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Species interactions can influence key ecological processes that support community assembly and composition. For example, coralline algae encompass extensive diversity and may play a major role in regime shifts from kelp forests to urchin‐dominated barrens through their role in inducing invertebrate larval metamorphosis and influencing kelp spore settlement. In a series of laboratory experiments, we tested the hypothesis that different coralline communities facilitate the maintenance of either ecosystem state by either promoting or inhibiting early recruitment of kelps or urchins. Coralline algae significantly increased red urchin metamorphosis compared with a control, while they had varying effects on kelp settlement. Urchin metamorphosis and density of juvenile canopy kelps did not differ significantly across coralline species abundant in both kelp forests and urchin barrens, suggesting that recruitment of urchin and canopy kelps does not depend on specific corallines. Non‐calcified fleshy red algal crusts promoted the highest mean urchin metamorphosis percentage and showed some of the lowest canopy kelp settlement. In contrast, settlement of one subcanopy kelp species was reduced on crustose corallines, but elevated on articulated corallines, suggesting that articulated corallines, typically absent in urchin barrens, may need to recover before this subcanopy kelp could return. Coralline species differed in surface bacterial microbiome composition; however, urchin metamorphosis was not significantly different when microbiomes were removed with antibiotics. Our results clarify the role played by coralline algal species in kelp forest community assembly and could have important implications for kelp forest recovery. [ABSTRACT FROM AUTHOR]

Coralline algae are ecologically important macroalgae providing settlement cues and habitat for a number of marine organisms. They are abundant in a wide range of habitats in tropical to polar systems and are at severe risk from a number of local and global anthropogenic stressors. Despite their ecological importance, there are large gaps in understanding coralline algae diversity patterns and abiotic drivers of this diversity. Furthermore, recent advances in molecular identifications have revealed larger diversity than previously estimated through morphological approaches. The aim of this study was to explore coralline algal community composition in a modern context using DNA-based species identifications and to examine broad-scale environmental variables driving community structure around the New Zealand coastline. Hierarchical cluster techniques identified seven distinct coralline algal communities in the region. Sea surface temperature (SST) and light at seabed explained significant variation in coralline algal community composition, with SST explaining the highest amount of variation. Both SST and light are important for the growth, calcification and survival of coralline algae, and responses to these environmental parameters can vary widely among species. Wave exposure indices and turbidity were not found to have a large influence in shaping coralline community structure, most likely due to the ability of coralline algae to tolerate mechanical stress and periods of intense sand scouring and burial. Understanding the drivers structuring diversity, particularly for ecologically important primary producers, provides insights into community and ecosystem functioning and contributes to our baseline understanding of current species distribution. [ABSTRACT FROM AUTHOR]