The role of environmental variables in shaping temperate coralline algae communities across large spatial scales

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.