Factors influencing the epibiota of granite blocks used in coastal defense structures: An observational experiment

Anthropogenic pressures exerted on coastal areas, combined with climate change, and accelerated sea level rise, result in the loss of natural habitats. At the same time, the number of coastal protection and defense structures have increased considerably and may become the dominant habitat in many intertidal regions. In this study we characterize the epibiota of artificial substrates (granite blocks) from coastal defense structures used to contain the effects of rising sea levels, and the same type of artificial substrate found in contact with coastal reefs located on Serrambi beach, northeast of Brazil. We mapped, measured, and photographed 45 artificial granite blocks and their surroundings in the field. A gradient was found for the number of taxa on the blocks, from high coverage by macroalgae to extensive non-colonized areas. However, it was observed that the closer to the natural reef the greater the taxonomic richness. Our results also indicate that the factor that best explains the colonization of blocks is the coverage of their surroundings, thus, few organisms were found on blocks whose surrounding unconsolidated sediment area was very extensive. In our wind and sun exposure simulation, granite rock demonstrated a more accelerated and greater level of heating (up to 3 °C) compared to fragments of natural substrate, although the variation in the slightly lighter colored and smaller granite block resembles the variation in the reef fragment. We suggest that the use of lighter-colored granite blocks at mid-coastal levels can increase the colonization rate of these substrates.