Taxonomic Variation in Coral Depletion at Orpheus Island (Inshore Great Barrier Reef, Australia) Linked to Unprecedented Rainfall and Hyposalinity

Acute disturbances caused by changing environmental conditions are increasingly affecting the structure and function of coral reef ecosystems. Notably, changing rainfall patterns are leading to increasing incidence of hyposalinity. This study explored interannual changes in the overall cover and composition of hard corals (order Scleractinia) in Pioneer Bay, Orpheus Island, which was subject to hyposalinity during unprecedented high rainfall in February 2025. Hard coral cover declined 66.60%, from 41.66% (±1.22 SE) in September 2024 to 13.92% (±0.92 SE) in October 2025, with coral loss mostly apparent on the reef flat and reef crest. Coral loss was not equally apportioned among different coral taxa (genera), possibly reflecting differential susceptibility to hyposalinity based on specific physiology and habitat associations. The most pronounced declines were recorded among Merulinidae, which were the predominant hard corals on the reef flat in 2024. Soft corals were similarly affected and exhibited major declines in abundance on the reef flat. Hyposalinity is rarely considered among the suite of climatic disturbances that impact on coral assemblages and reef ecosystems, but this research shows that the potential ecological effects are very extensive, adding to the diversity of acute disturbances that will influence the structure coral reef ecosystems in the Anthropocene.