Cool barnacles: Do common biogenic structures enhance or retard rates of deterioration of intertidal rocks and concrete?

Sedentary and mobile organisms grow profusely on hard substrates within the coastal zone and contribute to the deterioration of coastal engineering structures and the geomorphic evolution of rocky shores by both enhancing and retarding weathering and erosion. There is a lack of quantitative evidence for the direction and magnitude of these effects. This study assesses the influence of globally-abundant intertidal organisms, barnacles, by measuring the response of blocks of limestone, granite and marine-grade concrete colonised with varying percentage covers of Chthamalus spp. to simulated intertidal conditions. Monitoring temperature regimes at 5 and 10 mm below the surface of each material type demonstrates a consistent and statistically significant negative relationship between barnacle abundance and indicators of thermal breakdown. Subsurface peak temperatures of blocks with a 95% cover of barnacles were reduced by 1.59°C for limestone, 5.54°C for concrete and 5.97°C for granite in comparison to those without any barnacles. The amplitude of internal thermal fluctuations was also buffered by 0.70°C in limestone, 1.50°C in concrete and 1.63°C in granite. Furthermore, concentrations of potentially damaging salt ions were consistently lower under barnacles in limestone and concrete. These results indicate that barnacles do not enhance, but rather reduce rates of mechanical breakdown on rock and concrete by buffering near-surface thermal cycling and reducing salt ingress. In these ways, we highlight the potential value of barnacles as agents of bioprotection. These findings support growing international efforts to enhance the ecological value of coastal structures by facilitating their colonisation (where appropriate) through design interventions.