Sediment accumulation by coastal biogenic structures sustains intertidal flats facing sea level rise in the German Wadden sea.

Biogenic structures such as oyster reefs and mussel beds can enhance sedimentation and promote the expansion of intertidal flats in the German Wadden Sea. This study presents large-scale measurements of sedimentation at biogenic structures that depend on biological activities, the hydrodynamic environment and surface roughness. As the surface roughness of intertidal flats has changed with the bioinvasion of the Pacific oyster and the resulting transformation of mussel beds into oyster reefs, we hypothesised that sedimentation rates vary between these structures. To quantify the extent of sedimentation over time, we surveyed bivalve-covered intertidal flats in 2020 and 2022 and generated digital models of the structures and surrounding terrain. The sediment accumulation rates for the mussel bed and oyster reefs reached 3.9 cm³/(cm²*y), with higher rates observed within the mussel bed than within the oyster reefs. Generally, biogenic structures at lower elevations experienced higher sediment accumulation rates. All accumulation rates exceeded previously reported rates for intertidal flats and the current rate of sea level rise in the Wadden Sea. Our findings suggest that oyster reefs and mussel beds have comparable effects on sediment accumulation, supporting the persistence of intertidal flats and thus helping to stabilise the Wadden Sea.