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Biomass and Growth of Waterhyacinth in a Tidal Blackwater River, South Carolina
Amanda R. Rotella and James O. Luken

Southeastern Naturalist, Volume 11, Issue 3 (2012): 361–374

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2012 SOUTHEASTERN NATURALIST 11(3):361–374 Biomass and Growth of Waterhyacinth in a Tidal Blackwater River, South Carolina Amanda R. Rotella1 and James O. Luken1,* Abstract - Eichhornia crassipes (Waterhyacinth) occurs in isolated populations along the Waccamaw River in northeast South Carolina. Although actively managed with herbicides, plant biomass and growth in this coastal, blackwater river have not been measured. We located three persistent populations in protected backwaters during spring 2009 and used sequential harvests to measure biomass accumulation and allocation. Relative growth over one month in existing populations and in two downriver sites was measured by placing plants in floating cages. A separate experiment was conducted to determine salinity tolerance. Mean total biomass in the persistent populations was relatively low but increased from 202.9 g/m2 in spring to 380.1 g/m2 in fall, with leaves as the largest biomass component (72%). Absolute growth and leaf nutrient content for nitrogen, phosphorus, and potassium were highest for caged plants placed in a downriver site influenced by the Pee Dee River, a redwater system. Our results suggest that Waterhyacinth extent and growth in the Waccamaw River are limited by nutrient availability, but other factors may also be involved. Introduction Although Eichhornia crassipes (Martius) Solms-Laub (Waterhyacinth) is found in a wide range of aquatic systems throughout the world, the vast majority of research has focused on performance in eutrophic tropical lakes and impoundments (Gopal 1987, Gopal and Sharma 1981). While it is clear that Waterhyacinth is a successful invader (Center and Spencer 1981, Gopal and Sharma 1981, Penfound and Earle 1948) and potent ecosystem engineer in tropical or subtropical environments, less information is available for the plant growing under limiting temperature or nutrient conditions. Measuring variation in performance and assessing limiting factors of a known invader such as Waterhyacinth are critical for efficient allocation of management resources and are also useful for predicting where the plant might spread and dominate a system through growth of extensive floating mats (Luken 1997, Madsen 1997) Coastal rivers of the southeastern US provide suitable habitat for Waterhyacinth, but invasion may be limited by water quality, physical factors such as tide and current, and temperature (Wilson et al. 2001). The nutrient environment of coastal rivers is shaped by water origin, with redwater systems carrying relatively high nutrient loads and blackwater systems carrying relatively low nutrient loads (Hopkinson 1992, Hupp 2000, Laurie and Chamberlain 2003, Smock and Gilinsky 1992). The physical environment for invasion is shaped by 1Coastal Marine and Wetland Studies Graduate Program, PO Box 261954, College of Science, Coastal Carolina University, Conway, SC 29528-6054. *Corresponding author - joluken@coastal.edu. 362 Southeastern Naturalist Vol. 11, No. 3 river geomorphology, with backwaters and oxbow lakes forming protected sites conducive to plant introduction and growth (Laurie and Chamberlain 2003). Inevitably, coastal rivers end in estuaries where high salinities limit the distribution and growth of all freshwater aquatic species (Conner et al. 2007). Variation of Waterhyacinth growth within these coastal rivers is not well studied, although it is likely that some rivers are more susceptible to invasion than others (Tellez et al. 2008). The ability of Waterhyacinth to form extensive mats in coastal rivers may involve an interaction between limiting nutrients (i.e., nitrogen and phosphorus) and salinity (Muramato et al. 1991). Reddy et al. (1989) found that tissue nitrogen was related to the concentration of nitrogen in the water, with maximum biomass yield at 5.5 ppm. Knipling et al. (1970) showed that root-to-shoot ratios changed depending on phosphorus concentration of the water. Optimal phosphorus concentrations in the water ranged from 1 to 20 ppm (Haller and Sutton 1973, Reddy et al. 1990). Although Waterhyacinth is efficient at nutrient uptake, excess nutrient loads may inhibit growth (Reddy and Sutton 1984). While some aquatic plants are adapted to high salinities, Waterhyacinth is intolerant. Penfound and Earle (1948) measured lethal salinity at 2.2 ppt, while other studies found lethal levels of salinity ranging