Homogenization and distinction of coral recruit communities between natural and artificial substrates at Koh Tao a decade after deployment

Thousands of tonnes of artificial substrate have been deployed across the Gulf of Thailand in recent decades for the purpose of habitat creation and reef restoration. At the island of Koh Tao, these were mostly deployed between 2011 and 2017. Field surveys conducted in 2018–19 (an average of 44 months post deployment) at the island revealed considerable disparity in the community structures of scleractinian corals with recruit communities from artificial reefs and natural giant clam shells differing significantly from mature communities (reefs and pinnacles). Despite multiple biotic and abiotic variables being tested, no single explanation was found, and thus the findings were evaluated in the context of multiple hypotheses. In this study, we present findings from a repeated assessment of the same sites and substrates, carried out in 2023 (an average of 102 months post deployment), with significant differences in scleractinian recruit communities between survey periods, and again when compared with natural reefs and pinnacles. Based on a new dataset of 5442 coral recruits (clam shells and artificial reefs) and 2.46 km of reef and pinnacle transect data, we observed a significant increase in the heterogeneity of recruit communities on artificial reefs, with clear indications of convergence with mature reef communities. Meanwhile on natural substrates (reefs, pinnacles and shells), we observed proportionally little difference in communities between surveys (with some exceptions such as an increase in the proportion of Platygyra corals). Our findings support the assertion that small changes in depth may significantly influence the rate and direction of community change.