MEPS 717:17-35 (2023)  -  DOI: https://doi.org/10.3354/meps14380

ABSTRACT: High macroalgae cover on degraded coral reefs may reduce settlement of coral larvae via both direct and indirect mechanisms, limiting coral recovery potential following disturbances. We tested the allelopathic effect of a Lobophora sp., a common fast-growing brown macroalga, on the settlement of 9 coral species from 5 taxonomically diverse genera (Acropora, Coelastrea, Goniastrea, Montipora and Platygyra) common on inshore reefs of the Great Barrier Reef. Larval settlement of 2 species, Montipora aequituberculata and Platygyra daedalea, was reduced at the highest concentration of Lobophora-conditioned seawater (equivalent to ~15 g Lobophora per 100 cm²). Lobophora surface extracts were mildly toxic and elicited varied responses, increasing settlement in Coelastrea aspera, Acropora hyacinthus and M. aequituberculata and decreasing settlement in A. millepora, A. elseyi and A. intermedia, but overall had minimal effects. Lobophora non-polar extracts inhibited larval settlement in 7 species at high concentrations, whereas lower concentrations increased settlement for Goniastrea retiformis and C. aspera. Polar chemical extracts of Lobophora thalli reduced larval settlement in all 9 species and proved lethal at high concentrations for 7 species. The amplified effects of extracts compared with conditioned seawater suggest that experimental studies of extracts may overestimate allelopathic effects on coral settlement. This study adds to the growing evidence that Lobophora can reduce coral settlement via chemical allelopathy in laboratory experiments. Further efforts are required to confirm the transferability of these results to the field. Our results highlight coral inter-specific variation in Lobophora sensitivity, which may contribute to future changes in coral community composition.

KEY WORDS: Macroalgae · Recruitment · Coral reef · Recovery · Resilience