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Research Article
Two invaders, one reservoir: Hydrilla shapes the distribution of zebra mussels and may facilitate their growth
expand article infoEmily Lorkovic‡, Jason P. Martina‡, Monica E. McGarrity§, Astrid N. Schwalb‡
‡ Texas State University, San Marcos, United States of America
§ Texas Parks and Wildlife Department, Austin, United States of America

Corresponding author: Astrid N. Schwalb ( schwalb@txstate.edu )

Academic editor: David Wong
© 2025 Emily Lorkovic, Jason P. Martina, Monica E. McGarrity, Astrid N. Schwalb.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Lorkovic E, Martina JP, McGarrity ME, Schwalb AN (2025) Two invaders, one reservoir: Hydrilla shapes the distribution of zebra mussels and may facilitate their growth. Aquatic Invasions 20(3): 273-290. https://doi.org/10.3391/ai.2025.20.3.156675
Open Access

Abstract

Aquatic invasive species can alter ecosystem processes, detrimentally affect native species, and facilitate the invasion of other species. One infamous aquatic invader, the zebra mussel (Dreissena polymorpha), is known to cause declines in phytoplankton through their filtering activity and facilitate the subsequent growth of macrophytes by increasing water clarity. In turn, submerged macrophytes may provide substrate for settlement of zebra mussels. The goal of this study was to examine variation in the distribution of zebra mussels and hydrilla (Hydrilla verticillata subsp. verticillata) in relation to sediment composition, each other (including potential facilitation), and with season (summer vs. fall) in a subtropical reservoir. Surveys of zebra mussels and hydrilla showed that zebra mussel densities tended to be higher in rocky habitats where they were found on hydrilla and rocks (gravel and cobble), compared to muddy habitats where they were found only on hydrilla. Within the rocky habitat, zebra mussels attached to hydrilla had significantly higher densities and a smaller size than those attached to rocks. However, spring populations may be largely transient because only a small fraction of zebra mussels remained on hydrilla in early fall, almost exclusively representing a new settlement cohort based on their size distribution. Nevertheless, hydrilla may directly facilitate zebra mussel dispersal, especially in spring, as mussels attached to plant fragments can be transported downstream by currents or by human activities, such as entanglement in boat propellers and trailers. Laboratory experiments did not detect any significant impact of zebra mussels on the growth, biomass, or nutrient content of hydrilla. However, zebra mussel biomass was higher when hydrilla was present, suggesting that hydrilla may facilitate zebra mussel growth, although the difference was only statistically significant at low hydrilla densities. This study illustrates the complexities of interactions between multiple introduced species which can lead to facilitation of invasion of aquatic ecosystems.

Key words:

Invader facilitation, invasional meltdown hypothesis, invasive species distribution, plant-mussel interactions, reservoir ecology

Introduction

Modern human globalization has increased the rate of species introductions, leading to aquatic ecosystems being colonized by multiple non-indigenous and potentially invasive species (Ricciardi 2007; Hulme et al. 2008). The Invasional Meltdown Hypothesis (IMH) suggests that when introduced species disrupt ecosystems, these disruptions can lower community resistance and facilitate further invasions by other species (Simberloff and Von Holle 1999). An already present invader can benefit a novel one in diverse ways. For example, an invasive reef-building tubeworm (Ficopomatus enigmaticus) altered estuarine community assemblages by increasing substrate complexity which favored non-native fauna (Heiman and Micheli 2010). Another mechanism is when invaders reduce competition. For instance, an invasive snail (Pomacea maculata) may facilitate establishment of an invasive wetland plant (Alternanthera philoxeroides) by preferentially consuming native plants, thereby reducing the native plant’s competitive ability and increasing the biomass of A. philoxeroides (Meza-Lopez and Siemann 2015).

Zebra mussels (Dreissena polymorpha) are considered one of the most problematic invasive species on multiple continents because of their large-scale negative ecosystem effects coupled with their ability to encrust submerged infrastructure (e.g., water supply and control), causing large economic costs (Strayer 2009). In North America, they have primarily spread via attachment to watercraft (Strayer 2009; De Ventura et al. 2016; Cole et al. 2019), although downstream dispersal of free-swimming larvae, called veligers, also occurs (Olson et al. 2018). Zebra mussels are “ecosystem engineers” that cause benthification of ecosystem energy via the transfer of nutrients from the water column to the benthos by filtration and excretion (Sousa et al. 2009; Higgins and Vander Zanden 2010). The excretion of nutrients on the benthos can benefit aquatic plants and algae in the littoral zone, which can increase primary production as well as macrophyte and bacterial respiration, causing fluxes in environmental oxygen and carbon dioxide concentrations (Zhu et al. 2006; Armenio et al. 2016; Jeppesen et al. 2016). Several studies have documented the facilitation of macrophytes by dreissenid mussels. Increased water clarity caused by high filtration activity of zebra mussels and a subsequent decline in phytoplankton were associated with lake-wide increases in macrophyte production and diversity in Lake Oneida, NY (Zhu et al. 2006). Reduced turbidity from plant and mussel activity can shift ecosystem equilibria to a clear water state in shallow lakes (Scheffer et al. 1993). Also, quagga mussels (D. rostriformis bugensis), a congener of zebra mussels, appeared to facilitate rapid range expansions of an invasive macrophyte, Elodea nuttali, into deeper waters in a reservoir in Germany (Wegner et al. 2019) and, in laboratory conditions, zebra mussel presence promoted the growth of E. nuttali when grown in monocultures and mitigated the competition between this species and an invasive congener, E. canadensis (Crane et al. 2020).