from 3.4–8.8 ppt (De Casabianca and Laugier 1995, Haller et al. 1974, Muramoto et al. 1991, Olivares and Colonnello 2000, Zhenbin et al. 1990). Blackwater rivers of the southeast are not well studied in terms of susceptibility to Waterhyacinth invasion. One such river, the Waccamaw, is an unregulated, tidally influenced, low-nutrient, low-oxygen, blackwater system in northeastern South Carolina that also occurs at the northern limit of Waterhyacinth distribution. As such, this system offers an opportunity to examine potential for Waterhyacinth invasion when conditions are suboptimal. The objectives of this study were to measure biomass, to measure growth in response to variation in water quality, and to determine salinity tolerance of plants currently growing in this system. Field-Site Description This study was conducted in the Waccamaw River, SC (Fig. 1). This river begins at Lake Waccamaw in North Carolina and then roughly parallels the coast, eventually entering the ocean at Winyah Bay near Georgetown, SC. Our study sites were located between the tidal freshwater forest/marsh zone and the oligohaline zone (Conner et al. 2007), a stretch of about 14 km. Here, Waterhyacinth persists and forms mats in backwaters and bays where the plants are relatively protected from current. Plants may also be found as transient populations floating downriver or snagged on trees. Persistent populations of Waterhyacinth located in the freshwater forest/ marsh zone of the river were selected for estimates of biomass and growth. These populations were located upriver from the confluence of the Waccamaw River 2012 A.R. Rotella and J.O. Luken 363 and Atlantic Intracoastal Waterway (AICW). Two downriver zones, one below the confluence of the Waccamaw River and the AICW and the other below the confluence of the Waccamaw River and Big Bull Creek, were selected for measurements of Waterhyacinth growth. These zones characterized by water sources are hereafter referred to as upper, middle, and lower river zones (Fig. 1). Figure 1. Location of Waterhyacinth study sites in the Waccamaw River, SC. 364 Southeastern Naturalist Vol. 11, No. 3 Methods Biomass of persistent populations In spring (10 June 2009) and fall (14 October 2009), harvests were performed to assess biomass and biomass allocation (% tissue in leaves, roots, stem bases [i.e., leaf bases and buds], and stolons). Plants were collected from four 0.25-m2 frames haphazardly placed at the front (leading edge adjacent to open water) and four frames similarly placed at the back (trailing edge adjacent to the shoreline) of floating mats produced by persistent populations (n = 3). Application of herbicide by the SC Department of Natural Resources in September killed plants at mat fronts, and samples were collected only from the backs in October. Whole plants and loose parts were collected within each frame by using hedging sheers and then rinsed with water to remove periphyton, macroinvertebrates and attached organic and inorganic matter before separating into biomass components. Biomass here is defined as living, green tissue. The four samples collected at different mat positions were summed, yielding a single biomass sample for each position in each population. Subsamples of plant parts were removed to determine ash content (500 °C for 10 hrs) and ash-free dry mass (70 °C for 48 h). Growth of caged plants Duplicate plant-growth cages were deployed in 6 persistent populations of Waterhyacinth, and 6 plant-growth cages were deployed in each of the 2 downriver zones. There were no persistent Waterhyacinth populations downriver, and thus cage-placement sites were selected based on water depth and presence of Nuphar sagittifolia (Walter) Pursh (Narrowleaf Pondlily). Cages were constructed from ¾” PVC pipe and nylon netting as in Greco and de Freitas (2002). Each cage measured 1.0 m2 and was anchored to the river bottom while allowing for variation in water levels due to tidal fluctuation. Upper-river cages were deployed on 19 May 2009, while middle- and lower-river cages were deployed on 1 June 2009. Each cage was stocked with 6 tagged Waterhyacinth ramets of ca. equal size (longest leaf = 7 to 25 cm). Initial measurements included number of leaves, root length, longest leaf length, widest leaf length, and stem base diameter. After one month, all plant parameters were remeasured. Duplicate cages in the upper river were combined, and means were calculated to yield 6 samples. The absolute growth rate of all plants in cages was calculated as the difference between the total (or total mean) final and total (or total mean) initial values divided by the number of growth days. Nutrient content of plants Ten plants were randomly selected from cages in each river zone and separated into leaves and roots. In order to obtain enough material for nutrient analyses, randomly paired plants were combined, producing a total of 5 samples of each plant part for each river zone. These were sent to Clemson University’s Agricultural Lab for analysis of nitrogen, phosphorous, potassium, and calcium, except for 2 root samples with insufficient material for complete analysis. 