Are artificial reefs surrogates of natural habitats for corals and fish in Dubai,United Arab Emirates?

Artificial reefs are often promoted as mitigating human impacts in coastal ecosystems and enhancing fisheries; however, evidence supporting their benefits is equivocal. Such structures must be compared with natural reefs in order to assess their performance, but past comparisons typically examined artificial structures that were too small, or were immature, relative to the natural reefs. We compared coral and fish communities on two large (>400,000 m³) and mature (>25 year) artificial reefs with six natural coral patches. Coral cover was higher on artificial reefs (50%) than in natural habitats (31%), but natural coral patches contained higher species richness (29 vs. 20) and coral diversity (H′ = 2.3 vs. 1.8). Multivariate analyses indicated strong differences between coral communities in natural and artificial habitats. Fish communities were sampled seasonally for 1 year. Multivariate fish communities differed significantly among habitat types in the summer and fall, but converged in the winter and spring. Univariate analysis indicated that species richness and abundance were stable throughout the year on natural coral patches but increased significantly in the summer on artificial reefs compared with the winter and spring, explaining the multivariate changes in community structure. The increased summer abundance on artificial reefs was mainly due to adult immigration. Piscivores were much more abundant in the fall than in the winter or spring on artificial reefs, but had low and stable abundance throughout the year in natural habitats. It is likely that the decreased winter and spring abundance of fish on the artificial reefs resulted from both predation and emigration. These results indicate that large artificial reefs can support diverse and abundant coral and fish communities. However, these communities differ structurally and functionally from those in natural habitats, and they should not be considered as replacements for natural coral and fish communities.     

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.     

Benthic development on large-scale engineered reefs: A comparison of communities among breakwaters of different age and natural reefs

Breakwaters represent large-scale engineered artificial reefs that can develop diverse and abundant communities and are likely to play an increasing role in marine ecosystems as human populations grow in coastal urban areas. Information on how these communities develop and if and when these communities begin to resemble those on natural hard-bottom habitat is essential for marine management, but is not well understood. In this study, benthic communities on six breakwaters ranging from 1 to 31 years of age were compared to provide an understanding of patterns of community development on engineered coastal defenses, and these were compared to communities on natural reefs to gain an understanding of how communities develop on artificial structures relative to those in natural habitats. Multivariate analyses indicated that benthic communities on breakwaters became more similar to natural reefs with increasing age, but that communities on even the most mature (31 years) breakwater were distinct from those on natural reefs (ANOSIM p < 0.001). Generally, breakwaters ≤5.5 years had higher abundance of turf algae, sponges, bivalves, and bare pavement, while more mature (≥25 years) breakwaters were dominated by corals. Coral cover on 25 and 31 years old breakwaters (46% and 56%, respectively) was significantly higher than on natural reefs (37%; HSD test p < 0.05 and p < 0.001, respectively). These results indicate that breakwaters develop benthic communities that continue to change over periods exceeding 31 years, and that although they become more similar to communities on natural reefs with increasing age, these communities remain distinct.     

Mangrove restoration without planting

Mangrove forests are disappearing at an alarming rate. Mangrove planting is the most common method of restoring these forests. However, this approach is not often successful, especially when the reasons for mangrove degradation were not removed prior to planting new seedlings or propagules. A successful mangrove restoration project may not necessarily include a planting phase. When the stressors are removed and suitable environmental conditions such as correct hydrology and calm area, particularly on exposed coasts, are provided, natural regeneration processes could recover mangroves from degradation. This paper describes an approach to mangrove restoration applied to an exposed shoreline on the west coast of Peninsular Malaysia. In this method a detached breakwater was erected seaward at the shoreline to shelter the restoration area from wave action, prevent the ongoing erosion and promote sediment deposition raising the elevation of the substrate to the target elevation. Survival of Avicennia marina seedlings transplanted in the sheltered area was significantly affected by sediment burial (P < 0.05). However, about eight months after construction of the breakwater, the sedimentation rate decreased and the substrate became stable. Subsequently, natural recruits appeared on the site. We postulate that waterborne seeds or propagules were available from adjacent stands in the study area but wave exposure reduced the possibility of natural recruitment. Thus the breakwater presence provided favourable conditions which could attract mangrove recruits, facilitating reestablishment and natural recovery of the mangrove ecosystem without planting.