Macrophytes can provide biotic surfaces for the attachment of zebra mussels, which may be especially important in areas with soft substrates, where zebra mussels otherwise would not be able to attach (Londo et al. 2022). Attachment of a morphologically similar invasive bivalve, the golden mussel (Limnoperna fortunei) has been observed on hydrilla (Hydrilla verticillata), possibly facilitating the spread of the golden mussel in Brazilian reservoirs (Michelan et al. 2014). However, plant surfaces may not be sites of long-term settlement for zebra mussels in temperate regions where submerged macrophyte senescence can trigger mussel abandonment of plant tissues (Martel 1993; Londo et al. 2022). Additionally, zebra mussels attached to plants may release and fall to sediments or engage in a process called “post-metamorphic drifting,” which is a form of passive locomotion where mussels detach from plant surfaces and drift on a mucosal filament in the water column to encounter new substrates for permanent settlement (Martel 1993; Ricciardi and Hill 2023). Plant fragments can readily disperse to unoccupied habitats by currents or through human-mediated overland transport (Havel et al. 2015), which increases the risk of zebra mussels spreading to previously unaffected waters.

Research on interactions between zebra mussels and hydrilla is lacking. Hydrilla is an obligate submerged aquatic plant native to Southeast Asia that has spread to multiple continents, including North America, where established populations persist in eastern and southern river basins (Tippery 2023). Hydrilla spreads easily through multiple propagation methods, including fragmentation and turions, which can facilitate the dispersal of propagules by birds and boats (Langeland 1996; Zhang et al. 2013; Patrick and Florentine 2021). Most aquatic macrophytes, including hydrilla, thrive in soft substrates due to the improved nutrient diffusion and exchange between roots and sediment as compared to coarser sediments such as sand (Barko and Smart 1986; Silveira and Thomaz 2015). However, hydrilla has been observed in sediments of large particle size, such as gravel and cobble, despite low organic matter (Lorkovic personal observations). In addition, the turions are resistant to desiccation and freezing, which makes the plant difficult to control via physical removal and water level manipulations once established (Langeland 1996). Growth rates of up to 486 cm/day per plant have been measured, with growth increasing exponentially over time, contributing to the rapid formation of large canopies that outcompete native plants and phytoplankton for sunlight (Bianchini 2010; Glomski and Netherland 2012). Hydrilla sequesters nutrients from sediments and water, facilitating rapid growth (Tang et al. 2019). This results in the engineering of ecosystems by increasing vertical habitat complexity and increasing water clarity (Langeland 1996). Hydrilla may also increase carbon sedimentation by increasing the amount of decomposing plant tissue (i.e., source of organic carbon) and reducing flow (i.e., source of inorganic carbon) (Rooney et al. 2003; Drexler et al. 2021). At a finer scale, environmental oxygen concentrations may oscillate diurnally in densely packed stands of hydrilla where daytime concentrations are high from photosynthesis and nighttime concentrations dip near anoxia due to respiration (Spencer et al. 1994; Sousa 2011).

The goal of this study was to examine variation in the distribution of both zebra mussels and hydrilla in relation to substrate, each other, and season (i.e., summer vs. fall) and to examine potential interactive effects in laboratory experiments. Our study had four main objectives. First, we examined how substrate composition affects the distribution of zebra mussels and hydrilla within a large reservoir. Second, we examined differences in density and size distribution of mussels between those attached to rocks versus hydrilla in field surveys. Third, we examined these same differences over time between summer and fall. Zebra mussels experience high mortality when water temperatures are high in summer (Schwalb et al. 2023), whereas hydrilla can tolerate higher temperatures, but seasonal population changes have not been observed in relation to each other. Fourth, we examined facilitation between zebra mussels and hydrilla in a laboratory setting by testing the possible effects of mussel presence on the growth and tissue carbon-nitrogen ratios of hydrilla, as well as hydrilla effects on zebra mussel body mass.

