Two invaders, one reservoir: Hydrilla shapes the distribution of zebra mussels and may facilitate their growth

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.