Macroalgal species richness and assemblage composition of the Great Barrier Reef seabed

Abstract

Understanding the drivers of broad-scale patterns of biodiversity is an overarching goal in ecology. We analysed environmental drivers of macroalgal species richness and composition on the continental shelf seabed of Australia's Great Barrier Reef (GBR), and mapped these patterns to show phycologically diverse and depauperate areas. Although shelf seabed habitats constitute similar to 61% of the GBR Marine Park area, previous floristic studies have been largely confined to intertidal and coral reef areas. Recognising the lack of knowledge of this habitat, the GBR Seabed Biodiversity Project (SBP) surveyed environmental variables and associated biodiversity across the shelf. We used SBP data for 1195 epibenthic sled sites, of which 639 sites recorded 370 macroalgal taxa, including 250 taxa not previously described in the GBR. Regression Random Forests were used to identify the environmental variables that most influence algal richness. Patterns of species composition, or assemblages, were investigated using partitioning around medoids (pam) clustering, and classification Random Forests identified the environmental variables most influential, and shapes of responses, for each assemblage. The 5 assemblages were distinguished based on taxonomy, dominant species, functional form or abundance and species richness. Overall, sediment grain size composition and light availability had the greatest influence on species richness and assemblages, with strong thresholds at 20% mud and at relative benthic irradiance of similar to 0.06 (equivalent to PAR approximate to 120 mu mol m(-2) s(-1)). This study is the first systematic analysis of the macroalgal communities of the GBR shelf seabed, providing valuable information to stimulate future research on taxonomy, productivity and ecosystem services of this habitat.