Two Invaders, One Ecosystem: Exploring Interactions Between Dreissena Polymorpha and Hydrilla Verticillata




Lorkovic, Emily

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Invasive species in aquatic ecosystems 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 to facilitate the subsequent spread and growth of macrophytes by increasing water clarity. In turn, submerged macrophytes may provide substrate for settlement of zebra mussels, which require a firm substrate to settle on (i.e., not mud or sand). The invasion of Canyon Lake, TX by zebra mussels has occurred relatively recently, and since then, the spread of an invasive submerged macrophyte, Hydrilla verticillata, has also been observed. The goal of this study was to examine variation in the distribution of both zebra mussels and Hydrilla in relation to sediment composition, to each other, and over time (summer vs. fall), and potential facilitation between the species in laboratory and field experiments. To investigate the distribution of each species, dive surveys were conducted in June 2022 and September 2022 using sampling quadrats along a transect at eight sites at Canyon Lake, TX. Field and lab experiments used a fully crossed experimental design to examine the impact of Hydrilla on zebra mussels and vice versa with controls (Hydrilla only and zebra mussels only), and treatment (Hydrilla + zebra mussels). For the surveys, Hydrilla densities tended to be higher in muddy compared to rocky sediment. In contrast, zebra mussel densities tended to be higher in rocky (on Hydrilla and rocks) compared to muddy habitat (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, only a small fraction of zebra mussels remained on Hydrilla in September, almost exclusively representing a new settlement cohort based on their size distribution. Hydrilla biomass did not change significantly between summer and fall. The experiments did not detect any significant impact of zebra mussels on growth and condition of Hydrilla and positive effects of Hydrilla on zebra mussels was limited to Hydrilla presence in low densities. Nevertheless, Hydrilla may directly facilitate zebra mussel dispersal, especially in spring, as mussels attached to plant fragments can be transported by currents downstream or by human activities, such as entanglement in boat propellers or trailer bunks.



zebra mussels, hydrilla, invasional meltdown hypothesis, aquatic invasive species, invader interactions


Lorkovic, E. (2023). Two invaders, one ecosystem: Exploring interactions between Dreissena polymorpha and Hydrilla verticillata (Unpublished thesis). Texas State University, San Marcos, Texas.


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