Zebra mussel (Dreissena polymorpha) population dynamics and associated water quality impacts along their southern United States colonization front

Zebra mussels represent one of the most pervasive and expensive non-native species to be introduced into new aquatic ecosystems, negatively impacting human structures and infrastructure and acting as ecosystem engineers. Zebra mussels have demonstrated thermal plasticity, enabling expansion to semi-tropical aquatic systems including Texas ca. 2009. In this study we described spawning and population dynamics and water quality changes after colonization of two central Texas reservoirs, Lake Austin and Lady Bird Lake, ca. 2017. Veliger concentrations peaked in spring and early summer (Julian days 120–170) when water temperatures were between 20–25 °C. Adult population densities were initially highest nearest the busiest boat ramps and peaked in 2019–2020. Densities declined thereafter in the lower sections, but generally increased upriver in Lake Austin. However, the decline throughout Lady Bird Lake was three orders of magnitude from the peak. After colonization, chlorophyll a and suspended solid concentrations significantly declined, concomitant with significant changes in water total phosphorus concentrations; changes in different nitrogen-form concentrations were mixed. However, water quality changes were exacerbated by changing discharge volumes. Recent drought conditions and reduced discharges after 2021 have also resulted in elevated water temperatures, notably in Lady Bird Lake, that may have contributed to observed declines in adult zebra mussel densities nearshore. We hypothesize that other southern United States reservoirs should expect similar variations in population dynamics which will impact municipal, recreational, and water quality attributes. Preventing introductions remains essential as the species continues to rapidly spread to new regions.