Coral recruitment and early benthic community development on several materials used in the construction of artificial reefs and breakwaters

Artificial reefs are increasingly being promoted as a means to mitigate impacts from human activities in coastal urban areas. Coastal defense structures such as breakwaters are becoming recognized as large-scale artificial reefs that support abundant and diverse marine communities and play important roles in coastal ecology and management. However, there is limited understanding of how substrate materials used to construct artificial reefs or breakwaters can influence the development of habitat-forming benthic organisms. To assess the influence of substrata on coral recruitment and overall benthic community development, we deployed standard-size tiles of materials used in the construction of breakwaters and artificial reefs (concrete, gabbro, granite, and sandstone), along with terra-cotta for comparative purposes, at two breakwaters (DDD, PRT) and two natural reef sites (NR1, NR2) in Dubai, United Arab Emirates, for one year. Kruskal-Wallis ANOVA with post-hoc Mann-Whitney U-tests were used to examine differences in coral recruitment among sites and materials. Coral recruitment was highest at the DDD (4.9 ± 0.5 recruits 100 cm^{- 2}), while recruitment was low and did not differ among other sites (PRT: 0.1 ± 0.04, NR1:0.3 ± 0.1, NR2: 0.1 ± 0.03 recruits 100 cm^{- 2}). There were significant differences in coral recruitment among materials at DDD, where gabbro had higher recruit densities than concrete and sandstone; sandstone also contained less coral recruits than terra-cotta. Variability associated with low coral recruit densities precluded significant differences among materials at other sites. Overall benthic community structure differed more as a result of differences among sites than among substrate materials. Higher community dissimilarity was observed among sites than among material in SIMPER analysis, and significant differences were only observed among sites in ANOSIM. Univariate comparison of the benthos correlated with community differences in NMS ordination also showed significant differences among sites but not material. Overall, these results indicate that site-specific differences in recruitment patterns are more important in determining early benthic community structure and coral recruitment than are differences among substrate material. However, where coral recruitment is high, these results suggest that gabbro should be used preferentially over concrete or sandstone where it is feasible, but that granite may be a suitable alternative where it is the dominant stone. Coral recruitment on terra-cotta was comparable to all materials but sandstone, supporting its continued use in recruitment studies. These results also indicate that using stone amenable to coral recruitment is unlikely to influence the wider benthic community.