Effect of upscaling nature-based coastal protection on estuarine biodiversity using foreshores and transitional polders

Nature-based solutions (NbS), integrating ecosystems and natural processes, offer a promising approach to deliver benefits to both ecosystems and human society. In estuarine and coastal regions, highly vulnerable to storm surges and large wave exposures, NbS schemes are often primarily evaluated for flood risk. Comprehensive assessments of their broader impacts on biodiversity are frequently overlooked. This study presents an integrated modelling approach to compare the long-term estuarine biodiversity outcomes of two nature-based coastal protection schemes: (i) a seaward foreshore and (ii) a landward transitional polder (i.e. a temporary de-embankment). These schemes involve the creation of coastal wetlands, each subjected to different environmental and landscape settings. We also assess the influence of sea-level rise (SLR), sediment availability (i.e. suspended sediment concentration; SSC) and initial elevation on the temporal development of intertidal biodiversity, focusing on macrozoobenthos and tidal marsh vegetation. The findings demonstrate that the effectiveness of different NbS schemes in enhancing biodiversity is strongly dependent on the initial environmental conditions and, consequently, on how the NbS is integrated into the landscape. In accreting environments, existing sloped foreshores facilitate rapid vegetation establishment and development, while initially flatter, lower-elevation transitional polders better support benthic biodiversity. However, flat transitional polders initiated at elevations above mean sea level rapidly become dominated by vegetation, reducing their benefits to benthos. Over time, biodiversity outcomes in two schemes gradually converge as accretion progresses. SLR and SSC are key factors influencing the temporal development of biodiversity and scheme comparison. Higher SSC levels accelerate convergence, while SLR decelerates it. Synthesis and applications. Our study provides a critical toolset for designing, comparing and planning nature-based solutions with respect to biodiversity effects, supporting coastal management strategies that integrate flood safety with optimal biodiversity outcomes. By considering the distinct biodiversity trajectories of different nature-based solutions schemes-shaped by sea-level rise and suspended sediment concentration-we highlight time-sensitive trade-offs and long-term ecosystem developments. These insights are particularly relevant given practical constraints, such as hydrodynamic challenges for seaward foreshores and societal resistance to land-use changes for transitional polders. This study facilitates informed decision-making for sustainable and adaptive coastal management.