Macroalgal removal increases calcifier abundance and promotes coral settlement on inshore reefs

Abstract The indirect effects of macroalgal biomass on coral settlement remain poorly understood, particularly on degraded inshore reefs characterised by high algal cover. We investigated how macroalgal removal influences benthic community composition and subsequent coral settlement and persistence by deploying 240 terracotta settlement tiles across experimental plots at two inshore reefs (Yunbenun, Great Barrier Reef, Australia). On each reef, six plots underwent regular macroalgal removal while six served as untreated controls. Tiles were retrieved 3, 6, and 13 months after deployment, photographed to quantify benthic composition, analysed for coral abundance, and returned to their original location to maintain consistent microhabitat conditions. Macroalgal removal significantly altered early successional trajectories on bottom tile surfaces, increasing cover of crustose coralline algae (CCA) and bryozoans. These calcifier-rich assemblages hosted the highest abundance of newly settled corals. Generalised linear mixed models identified CCA cover as a significant positive predictor 1 month after spawning, whereas bare tile negatively affected coral persistence 4 months after spawning. In contrast, top tile surfaces were dominated by turf algae and had low coral settlement regardless of treatment, underscoring the importance of orientation, light, and grazing in structuring microhabitat outcomes. Despite enhanced initial settlement in removal plots, coral abundance declined sharply over 13 months across all treatments. Our findings indicate that macroalgal biomass can indirectly suppress coral settlement by limiting calcifier development, and that managing macroalgae may create transient “settlement windows” that enhance coral settlement opportunities.