Materials and methods

Surveys

This study was conducted in Canyon Lake, a eutrophic reservoir with over 24 kilometers of shoreline located in the Guadalupe River basin in south-central Texas (Fig. 1). Hydrilla (pistillate dioecious biotype) stands were first described in Canyon Lake in 2019 and increased in coverage by 2022 (Ireland and Farooqi 2019, 2024). Zebra mussels were first detected in Canyon Lake in 2017, and the population has been monitored since soon after their initial detection (Schwalb et al. 2023), but not in relation to hydrilla. A total of eight sites were surveyed between the 16th and 23rd of June 2022. Four sites consisted mostly of muddy sediment (i.e., a mixture of organic particles, silt, and Asian clam shells (Corbicula fluminea, a naturalized mussel)), and the other four sites were dominated by rocky sediment (i.e., larger pebbles and cobbles) (Wentworth 1922; Londo et al. 2022). Two sites (BR 1 G and BR1 Cove H) were near the dam, where water depth can reach 42 meters (Fig. 1, USACE Bathymetric Map 2000). Another four sites (Potters C, Potters A, Marina E, and BR 23) were in the transitional zone of the reservoir, with much shallower water than sites near the dam (i.e., maximum depth 16 to 24 meters, Fig. 1, USACE Bathymetric Map 2000). The last two sites (Crane A and Crane B) were located near a major channel bend (maximum depth up to 16 m) and were the furthest upstream sites in our study. Between the 15th to 18th of September 2022, 6 sites were surveyed again (i.e., all except Marina E and BR 23, due to time constraints) in transects adjacent to those measured in June. An “exceptional drought” occurred that year (Data accessed on this website: https://www.drought.gov/states/texas), which resulted in less flow and subsequent reservoir drawdown, decreasing water levels by 1.2 m between June and September during the survey period (Data accessed on this website; https://www.waterdatafortexas.org/reservoirs/individual/canyon), TWDB 2022).

Figure 1. 

The location of Canyon Lake in Texas, USA, showing dive survey sites with muddy (triangles) and rocky (squares) sediment. The location of hydrilla and zebra mussel collection for the lab experiment (rectangle) and dam (black line) are also indicated on the map.

During each field survey, temperature (°C), dissolved oxygen (mg/L-1 and %), and specific conductance (μS cm-1) were measured with a multisonde (YSI model ProDDS). Water temperature averaged 29.3 ± 0.7 °C (mean ± SD) in June and 28.7 ± 0.9 °C in September (Table 1). In a hydrilla stand located in an upstream cove, dissolved oxygen (DO) during the day was (≥ 7.1 mg/L), but when measured during 2 nights at ~30 °C, decreased to almost 3 mg/L in the same location. The water depth of each sampling quadrat was determined to the nearest 30 cm (~1 foot) during diving with a SCUBA depth gauge (Cressi miniconsole 2 imperial gauge). Secchi transparency was not measured because the transparency of the water exceeded the depth of the survey locations. During surveys, 2-L water samples were collected at the surface and 3- and 2-m depths in June and September, respectively; September depth was less due to decreased water levels. Water samples were analyzed for turbidity (NTU) and relative chlorophyll-a (Chl-a) concentrations (µg/L) in the laboratory using a fluorometer (Trilogy, model 7200-000). Chlorophyll-a was very low at most sites for both months (≤ 1.5 μg/L), consistent with oligotrophic designation by the Texas Commission on Environmental Quality (https://www.tceq.texas.gov/assets/public/waterquality/swqm/assess/20txir/2020_trophic.pdf, Table 1). Turbidity was low (≤ 8 NTU) except for Crane B and Cove G in September and Potters C in both June and September (15.4–22.7 NTU, Suppl. material 1: table S1). Total suspended solids (TSS) were measured by filtering a volume of lake water ranging from 500 to 1000 mL (until the filter was stained brown) through an ashed 1 µm filter (Pall A-E Glass-Fiber Filters), and samples were dried in the oven at 101 °C for 1 hour. After dehydration, suspended solids were weighed on a microbalance (Sartorius Lab Instruments ENTRIS124-1S).

Table 1.
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Nested analysis of variance for benthic composition with site as a covariate for surveyed hydrilla and zebra mussel densities in June 2022.

