Bivalve reefs modify sediment properties within and beyond their boundaries

Reef-building bivalves such as mussels and oysters are ecosystem engineers that strongly modify sediment properties by attenuating water flow and accumulating fine biodeposits. Yet, despite advances in understanding reef–sediment dynamics, it remains partially resolved how reefs and their environment interact to control sediment characteristics within and beyond reef boundaries. We explored how the extent and spatial scale of the sediment modification depend on the prevailing environmental conditions (e.g., bathymetry, orbital and flow velocity) and reef characteristics (e.g. size, density) in the intertidal Dutch Wadden Sea. We sampled sediment silt and total organic matter content at three sediment depths (0-1 cm, 9-11 cm, 25-28 cm) on twelve reefs from their centre up to 400m beyond their borders, and used linear models to assess relationships. We found that within reefs, organic matter and silt content were higher compared to the surroundings, and that denser reefs had a greater impact, with only a marginal influence of environmental conditions. Effects on sediment properties beyond reef boundaries were found to extend up to 100m at the surface, but were detected up to 200m in deeper layers, with total organic matter and silt content decreasing nonlinearly with distance. These effects were more pronounced around larger reefs and correlated negatively to hydrodynamic conditions that increase sediment resuspension, such as orbital and flow velocity. Overall, our findings highlight that intertidal bivalve reefs modify their environment well beyond their physical borders. However, the scale of the extended-engineering footprint is context dependent as it varies with reef size and density, and current and wave action.