Reconceptualising the role of organisms in the erosion of rock coasts: A new model

Attempts to understand the morphodynamics of rock coasts focus on the nature and rate of erosion and how this influences the evolution of coastal profiles over century to millennial timescales. Biological contributions to this process are often ignored or treated in a rather simplistic way as merely producing microscale decreases in rock strength. Using a combination of field observations and literature review, we address two key issues hampering a more rounded assessment of biological impacts erosion of coastal rocks. Firstly, we reconceptualise the biological contributions to erosion of rock coasts into direct and facilitative types, and secondly we discuss the need for more appropriate reporting of rates of bioerosion. For direct bioerosion, this means clearly documenting the percentage of platform surfaces affected by bioerosive agents and quantifying the volume of material eroded and the amount of fine sediment produced. For facilitative bioerosion, we must quantify the roles of biota in changing the material properties of rocks, creating or altering surface morphologies and/or weakening joint boundaries. In this way, facilitative bioerosion is one of several processes mediating geomorphic alteration of rock surfaces, contributing to the production of landforms and/or reducing the resisting force of rock. Subsequently, we build on these ideas to produce a new conceptual model which more accurately portrays the multiple and cross-scalar ways that biological processes, notably bioerosion, influence the resistance of coastal rocks to erosive forces. Such a model provides a stronger basis for numerical modelling and understanding the morphodynamics of rock coasts in the future and allows a quantitative assessment of the role of biota.