Source df F-value p-value Percent explained
Hydrilla
Substrate 1 24.36 2.39 e-05 0.13
Substrate: Site 6 21.85 4.90 e-10 0.70
Residuals 32 0.17
Zebra Mussels
Substrate 1 19.62 1.03 e-04 0.22
Substrate: Site 6 6.549 1.39 e-04 0.43
Residuals 32 0.35

Hydrilla and zebra mussel surveys were carried out by scuba diving using five sampling quadrats (25 × 25 cm), each placed at two-meter intervals along 10-meter transects located parallel to the shore at a water depth between 2.7 and 4.0 m in June and 1.7 and 2.7 m in September. Inside each sampling quadrat, all hydrilla tissues present in the water column (i.e., from surface to lake bottom) were removed by hand using garden shears. Immediately after collection, hydrilla tissue samples were placed in 0.5-millimeter mesh bags. Rocks colonized by zebra mussels were sampled by collecting from 1/4 of the sampling quadrat (156 cm2) for rocky sites in June only due to time constraints. Plant and mussel samples were transported in aerated lake water, to reduce stress, to the laboratory and immediately frozen at -12 °C.

Hydrilla samples were thawed at room temperature for processing, after which zebra mussels were carefully removed from plant tissue and washed with deionized water. Subsampling was performed by removing zebra mussels from a random 1/3 of each plant sample (derived from one sampling quadrat). If fewer than 50 mussels were found, an extra 1/3 of the plant sample was inspected for more individuals. If a total of 50 mussels was still not reached in the subsamples, then the entire plant sample was inspected for individuals. All zebra mussels removed from hydrilla and collected from rocks and hydrilla samples were oven-dried at 60 °C for 24 and 48 hours, respectively, until a constant weight was achieved. Zebra mussel individuals were counted and measured with calipers to the nearest 0.1 mm, and hydrilla samples were cooled to room temperature and further weighed to determine their dry biomass.

Laboratory experiments

A laboratory experiment was conducted to test the effects of zebra mussels on hydrilla growth and vice versa under controlled conditions. The experiment was carried out using a 3 × 2 fully crossed design which included three levels of hydrilla density (without hydrilla, low, or high) and two levels of zebra mussels (without mussels or with mussels) with five replicates of the six treatment combinations, except for the water only control, which had three replicates. Treatment combinations were: hydrilla high density + zebra mussels, hydrilla low density + zebra mussels, zebra mussels only, hydrilla only in low or high density, plus water only to control for water parameters. Each independent experimental unit consisted of a 1.5 L tank filled with filtered (50 μm mesh) lake water from Canyon Lake. The experiment was carried out for 16 days, from November 7th to November 23rd, 2022. Zebra mussels and hydrilla were collected from Canyon Lake at the start of the experiment near boat Ramp 23 (Fig. 1). Apical stems of hydrilla were clipped at 5 cm total length, and the bottom 0.5 cm wrapped in cotton gauze, where they were attached individually to rocks using cotton thread to counter the buoyancy of clipped stems. Hydrilla uses fragmentation as a propagation method and survives clipping as employed by this study and Crane et al. 2020. High-density hydrilla tanks received nine plant strands and low-density hydrilla tanks received three strands. All tanks received three rocks (2–3 cm intermediate axis) for standardization. Initially, 10 mussels (three each size 7 and 12 mm and two each size 8 and 13 mm) were placed on the tank bottom in all mussel-inclusive treatments. On day five the number of zebra mussels was increased to three times the initial densities (i.e., 30 mussels per tank) to buffer mortality and increase treatment effects; zebra mussel wet biomass was extrapolated to match low-density treatments in Crane et al. (2020).

Temperature was continuously measured using temperature loggers (HOBO pendant Temperature/Light 64K data logger) in four randomly selected tanks across the treatments. The average water temperature during lab experiments, measured continuously at 1-hour increments, was 20.6±1.4 °C over both light and dark periods. The experiment was conducted at ambient air temperature, selected as a moderate compromise to support the metabolic requirements and health of both zebra mussels and hydrilla. Photosynthetic active radiation (PAR) was calculated by regression analysis (y = 0.0138x, R2 = 0.677)from lux (lumens per m2) measured in the tanks using a PAR meter (LI-190R Quantum Sensor, LI-COR, Lincoln, NE, USA). Three Hygger Programmable Aquarium LED Lights (HG-957, 36 watts) were placed between each set of two rows and set to a 16:8 day/night schedule. Light intensity (PAR) at the center of the light fixture averaged 61.4±3.86 μmol/m²/s during daytime, which is well above sufficient light (30 μmol/m²/s) for photosynthesis in similar submerged macrophytes (Mielecki and Pieczyńska 2005). All treatments were aerated using aquarium pumps (1 per 5 treatments), 4 mm silicone tubing, and 1-inch air stones. Temperature, dissolved oxygen, and specific conductivity were measured once daily, at noon, in each tank using a multisonde (YSI proDDS) and pH was measured with a smaller probe (Hannah HI 98195/10). Mussels were inspected daily for mortality, as evidenced by wide-open shells and zero response to repeated agitation of shells and were replaced if found dead. Daily mussel filtration rates were estimated to be approximately 3 L per day based on biomass (Bunt et al. 1993). Two water changes were conducted on days 2 and 4 to feed the mussels. On day five and every other day for the remainder of the experiment, mussels were switched to a mussel food consisting of 2.4 mL of diluted 2:1 shellfish diet and Nannochloropsis liquid food (Reed Mariculture) to ensure adequate food was provided. On the last day of the experiment, all hydrilla was measured for length (0.1 mm) and both zebra mussels and hydrilla were separated and rinsed with de-ionized water before freezing. Hydrilla plants were processed in the same manner as described for the field survey samples (i.e., thawing and drying). Carbon and nitrogen analysis were performed on hydrilla, where all stems per sample were homogenized using a grinder (IKA Works A 11 Basic Analytical Mills) and measured using a FlashEA 112 Series – NC Soil Analyzer. Zebra mussel samples were not processed because mortality was too high over the duration of the experiment. Carbon/Nitrogen levels were selected as an indirect measure of fertilization from zebra mussel excrement. Mussel samples were dried using the same methods as described for field survey samples. Additionally, dried mussels were measured for length, weighed to the nearest 0.001 μg on a microbalance, burned in the Muffle Furnace (Barnstead Thermolyne 600 Furnace) at 500 °C for 4 hours, then weighed again to determine Ash Free Dry Mass (AFDM).

