2.3.5 Coral reefs

Coral reefs are the most iconic of the Great Barrier Reef’s ecosystems and support the greatest diversity of marine species.151,152 The Region’s coral reefs have long provided sustenance, connection, and protection to First Nations peoples, and they provide vital ecosystem services to coastal communities, including productive fisheries, protection from damaging ocean swells and storm surges,153 and a vibrant tourism industry. They are central to the Reef’s outstanding universal value as a World Heritage Area.154 This section assesses the condition (and trend in condition) of both emergent and shallow submerged reefs, and deeper coral communities. Since 2019, important advances have been made to improve baseline understanding of the distribution of coral reef habitat globally 155 and across the Region (Boxes 2.2 and 2.4).

A variety of calcifying organisms contribute to reef growth (most notably hard corals themselves). Complex habitat structure is also supported by older reef framework, associated coral rubble and sediments, and the physical and ecological processes which help to redistribute carbonate material within the reef system. In this section a focus has been placed on the coral components of coral reef habitats, even though other calcifying species are also important for reef building (Section 3.4.8).

 

Box 2.2

New habitat maps for the Great Barrier Reef

Detailed maps of benthic composition, geomorphic zonation, and bathymetry at high spatial resolution for every individual reef establish an improved baseline for monitoring, mitigation, management, permitting, and policy development across the Region.

The three-dimensional Great Barrier Reef habitat mapping project led by the University of Queensland is creating maps using a combination of field data, ecological knowledge, satellite imagery and machine learning. Offshore reef maps have been publicly available 156 since 2021, and maps of the Region’s inshore reefs will be released in early 2025. The collective data may also be used to update the Reef Authority’s indicative reef outline dataset displayed on zoning plan maps.157

Current applications of these maps include improved planning for the Crown-of-thorns Starfish Control Program, tracing coral larvae and finding their sources and sinks, hydrodynamic modelling to predict the movement of water and key environmental conditions over the Reef, and selecting intervention sites at reef and intra-reef scales.

Three-dimensional bathymetric, geomorphic and benthic habitat maps of Unnamed Reef 21-110.
Three-dimensional bathymetric, geomorphic and benthic habitat maps of Unnamed Reef 21-110, southern Great Barrier Reef. © The University of Queensland 2023

Region overall

After a decade of cumulative impacts, the Reef experienced fewer major disturbances from 2018 to 2023 (prior to the 2023–24 summer). Only 3 cyclones tracked through the Region during this period (Section 3.2.2). Mass coral bleaching events occurred in 2020 and 2022. The 2022 event was the first recorded during a La Niña year. The 2020 event was the most widespread on record, with Reef-wide median temperature stress approaching levels associated with severe coral bleaching (Figure 8.3).158,159 Despite the broad extent of bleaching, estimated coral mortality was low in both 2020 and 2022.160,161,162 Observations of coral disease during this period 163 signal potential environmental stressors at affected reefs.164

A further two cyclones occurred early in summer 2023–24 (Jasper in December and Kirrily in January). Record global surface temperatures in 2023 and a transition to El Niño conditions 165,166 contributed to a widespread coral bleaching event in the Region in early 2024 (Box 2.3). At the end of summer 2023–24, the full extent and impacts of this latest coral bleaching event and the season’s cyclones were yet to be determined.167 These summer events are not considered in the assessment at Section 2.5.1. 

