Ecosystem engineers enhance the multifunctionality of an urban novel ecosystem: Population persistence and ecosystem resilience since the 1980s.

In degraded urban habitats, nature-based solutions aim to enhance ecosystem functioning and service provision. Bivalves are increasingly reintroduced to urban environments to enhance water quality through biofiltration, yet their long-term sustainability remains uncertain. Following the restoration of the disused South Docks in Liverpool in the 1980s, natural colonization of mussels rapidly improved dock-basin water quality and supported diverse taxa, including other filter feeders. While the initial colonization phase has been well documented, there has been limited published research since the mid-1990s, despite ongoing routine water quality monitoring. Here, we assessed the long-term persistence of mussel populations, their associated biodiversity, and physico-chemical parameters of the water in Queens and Albert Docks by comparing historical (1980s to 1990s) and contemporary data from follow-up surveys (2012,2022). Following an initial period of poor water quality (high contamination and turbidity, low oxygen), the natural colonization of mussels from Albert Dock in 1988 extended throughout the South Docks. By the mid-1990s, the environment of the South Docks and its mussel populations had stabilized. The dock walls were dominated by mussels which provided important complex secondary substrate for invertebrates and macroalgae. Surveys conducted in 2012 and 2022 confirmed the continued dominance of mussels and estimates of mussel biofiltration rates confirm that mussels are continuing to contribute to maintaining water quality. A decline in salinity was observed in both docks in 2022, with evidence of recovery. While these ecosystems appear relatively stable, careful management of the hydrological regime is crucial to ensuring the persistence of mussels and resilient ecosystem service provision through biofiltration.