Data analysis

Normality of data was assessed using Shapiro-Wilk’s test, and homogeneity of variances was evaluated using Levene’s test. Several response variables exhibited normal distributions, including hydrilla biomass from surveys, hydrilla C/N ratios, hydrilla growth, and zebra mussel AFDM from the lab experiment. Square-root, logarithmic, or cube-root transformations were applied to non-normal data to meet the normality assumptions. Zebra mussel densities were cube-root transformed for both total datasets and subsets (i.e., June vs September, substrate rocks vs hydrilla). Pooled zebra mussel densities and lengths as well as experimental hydrilla biomass did not respond to transformation and were analyzed using non-parametric tests. To examine the effect of benthic composition type on hydrilla biomass and the densities of zebra mussels attached to hydrilla, a nested Analysis of Variance (ANOVA) was conducted, with site serving as a covariate. One-way ANOVA was employed to evaluate differences in hydrilla C/N ratios for zebra mussel AFDM for lab experiments. Tukey’s post-hoc tests were performed on all ANOVAs to test for significant differences among groups. A Pearson correlation coefficient was computed to examine whether there was a significant correlation between total zebra mussel densities and hydrilla biomass at rocky and muddy sites. Pairwise t-tests were used to compare summer and fall surveyed hydrilla biomass, assess differences between hydrilla-bound and rock-bound mussels at rocky sites, and evaluate treatment effects of zebra mussels on hydrilla change in biomass and growth in the field and laboratory experiment. Non-parametric Mann-Whitney U tests were employed to test for zebra mussel density changes between summer and fall, size differences between mussels found on hydrilla tissues and rocks, and pairwise comparisons between controls and associated treatments plus low-density and high-density hydrilla biomass in the laboratory experiment. All statistical analyses were performed in R version 4.1.0 (2021).

Results

Distribution depends on substrate

In the field surveys, substrate and site (nested) influenced the density of zebra mussels and biomass of hydrilla (Fig. 2, Table 1). Substrate alone explained 13% of variation for hydrilla and 22% for zebra mussels, while site (nested within substrate) explained 70% and 43%, respectively (Table 1). Zebra mussel densities tended to be higher at rocky sites, whereas hydrilla biomass tended to be higher at muddy sites (Fig. 2, Suppl. material 1: fig. S1). A wide range of hydrilla biomass was observed at both muddy and rocky sites; however, only muddy sites had extremely high hydrilla biomass (> 400 g m-2) and mussel densities below 10,000 individuals m-2, except for one quadrat at Marina E, a rocky site. Total zebra mussel densities were significantly correlated with hydrilla biomass at muddy sites (r = 0.73, p < 0.001) but not at rocky sites (r = 0.29, p = 0.21). The highest hydrilla biomass was observed at a muddy site in the riverine zone of the reservoir (Crane A), where biomass was 3.5 times higher than at Potters D, the lowest biomass muddy site near the main channel (Fig. 2). The lowest hydrilla biomass was observed at sites located near the main channel of the reservoir, where hydrilla was restricted to a narrow strip in shallow water near the shore (Crane B, rocky site) or was found in isolated patches (Potters D, muddy site, Fig. 2). Like hydrilla, zebra mussel densities were also lowest at the muddy site Potters D ranging between 64 to 1,024 individuals m-2 attached to hydrilla. The highest densities of mussels attached to hydrilla were found at rocky site BR 1H, where values ranged from 11,156 to 46,729 individuals m-2 (Fig. 2).

Figure 2. 