2012 A.R. Rotella and J.O. Luken 365 Growth and salinity Tolerance of Waterhyacinth to different salinities was assessed in a mesocosm experiment completed in September of 2009 on the Coastal Carolina University campus. Waterhyacinth ramets collected from the Waccamaw River were rinsed and new offshoots removed before placing one plant in each of forty 22-L buckets. Each bucket was filled with 15 L of tap water, to which 30 mL of a standard solution of Miracle-Gro® Liquid Plant Food was added. Instant Ocean® was used to vary salinity. The experiment was designed to block for potential but unmeasured environmental variation in the greenhouse area. Ten randomized blocks were established, each with four levels of salinity: 0, 1.5, 3, and 4.5 ppt. Initial leaf number, initial root length, longest leaf length, widest leaf length, and stem base diameter were measured for each plant. After one month, dissolved oxygen, conductivity, and temperature were measured and all plant parameters were remeasured. Statistical analyses Spring Waterhyacinth biomass from fronts and backs of mats was compared with a paired samples t-test (samples paired by plant population). Biomass from backs of mats in spring and fall was also compared using a paired samples t-test (samples paired by plant population). One-way ANOVA was used to determine significant differences in growth and plant nutrient content among the three different river zones and among the salinity treatments. A Tukey HSD test was used to compare each group mean with every other group mean in a pair-wise manner. Data were square root transformed to meet ANOVA assumptions, but untransformed data were presented in all figures. SPSS version 17.0 was used for all statistical tests, with P < 0.05 chosen as a level of significance. Results Biomass Mean total spring biomass of 157.3 g/m2 in mat fronts was significantly lower (t = -9.125, P < 0.05, df = 2) than the mean total spring biomass of 202.9 g/m2 at mat backs (Fig. 2). In spring, leaves comprised 62% of total biomass, with 31.4%, 4.6%, and 1.6% for roots, stem bases, and stolons, respectively. At mat backs, overall biomass allocation was similar to mat fronts, with a slightly higher allocation to leaves. Mean total biomass increased significantly (t = -5.369, P < 0.05, df = 2) at mat backs, nearly doubling from 202.9 g/m2 in spring to 380.1 g/m2 by fall (Fig. 3). Biomass allocation also shifted from spring to fall with 72.1%, 20.9%, 8.1%, and 1.1% of biomass in leaves, roots, stem bases, and stolons, respectively (Fig. 3). Growth in different river zones Waterhyacinth grew best when moved to the lower river zone (Fig. 4). The absolute growth of all plant parts was significantly higher (Tukey HSD, P < 0.05) in the lower river compared to the upper and middle river zones, with the exception 366 Southeastern Naturalist Vol. 11, No. 3 Figure 2. Mean biomass (n = 3, ± SE) of Waterhyacinth floating mats sampled on 10 June 2009. Biomass differences between the front and back of the mats are compared. (*= P < 0.05; N.S.= P > 0.05). Figure 3. Mean biomass (n = 3, ± SE) of Waterhyacinth floating mats sampled on 10 June and 14 October 2009. Biomass differences between spring and fall are compared for the backs of the mats. (*= P < 0.05; N.S.= P > 0.05). 2012 A.R. Rotella and J.O. Luken 367 of roots (ANOVA: F2, 15 = 2.176, P > 0.05). The number of leaves produced per cage increased from an average of 5 per day in the upper river zone to 11 per day in the lower river zone (Fig. 4). Leaf length (measured as total length of longest leaves per cage) was the component of growth with greatest response to river zone (Fig. 4). Total growth in length of the longest leaves per cage increased from 10.2 cm/day in the upper river zone to 43.0 cm/day in the lower river zone. Nutrient accumulation Patterns of variation in nutrient (N, P, K, Ca) contents were different for river zones and for plant parts. Generally, leaves produced in the lower river zone accumulated more N, P, and K (Fig. 5). Calcium was an exception to this trend. Nutrient contents of roots were generally lower than that of leaves, and only phosphorus varied significantly relative to