Hidden biodiversity in cryptic habitats provided by porous coastal defence structures

In response to flood risk from rising and stormier seas, increasing amounts of natural coastline worldwide are being replaced by a proliferation of coastal defence structures. While the primary role of defence structures is protecting the coastline, consideration should be given to the biological coastal communities they support. Artificial structures are currently seen as poor habitats for marine organisms. They are constructed in harsh coastal environments, lack structural complexity, and are subjected to episodic disturbance from maintenance, reducing their suitability as habitats for coastal species. Recent work has focused on mitigating the impacts of coastal defence structures, through secondary routes such as enhancing biodiversity by encouraging colonisation of marine biota. Research thus far has focused on enhancements to improve structural complexity on the external surfaces of coastal defences. Many structures are porous with internal compartments. To date no work has been undertaken on the habitat provided by the internal surfaces of the blocks used in building structures.We investigated the role of porous coastal defence structures in habitat provision. Taking advantage of a groyne reduction from 45 m to 20 m length, we surveyed the internal environment of the structure. We also considered the impacts of maintenance activity on coastal assemblages. Our work shows that the internal environment of artificial structures provides functional habitat space supporting higher species richness and diversity than external surfaces. The more benign environment of internal surfaces protects from desiccation stress and is probably less scoured by mobile sediments, and as such is of unrealised importance to coastal assemblages. External surfaces are also subject to high levels of disturbance from maintenance activities, further limiting the potential ecological contribution this area of the artificial habitat might otherwise develop. These findings reveal the multifunctional role of porous coastal defence structures, acting as engineering protection and habitats for coastal assemblages