Hydrilla biomass (a, c) and zebra mussel density on hydrilla (b, d) at different sites (a, b) and mean by treatment at muddy vs. rocky sites (c, d). The lower and upper lines of the boxplots represent the 25th and 75th percentiles and the bold line represents the median. Data falling outside the percentile range are plotted as outliers. Different letters denote significant differences (Tukey HSD pairwise comparison, p-values < 0.05). Differences between muddy vs. rocky sites (c, d) were not significant.

Attachment of zebra mussels on hydrilla and seasonal changes

Zebra mussel densities and size differed between those attached to rocks and those attached to hydrilla. At rocky sites in June, zebra mussel densities were significantly higher attached to hydrilla compared to those attached to rocks at all sites (Fig. 3). The greatest difference was found at rocky site BR1 H, where zebra mussel densities on hydrilla were 14 times higher compared to rocks (T38 = 11.19, p < 0.001, Fig. 3).

Figure 3. 

Zebra mussel density on rocks (white boxplots) and on hydrilla (grey boxplots) and mean by treatment for Hydrilla vs Rock as substrates in June 2022 for sites where rocky substrates were dominant. The lower and upper lines of the boxplots represent the 25th and 75th percentiles and the bold line represents the median. Data falling outside the percentile range are plotted as outliers. Asterisks above the diagram denote statistically significant differences between attachment surfaces (rocks vs. hydrilla) at each site (paired student’s t-test, * p-value < 0.05, ** p-value < 0.01, *** p-value < 0.001).

At all rocky sites, zebra mussels on rocks were significantly larger (6.1 ± 3.1 mm, mean ± SD) compared to mussels on hydrilla (3.4 ± 0.8 mm) (Fig. 4; compare boxplots for rocks in June vs. hydrilla in June). Mussel size on hydrilla tissues did not significantly change at resampled sites between June (3.4 ± 1.4 mm) and September (3.0 ± 1.5 mm, Fig. 4; compare boxplots for hydrilla in June vs. September). However, both month and site significantly affected mussel length (p-value < 0.001). This was observed at site Crane A and BR 1 Cove G, where zebra mussels were approximately 1.2 mm and 1.4 mm larger in September (4.2 ± 1.5 mm) than in June (3.5 ± 1.6 mm) at site Crane A. The opposite was true for BR1 Cove G, where zebra mussels were approximately 1.4 mm smaller in September (2.3 ± 0.1 mm) than in June. Zebra mussel densities declined by an average of 94.7% over the summer period, whereas hydrilla biomass did not change significantly. The decline in zebra mussel densities was statistically significant at all sites except Crane B.

Figure 4. 

Length of zebra mussels in June attached to rocks (white boxplots) and hydrilla (light grey) and attached to hydrilla in September (dark grey). The lower and upper lines of the boxplots represent the 25th and 75th percentiles and the bold line represents the median. Data falling outside the percentile range are plotted as outliers. Asterisks above the boxplots denote statistically significant differences between mussels on rocks vs. hydrilla in June (Mann Whitney U tests with Bonferroni corrections, * p-value < 0.05, ** p-value < 0.01. There were no significant differences between June and September except for Crane A and BR1 Cove G (denoted by a plus, pairwise t-test, p-value < 0.05).

Facilitation

Zebra mussel presence did not have a significant effect on hydrilla biomass (Low Density, W = 20, p = 0.14; High Density, W = 9, p = 0.53; Fig. 5a) or growth (Low density T = -0.25, p = 0.90, high-density T=-0.69, p = 0.51; Fig. 5b). Rather, hydrilla biomass increased significantly at higher densities compared to lower densities, regardless of zebra mussel presence (W = 100, p < 0.001; Fig. 5a), while growth generally increased for low-density hydrilla treatments but was not statistically significant (T8 = -0.69, p = 0.51; Fig. 5b). Additionally, carbon to nitrogen (C/N) ratios did not differ significantly among treatments or compared to the initial measurement (F4,18 = 0.70, p-value = 0.60; Fig. 5c).

Figure 5. 

Delta biomass (i.e., increase in biomass during the experiment), growth in length and carbon/nitrogen ratios of hydrilla tissue in low density (LD) and high density (HD) hydrilla treatments (grey boxplots) and controls (white boxplots).

Mussel biomass was higher when hydrilla was present (F2,11 = 4.3, p = 0.04; Fig. 6). The difference was statistically significant for the low-hydrilla density treatment (p = 0.04), but only marginally significant at the high-density treatment (p = 0.09), where variation was higher. Mortality during the 16-day lab experiment ranged between zero and three mussels per tank, except for one mussel-only control tank, which experienced a mortality of 19 mussels and was subsequently excluded from analysis. That tank was located closest to the air outtake for the electric air pumps but experienced only slightly higher temperatures (+0.8 °C) and conductivity (+14.7 μS/cm) than the other four tanks.