zone (ANOVA: F2, 10 = 4.971, P < 0.05). Growth and salinity Salinity treatments represented mean conductance of 568.7 μS/cm for 0.0 ppt, 2852.7 μS/cm for 1.5 ppt, 7884.0 μS/cm for 3.0 ppt, and 9522.0 μS/ cm for 4.5 ppt. High baseline conductance resulted from adding nutrients to all of the buckets. Mean dissolved oxygen ranged from 10.12 mg/L to 12.15 mg/L across treatments. There was a general decrease in plant productivity as salinity increased (Figs. 6, 7). Plants at 0.0 ppt grew better than in other treatments. Relative to the 0.0 ppt Figure 4. Mean absolute growth of Waterhyacinth in three different river zones (n = 6, ± SE). Letters indicate significant differences among river zones for a growth component. Bars with different letters are significantly different. Data for leaves (units = #/day, P ≤ 0.05, n = 6). 368 Southeastern Naturalist Vol. 11, No. 3 treatment, all growth components showed significant decreases in number and length at 4.5 ppt (Tukey HSD, P < 0.05). Most growth in the salinity experiment occurred as the production of new offshoots, but all salinity treatments had negative impacts on shoot production (ANOVA: F3,36= 28.80 , P < 0.05) and on leaf production from these offshoots (ANOVA: F3, 36 = 25.13 , P < 0.05). Discussion Wilson et al. (2001) modeled Waterhyacinth populations with the assumption that limitations to successful invasion are set by low temperature, nutrients, salinity, disturbance, and natural enemies. However, due to evolution in highly variable tropical rivers, the plant may acclimate to a wide variety of aquatic conditions (Center and Spencer 1981). We studied Waterhyacinth at the northern edge of its distribution in the eastern US and measured a maximum biomass Figure 5. Mean percentages of nutrients found in (A) leaves and (B) roots of Waterhyacinth plants from three different zones of the Waccamaw River. Letters indicate significant differences among river zones for a nutrient (n = 5, ± SE). 2012 A.R. Rotella and J.O. Luken 369 Figure 6. Waterhyacinth growth in mesocosms. Effect of different salinities on absolute production of existing structures found on plants grown in buckets. Bars with different letters are significantly different (n = 10, ± SE). Figure 7. Waterhyacinth growth in mesocosms. Effect of different salinities on number of new leaves produced on the original plants, total new leaves produced from offshoots, and the number of new offshoots for plants grown in buckets. Bars with different letters are significantly different (n = 10, ± SE). 370 Southeastern Naturalist Vol. 11, No. 3 of 380 g/m2, a quantity much lower than in other climatic zones (Table 1). The relatively low biomass and the absence of extensive Waterhyacinth mats in the Waccamaw River suggest that the plant is indeed limited with low potential for widespread invasion. Center and Spencer (1981) found that seasonal changes in biomass and size of Waterhyacinth in north-central Florida were directly related to intraspecific competition within developing mats. Rapid growth and overcrowding of Florida plants in late spring reduced leaf production and leaf longevity (Center and Spencer 1981, Center and Van 1989, Greco and de Freitas 2002). In contrast, Waccamaw River Waterhyacinth plants achieved maximum leaf lengths and maximum plant densities that were about half of plants growing in Florida (Center and Spencer 1981), while at the same time maintaining roughly similar shoot/ root ratios (Reddy 1984). As such, it is possible that Waterhyacinth in the Waccamaw River does not reach levels of biomass where intraspecific effects begin to influence standing crop. Nutrient supply clearly modifies Waterhyacinth growth in the Waccamaw River. Aquatic free-floating plants, like Waterhyacinth, absorb all their nutrients from the water (Haslam 1978), and tissue nutrient concentrations are dependent on the mass of nutrient supply (Reddy et al. 1989), levels of herbivory (Moran 2006), and plant growth rates (Center and Van 1989). Concentrations of nitrogen and phosphorus in leaves and roots of Waterhyacinth in the Waccamaw River were generally similar to or higher than nutrient concentrations in plants from a Texas river (Moran 2006) and from experimental cultures in Florida (Center and Van 1989). Nitrogen concentrations were clearly above the minimum required for growth (Reddy et al. 1989). Although the Waccamaw River is generally nutrient poor, nutrient supply to floating mats may be increased as a result of current and fluctuations in water levels due to tides. Plants may also absorb nutrients from sediments during extremely low tides. Plants in the middle and lower river zones showed