Geomorphic invaders: Geomorphic potential and landscape controls on the biogeomorphology and ecosystem engineering of Dreissenid mussels

Zebra mussels (Dreissena bugensis) and quagga mussels (Dreissena polymorpha) are invasive throughout much of the northern hemisphere. Whilst much attention has been paid to their role in altering aquatic systems via their filter feeding, little attention has been given to their role as geomorphic agents, or the relationships between geomorphology and their ecosystem engineering. We aimed to understand the controls and feedbacks between fluvial geomorphology and Dreissenid mussel invasion, utilising field, laboratory, and numerical modelling approaches. We found important consequences for both geomorphology and ecosystems, with mussel invasion significantly impacting annual sediment transport rates, and positively facilitating the invasion of further priority invasive species.

Quagga mussels attach to benthic sediments using byssal threads, which affects sediment stability and thereby broader river geomorphology. At an invaded gravel bed river, quagga mussels attached >500 g m^-2 of mineral sediments together. In ex situ flume experiments, this process increased critical shear stress by 40%. Numerical modelling of flow at the study river was used to upscale these stresses to estimate changes to sediment transport over a recorded five-year flow period, which indicated that typical densities of quagga mussels may reduce the occurrence of a geomorphically active flood event from Q₃₀ to Q₂, and reduce sediment transport by 74%. Thus, substantial alterations to bedload sediment transport may occur following quagga mussel invasion.

Dreissenid mussels are also ecosystem engineers, where their shells provide a unique stable habitat in fine-grained rivers. Field surveys found that mussel shells positively facilitate macroinvertebrate communities, but preferentially facilitate co-evolved, high-priority invasive amphipods. The construction of a spatial model of riverbed grainsize across England and Wales, combined with an analysis of Environment Agency nationwide presence/absence records, identified that ecosystem engineering by zebra mussels was particularly powerful in fine-grained river systems to other invasive taxa. Supporting mechanistic aquarium experiments indicated that the ecosystem engineering of zebra mussels may support the invasions of high-profile amphipod species into otherwise unfavourable habitats, which could not be invaded without mussel engineering. Channelisation and dredging, which simplify river channels, may benefit Dreissenid mussel ecosystem engineering and the facilitation of other invasive species. Instead, Nature-based Solutions could be employed to restore the geomorphic functioning of systems, which may improve resilience against high-priority invasive species.