of 2% at 2.0 m in depth (Fig. 4). Lavender Bladderwort also tended to be more common in the shallows, although the linear regression was not statistically significant (R2 = 0.01; Table 1). This attached macrophyte was absent at the 2.0-m and 2.5–m depths across all but 1 site. At the more exposed Second Lake site, it was a dominant species and had a frequency value of 60% at 2.5 m (Fig. 4c). Discussion Swollen Bladderwort and Common Bladderwort exhibited a difference in their asexual reproduction. While new Common Bladderwort branches grew longer, there were only 1–2 new branches per fragment; new Swollen Bladderwort branches were significantly shorter, but there were up to 12 new branches per fragment. Figure 3. Mean (± SE) percent values of deployed Swollen Bladderwort (■), Purple Bladderwort (●), and Common Bladderwort (▲) remaining in the original quadrats in relation to depth from 3 separate trials. Table 1. Summary results from linear regressions that tested the relationship of plant frequency and lake depth for 5 bladderwort species at vegetation sampling sites in the Adirondack Mountains of NY. *P < 0.05, **P < 0.01, ns = not significant. Plant Equation df R2 Lavender Bladderwort y = 25.40-3.0x 18 0.01 ns Flat-leaf Bladderwort y = 21.55-9.7x 18 0.23* Common Bladderwort y = 1.60-0.01x 23 0.05 ns Purple Bladderwort y = 0.94 + 0.01x 23 0.31** Swollen Bladderwort y = 1.01 + 0.15x 23 0.34** Northeastern Naturalist 416 R.A. Urban and M.E. Dwyer 2016 Vol. 23, No. 3 Each new branch has the potential to develop into a new individual as the original stolon decays. This mechanism may help explain why Swollen Bladderwort occurs at high frequencies in many Adirondack lake sites. The development of branches was likely stimulated when the removal of the apical bud resulted in a decline of auxin reaching the axillary buds (Raven et al. 2013). Swollen Bladderwort is unique among the other bladderworts mentioned in this paper due to its production of large radial floats that subtend its inflorescences. It is possible that these structures could aid the dispersal of this species; however, we believe that fragments are more important. We rarely observed Swollen Bladderwort flowering in Adirondack lakes, even in systems with large populations of this invasive species. Figure 4. Frequency of Swollen Bladderwort (■), Purple Bladderwort (●), Common Bladderwort (▲), Lavender Bladderwort (□), and Flat-leaf Bladderwort (○) in relation to depth at 5 different sites: (a) First Lake, (b) Seventh Lake site A, (c) Second Lake, (d) Seventh Lake site B, and (e) Third Lake. Lavender Bladderwort was not observed at Seventh Lake site B, while Flat-leaf Bladderwort was not observed at the Second Lake site. Northeastern Naturalist Vol. 23, No. 3 R.A. Urban and M.E. Dwyer 2016 417 The results of our field trials showed that Common Bladderwort was less likely to be displaced at depths of 1.0–2.5 m compared to Swollen Bladderwort and Purple Bladderwort. Although unattached bladderworts can sometimes form dense mats at the water surface, the basal ends of these bladderworts were covered in periphyton and in direct contact with the substrate during our vegetation sampling. The apical ends of these species were often turned upward towards the water surface, but the growing tips of Purple Bladderwort and Swollen Bladderwort were almost always positioned higher in the water column than Common Bladderwort. It appears that Common Bladderwort is less buoyant, which causes this species to better withstand water movement compared to the other 2 unattached species. How far a plant is carried in moving water can vary between species, with more-buo yant species less likely to remain in their original location (Riis and Sand-Jensen 2006). Our vegetation-sampling data showed that a zonation pattern existed for different bladderwort species: attached bladderworts were more common in the shallows, followed by Common Bladderwort, and finally Purple Bladderwort and Swollen Bladderwort. Aquatic plant communities typically exhibit a zonation pattern in which emergent species occur along the land–water interface, floating-leaved plants occupy relatively sheltered locations, and submersed macrophytes range from the shallows to much greater depths (Spence 1982). Light availability is the main determinant for depth limits of submersed macrophytes; temperature and type of substrate act as modifying variables (Spence 1982) and wave activity likely determines the upslope limit for aquatic plants (Chambers 1987). Although wave activity may prevent an unattached plant from thriving in the shallows, susceptibility to water movement facilitates the downstream spread of such species. We believe Swollen Bladderwort’s prolific asexual reproduction and high dispersal ability have positively influenced its spread, and may be part of the reason it is found at greater frequencies in Adirondack Mountain lakes compared to Common Bladderwort. Acknowledgments We thank John Titus, Jeff Bohner, Jim Doherty, and Timothy Doty for their assistance in the field. Thanks also to 2 anonymous reviewers for constructive comments on an earlier draft of the manuscript. This research was funded in part by 2 Lebanon Valley College Faculty Research Grants and the Lebanon Valley College Wolf Biology Research Fund. The vegetation sampling was originally conducted under a GRO (STAR) Research Assistance Agreement No. F6E61477 awarded by the US Environmental Protection Agency. This manuscript has not been formally reviewed by the EPA. 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