higher levels of tissue N and P, and these likely contributed to greater leaf growth. The Pee Dee River (a redwater system influencing middle and lower river zones) and the Waccamaw River (a blackwater system influencing all river zones) exhibit contrasts in Table 1. Maximum biomass (g/m2) of Waterhyacinth measured in different areas of the world. Location Biomass Reference South Carolina, USA 380 This study Gorakhpur, India 630 Singh and Sahai (1979) Gorakhpur, India 723 Sahai and Sinha (1970) Mississippi, USA 800 Luu and Getsinger (1990) Louisiana, USA 1500 Penfound and Earle (1948) Belo Horizonte, Brazil 2027 Greco and de Freitas (2002) Alabama, USA 2130 Boyd and Scarsbrook (1975) Florida, USA 2300 Center and Spencer (1981) Florida, USA 2500 Knipling et al. (1970) Louisiana, USA 2970 Wooten and Dodd (1976) 2012 A.R. Rotella and J.O. Luken 371 nutrient loads due to differences in watershed characteristics. The Pee Dee River expresses higher conductivity, turbidity, pH, and nitrate concentrations when compared with the levels obtained from the Waccamaw River (USGS 2009). Our results suggest that Waterhyacinth in the Waccamaw River can respond quickly to changes in nutrient supply and that areas of higher nutrient content may be most susceptible to mat development. Figure 8. Seasonal changes in structure of a Waterhyacinth mat growing in the Waccamaw River, SC. (A) 12 March 2009: most plants dead due to freezing temperatures; (B) 17 June 2009: population regenerated from the few surviving individuals. 372 Southeastern Naturalist Vol. 11, No. 3 Although Waterhyacinth plants may grow faster as they are transported downstream in the Waccamaw River, eventually these plants experience increasingly saline waters. Our results suggest that decreased growth will occur at salinities of 3.0 ppt, but 30% of plants receiving this level of salinity retained the ability to produce one new offshoot. These results were different than those presented by DeCasabianca and Laugier (1995), who measured a lethal level of salinity of 8.8 ppt, and Muramato et al. (1991), who reported a lethal level of 6.3 ppt. The results for plants taken from the Waccamaw River more closely resembled data obtained by Haller et al. (1974), Olivares and Colonnello (2000) and Zhenbin et al. (1990), who found the lethal level of salinity to be ca. 3.3 ppt or as low as 2 ppt (Wilson et al. 2001). Performance in lower river zones may depend on the interaction between the positive effects of increased nutrient supply and the negative effects of salinity (Muramato et al. 1991). Although not considered in this study, temperature determines the number of living emergent Waterhyacinth shoots that survive winter. In contrast to Waterhyacinth populations in Florida (Center and Spencer 1981), our study populations were greatly reduced by freezing temperatures and permanent populations were re-established by a relatively small number of plants that survived (Fig. 8). No plants from seed have been observed in the Waccamaw or in experimental tanks. Although considered a perennial, Waterhyacinth growing in 110-gal tanks on the Coastal Carolina University campus lost 92% of the population during winter 2011 (J.O. Luken and A.R. Rotella, unpubl. data). As such, winter resets plant density and plant biomass, two factors critical for predicting Waterhyacinth growth in temperate climates (Wilson et al. 2005). Invasion impacts With the recognition that Waterhyacinth forms persistent but low-biomass populations in the Waccamaw River, it is important to fully understand impacts, particularly if management with herbicides is an option. In this system, where plants experience several stresses, it is clear that an introduced species can be limited in growth and extent similar to native species (Crawley 1987, Hierro et al. 2005). Furthermore, there are few other floating aquatic plant species that produce suspended roots in this river system, a situation potentially providing new habitat for invertebrates and fish (Barker 2011, Toft et al. 2003). Future research should focus on the summation of ecological and economic factors associated with Waterhyacinth invasion in this coastal river. Acknowledgments This research was supported by a fellowship from the South Carolina Aquatic Plant Management Society and by a grant from the Coastal Carolina University Research Council. Literature Cited Barker, J.E. 2011. Invertebrate assemblages associated with Water Hyacinth (Eichhornia crassipes) roots in the Waccamaw River, South Carolina. M.Sc. Thesis. Coastal Carolina University, Conway, SC. 52 pp. 2012 A.R. Rotella and J.O. 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