Figure 6. 

Ash Free Dry Mass (AFDM) of zebra mussels in low density (LD) and high density (HD) of hydrilla treatments (grey boxplots) and control (white boxplot). The lower (25th) and upper (75th) lines of the boxplots represent respective percentiles and the thick bold line indicates median values. Different letters denote significant differences (Tukey HSD pairwise comparison, p-values < 0.05).

Discussion

This study examines variation in the distribution of both zebra mussels and hydrilla in relation to substrate, each other, and season (i.e., summer vs. fall). It also quantifies densities of zebra mussels attached to hydrilla. Our results support findings by previous studies that macrophytes support transient populations of seasonally high numbers of small zebra mussels (Bodamer and Ostrofsky 2010; Londo et al. 2022). This supports an increased risk of human-mediated transport of zebra mussels by watercraft via transport of infested hydrilla, at least during certain seasons.

The lower numbers of zebra mussels found on hydrilla in September were most likely the result of loss of the spring cohort and lower settlement occurring during summer months (Schwalb et al. 2023), because there was no significant size differences compared to June although growth should have occurred. The decline of juvenile mussels on hydrilla could have been caused by voluntary post-metamorphic drift, mortality, or inability to remain attached due to their size as mussels grew. Whether zebra mussels are physically able to remain attached to hydrilla as they grow larger remains to be tested. It is known that juvenile zebra mussels may detach and that those ≤ 2 mm in size may drift in water, especially at sites with high wave action from wind or boaters (Ackerman et al. 1994; Ricciardi and Hill 2023). However, such transport mechanisms are unlikely to explain the substantial decrease by an average of -95% (i.e., thousands of zebra mussels per m2) observed on hydrilla in Canyon Lake. A -56% decline was observed in the same year (2022) on cumulative settlement monitors (bricks) located near marinas on the same lake (Schwalb, unpublished data). In the past several years at Canyon Lake, declines up to -85% at one site in 2021 and an average of -54 to -58% in September 2019 and August 2018 were observed, which was attributed to high summer temperatures (Schwalb et al. 2023).

Other factors may have influenced population declines such as predation by fish (e.g., catfish) or dissolved oxygen depletion. Respiration by bacteria and macrophytes at night can cause oxygen levels in the water column to plummet due to the absence of photosynthetic activities (Sand-Jensen 1989; Spencer et al. 1994; Sousa 2011). In summer of 2022, dissolved oxygen was depleted to 3 mg L-1 in dense strands of hydrilla when water temperatures were approximately 30 °C. This is higher than reported by another study in Mississippi (1.5 mg L-1 at 27–29 °C; Miranda and Hodges 2000), but in combination with higher temperatures could have contributed to zebra mussel mortality as demonstrated by previous experiments (Gantz et al. 2023; Schwalb et al. 2023). However, declines in oxygen concentration in macrophyte beds are short-term and can oscillate to very high oxygen concentrations during the day, which may be tolerable by zebra mussels (Ventura et al. 2016).

Although zebra mussels were found attached to hydrilla, densities of mussels in this study were moderate compared to attachment on other macrophytes found in other studies. For example, we found up to 46,729 ind. m-2 located on hydrilla dry mass, whereas densities up to 750,000 ind. m-2 were found on clasping-leaf pondweed (Potamogeton perifoliatus) and Eurasian watermilfoil (Myriophyllum spicatum) in Lake Balaton, Hungary (Muskó and Bakó 2005). Juvenile settlement densities on artificial surfaces (i.e., bricks) were much lower than on hydrilla in June of the same year, averaging less than 100 ind. m-2. Settlement densities have consistently reduced by approximately 100 ind. m-2 each year from 2019 to 2021 and stayed consistent thereafter. Some lakes in Europe may have higher densities of zebra mussels than the southern United States. For example, Lake Balaton, Hungary had a maximal density of 220,000 ind. m-2 during the time of the Muskó and Bakó (2005) study, and Canyon Lake had a maximal density of 9,000 ind. m-2 in 2022. One study suggests that veligers selectively settle on plants with sturdy stems (Londo et al. 2022), which hydrilla lack, but future research should test whether zebra mussel densities are lower on native macrophytes as compared to hydrilla, ideally in the same waterbody.

Higher densities of hydrilla in muddy compared to rocky substrates were observed. However, the presence of hydrilla observed in rocky areas further corroborates past evidence that hydrilla is often found in rocky areas despite limited sediment nutrients (Jain and Kalamdhad 2018). In our study, the furthest upstream site had a relatively thick layer of fine sediment and supported the highest hydrilla biomass, possibly owing to greater sediment load in the riverine zone of this part of the reservoir. Also, the presence of submerged aquatic vegetation can slow current velocity and reduce resuspension which has been reported by several studies (Zhu et al. 2015; Li et al. 2016). Increased fine sediments on benthos are detrimental to zebra mussels, which require hard substrate for settlement and their attachment on hydrilla appeared transient in nature.

