Changes in coral reef health and status are commonly reported using hard coral cover, however such changes may also lead to substantial shifts in coral community composition. Here we assess the extent to which coral communities departed from their pre-disturbance composition following disturbance (disassembly), and reassembled during recovery (reassembly) along an environmental gradient across the continental shelf on Australia’s Great Barrier Reef. We show that for similar differences in coral cover, both disassembly and reassembly were greater on inshore reefs than mid- or outer-shelf reefs. This pattern was mostly explained by spatial variation in the pre-disturbance community composition, of which 28% was associated with chronic stressors related to water quality (e.g., light attenuation, concentrations of suspended sediments and chlorophyll). Tropical cyclones exacerbated the magnitude of community disassembly, but did not vary significantly among shelf positions. On the outer shelf, the main indicator taxa (tabulate Acropora) were mostly responsible for community dissimilarity, whereas contribution to dissimilarity was distributed across many taxa on the inner shelf. Our results highlight that community dynamics are not well captured by aggregated indices such as coral cover alone, and that the response of ecological communities to disturbance depends on their composition and exposure to chronic stressors.

Coral reefs are under increasing pressure from a variety of stressors, highlighting the need for information about the status of coral reef communities including the distribution, abundance and composition of juvenile and adult coral assemblages. This information is currently limited for the Great Barrier Reef (GBR) and is necessary for understanding the impacts of disturbances and the system’s potential for recovery. This study reports juvenile and adult hard coral abundance and composition from 122 reefs on the GBR during a period of limited acute disturbance. The data represent baseline observations for juvenile hard coral assemblages spanning the longitudinal cross-shelf gradient of the GBR and 12 degrees of latitude and augment reported distribution of adult coral assemblages over the same scale with inclusion of additional reefs. Juvenile and adult coral assemblages reflected broad differences imposed by the gradient of environmental conditions across the GBR. The mean density of juvenile hard corals was lower in the inshore reefs (5.51 m²) than at either the mid-shelf (11.8 m²) or outer shelf reefs (11.2 m²). The composition of juvenile and adult coral assemblages covaried overall, although there were different relationships between these two life stages across the continental shelf and among community types. Dissimilarity between juvenile and adult coral assemblages was greater on inshore and outer shelf reefs than on reefs in the mid-shelf, although, there were differences in community types both within these shelf positions and those that spanned mid- and outer shelf reefs. Dissimilarity was greatest for Inshore branching Acropora and high for Southern Acropora communities, although very high coral cover and very low juvenile densities at these reefs precluded interpretation beyond the clear competitive dominance of Acropora on those reefs. Dissimilarity was also high between juvenile and adult coral assemblages of Turbid inshore communities suggesting water quality pressures, along with synergistic effects of other stressors, pose ongoing selective pressures beyond the juvenile stage. Conversely, relatively low dissimilarity between juvenile and adult coral assemblages on mid-shelf and lower latitude outer shelf reefs suggests pressures beyond those influencing settlement and early post-settlement survival were having less influence on the composition of adult coral assemblages.

Anthropogenic climate change is fuelling repeated marine heatwaves and has triggered the fourth global mass coral bleaching event in 2023–2025, causing widespread coral mortality worldwide. Here, we document the impacts of this event on the Great Barrier Reef (GBR), where heat accumulation peaked in February–March 2024 and caused mass coral bleaching for the seventh time since 1998. We use data from the Australian Institute of Marine Science (AIMS)’s Long-Term Monitoring Program (LTMP), which during the 2024–2025 season recorded coral cover on 124 reefs distributed across the full latitudinal and longitudinal extent of the GBR. Coral cover declined to 30.0% (26.6–33.8% Credible Intervals [CIs]; a 24.8% relative decline since 2024) in the Northern GBR, to 28.6% (25.7–31.6% CIs; a 14% relative decline) in the Central GBR and to 26.9% (23.1–30.9% CIs; a 30.6% relative decline) in the Southern GBR. For the Northern and Southern GBR, this was the largest single-year decline in hard coral cover since monitoring began in 1986. Acropora corals contributed between 40 and 80% of the increases and 48–70% of the decreases in hard coral cover over the 39 monitoring years. Despite the pivotal role Acropora play in accelerating post-disturbance recovery through their rapid growth, their vulnerability to disturbance also contributes to an increasingly volatile ecosystem, where hard coral cover oscillates between record lows and record highs. Climate change is already fuelling increasingly frequent and intense heatwaves, and conditions for reef recovery will continue to deteriorate without urgent reductions of greenhouse gas emissions.