Understanding the conditionality of ecosystem services: The effect of tidal flat morphology and oyster reef characteristics on sediment stabilization by oyster reefs

Ecosystem-based coastal protection by means of conserving, restoring or creating intertidal ecosystems that attenuate waves and stabilize shorelines, offers a promising way to climate proof coastlines for the future. The Pacific oyster (Crassostrea gigas) is an ecosystem engineering species, which is known for its wave attenuating and sediment trapping ecosystem services, but it remains unknown to which extent this is conditional. We aim to test the hypothesis that the ecosystem engineering effect concerning sediment trapping and stability by oyster reefs is conditional, and can be predicted based on i) local physical forcing, ii) morphological characteristics of the tidal flat, and iii) biological characteristics of the oyster reef.Analyses of long-term sediment accretion patterns on natural intertidal oyster reefs at the Oosterschelde basin (The Netherlands) showed that this ecosystem engineering effect is strongest on tidal flats under erosional conditions, lower aspect ratio (i.e., relative long and narrow reefs), relatively closed reefs (i.e., few open patches) and higher coverage of oysters within reef patches. The ability of C. gigas to shape the environment thus depends both on biotic and abiotic conditions, meaning that oyster reefs only work under specific conditions for erosion control. Overall, our results provide baseline understanding for ecosystem management aimed at affecting sediment dynamics, thereby contributing to a better understanding for designing ecosystem-based solutions under different abiotic and biotic conditions. In addition, present study provides a clear example of how we need to gain a better understanding of the conditionality of ecosystem services in general, to be able to create and restore ecosystems for obtaining their services