Coral reef trajectories: Altered disturbance regimes correspond with divergence toward 'flattened' and...

Coral reefs rely on minimum thresholds of live hard coral cover to sustain key ecological and geomorphic functions, including habitat provisioning, carbonate production, and wave attenuation. While declining hard coral cover is widely recognised as a driver of reef degradation, the boundaries at which reefs transition between distinct functional states remain poorly defined-particularly across reefs with contrasting baseline conditions, where absolute cover values are not directly comparable. To address this gap, we quantified long-term changes in benthic composition from 1995 to 2022 across three eco-geomorphologically distinct reefs spanning the northern, central, and southern Great Barrier Reef. Hard coral cover was indexed relative to each reef's historical maximum, allowing reef state transitions to be parameterised using comparable thresholds. Three reef states were identified, driven predominantly by shifts in coral cover and corresponding changes in algal dominance, with secondary reef-specific reorganisation in coral morphological composition shaped by baseline community structure. 'Healthy' states were characterised by high structural complexity; 'coping' states by overall hard coral cover decline, with reef-specific shifts in dominant coral morphology; and 'struggling' states by near-complete coral loss and algal dominance. Indexed hard coral cover provides a practical benchmark-with transitions into 'coping' states occurring below ∼65% of historical maxima, and further declines below ∼35% associated with 'struggling' states. Regardless of baseline morphological composition, disturbance-driven transitions toward flatter, algal-dominated reef profiles represent a convergent trajectory of functional decline-one that is likely to accelerate as disturbance regimes intensify under climate change.