In Lake Lewisville, Texas, hydrilla above-ground biomass peaked in early August (Madsen and Owens 1998), but in Canyon Lake, hydrilla did not appear to grow between June and September in summer 2022. The early warming in Spring of 2022 may have triggered earlier spring growth of hydrilla. Optimum growing temperatures for hydrilla start at 20 °C, which was reached in early March 2022, compared to mid-April in 2020 and 2021 (True-Meadows et al. 2016; Schwalb et al. 2023); however, due to a combination of high air temperatures and low water levels associated with drought conditions, water temperatures may have been too hot over the mid-summer growth period. Studies suggest temperature upper threshold limits for survival of zebra mussels at 29 to 30 °C, but others suggest thermal tolerance may be even higher at the southernmost distribution (Garton et al. 2014; Schwalb et al. 2023).

Zebra mussels have been shown to improve the physiological condition of Eurasian Watermilfoil and Eelgrass (Zhu et al. 2006), but such a positive interaction was not found in our experiments with hydrilla. When grown together, the growth and biomass of hydrilla were not significantly affected in the lab, and these results were corroborated from a similar field experiment conducted in the Fall of 2022 (Lorkovic MS thesis, Texas State University, 2023). Laboratory experiments resulted in no change to hydrilla C/N ratios in treatments with zebra mussels as compared to the control (only hydrilla) or initial concentrations. However, field experiments showed a 7% increase in tissue nitrogen content over a 1-month period, suggesting zebra mussels may have competed with hydrilla for nutrients (Lorkovic 2023) in the laboratory study. It is possible that the removal of roots and stolons in the laboratory experiments may have affected the results. Additionally, we conducted experimental investigations during fall, which may have influenced the uptake ability of our plants.

Zebra mussels may benefit physiologically from the presence of hydrilla as shown in our laboratory experiment where mussels increased biomass when hydrilla was present, particularly at low density. One possible explanation is that macrophytes can provide attachment surface for biofilms and other bacteria which can be utilized as a food source for zebra mussels (Silverman et al. 1996). Our experiments did not detect a significant impact of zebra mussels on hydrilla growth, suggesting this may not be a mechanism of invasional meltdown in Canyon Lake. Still, zebra mussels (i.e., the primary invader) may have played a role in facilitating the invasion of hydrilla in Canyon Lake by reducing phytoplankton and increasing water clarity (Robertson et al. 2019), expanding the open niche for the invasive macrophyte, particularly in the absence of major competition from long-term established native macrophytes (Zhu et al. 2006; Wegner et al. 2019). Demonstrated benefits of hydrilla on zebra mussel biomass and potential for zebra mussels to have facilitated hydrilla invasion extent suggest these as possible mechanisms of invasional meltdown processes at work in Canyon Lake.

Reservoirs, albeit man-made ecosystems, provide habitat for plants and animals as well as sources of drinking water and hydroelectric power, but abundant recreational access points may result in species introductions, making it crucial to fully understand and effectively manage invasive species to preserve biodiversity and prevent economic impacts. This study shows that managing hydrilla is important as it may directly facilitate zebra mussel dispersal through transport of mussel-fouled hydrilla fragments downstream by currents or to new water bodies by human vectors, as plants with attached zebra mussels are transported on boat propellers or trailers (Horvath and Lamberti 1997). Additionally, this study has shown that hydrilla may contribute to the increased growth of zebra mussels under certain conditions. Continued research efforts are needed to further understand the potential impacts of invasive species on native species and ecosystems and the interactions between invasive species and their implications for invasive species management.

Acknowledgements

We would like to thank students and technicians in the Texas State Stream Ecology Lab for their assistance with field and lab work. We also extend our gratitude to the anonymous reviewers for their valuable feedback, which helped improve the clarity and quality of this manuscript. This work was funded in part by the US Army Corps of Engineers’ Aquatic Nuisance Species Program’s focus on Next Generation Ecological Modeling. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of the United States Army Corps of Engineers.

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Supplementary material

Supplementary material 1 

Supplementary data

Emily Lorkovic, Jason P. Martina, Monica E. McGarrity, Astrid N. Schwalb

Data type: docx

Explanation note: table S1: Physical characteristics of the eight survey locations arranged in order (Top to bottom) from far to near distance from the dam, stars denote sites with sediments dominated by rock; fig. S1: Relationship between total densities of zebra mussels (benthic and hydrilla attached) and biomass of hydrilla for each quadrat in June 2022.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
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