Natural recovery led by fast-growing corals drove rapid increases in cover

The period 2018 to December 2023, which was relatively free of disturbances that cause widespread coral mortality, saw a substantial increase in coral cover, despite the 2020 and 2022 bleaching events. Increases were highly variable between reefs. Natural recovery followed a typical trajectory 174 of fast-growing corals (especially Acropora species) driving rapid increases in coral cover.161,162 Despite their importance in leading ecosystem recovery, the susceptibility of these fast-growing corals to predation by crown-of thorns starfish, thermally induced bleaching, breakage and removal during cyclones, and outbreaks of disease, is well documented.175

Box 2.3

Another coral bleaching event affects the Great Barrier Reef

Heat stress accumulated in all 3 regions of the Reef over the 2023–24 summer.167 Sea surface temperatures peaked at 2.5 degrees Celsius above historical summer maximums in the southern region, leading to accumulation of the highest levels of heat stress in the satellite record for the Reef.168 

A mass coral bleaching event was confirmed in March 2024 following aerial surveys of over 1000 reefs.168,169 This is the third mass bleaching event since 2019 and the seventh recorded for the Reef. In 2024, extreme levels of bleaching (where greater than 90 per cent of corals on a reef are affected) were, for the first time, recorded within all 3 regions. 

The Reef Authority works with the Australian Institute of Marine Science, the Reef Joint Field Management Program, Crown-of-thorns Starfish Control Program, tourism operators, and researchers on gathering data from in-water surveys.170 These complement the aerial survey results and provide detail on the severity of coral colony responses, the prevalence of bleaching in different habitats and depths, and levels of coral mortality. 

At the end of summer, preliminary results indicated mortality had begun 167 in badly bleached areas. As it becomes available, further information on the severity of the 2024 mass bleaching event will be shared through the Reef Authority’s Reef Health webpage 171 and the Australian Institute of Marine Science’s Reef Monitoring webpage.172

The graph shows thermal stress accumulated on individual reefs during each of the 7 mass bleaching events on the Great Barrier Reef.
The graph shows thermal stress accumulated on individual reefs during each of the 7 mass bleaching events on the Great Barrier Reef. Thermal stress is measured in accumulated degree heating weeks (DHW), a measure of how long and by how much sea temperatures have exceeded a local temperature threshold. Source: Roff (2024)

Coral reefs within the shallow areas of the mid shelf and outer shelf have been systematically monitored for over 35 years.176,177 Inshore coral reefs have also been monitored for nearly 2 decades to complement offshore monitoring. While a variety of metrics are assessed and reported following each survey cruise,178 reef condition is primarily reported as changes in coral cover.161 Coral cover is commonly used to quantify the state of coral reefs, as it is a simple metric easily obtainable across large scales. Although a widely used and very useful metric, coral cover on its own is unable to provide a reliable indication of diversity, ecological function, or the state of ecological processes (Section 8.3.1).

The condition of coral reefs across the Region is highly variable, both on a regional scale and at the scale of individual reefs. Overall, condition is assessed to have improved since 2019 due to increases in total cover of key reef-building coral species across many reefs. Understanding condition more broadly is complex and it is unclear whether the recovery in hard coral cover has resulted in shifts in the composition of species within coral assemblages or included recovery of reef-associated species. Coral reefs remain vulnerable to the cumulative pressures brought by climate change, crown-of-thorns starfish outbreaks, and poor water quality in inshore areas – factors that amplify impacts and inhibit recovery from acute disturbances such as cyclones and floods.179 Little is known about the condition of deeper coral communities across the Reef due to the inaccessibility of these sites (Box 2.4), but they may provide critical refuge areas during periods of thermal stress.180,181

Measures of coral condition other than coral cover are poorly understood 

Trends in coral cover on offshore reefs are reported for 3 regions: the northern, central and southern Great Barrier Reef (Figure 2.5). These coral cover results and condition more broadly are explored below.

Figure 2.5
Trends in mean hard coral cover since 1986 for the northern, central and southern Reef

Mean hard coral cover (blue line) is shown for mid- and outer-shelf reefs. Blue shading represents
95 per cent credible intervals. Each graph shows trajectories of change over time (based on
manta tow surveys). Levels of coral cover are mostly driven by the cumulative impacts of the main
disturbances affecting surveyed reefs, including severe cyclones, outbreaks of crown-of-thorns
starfish and coral bleaching. Source: Australian Institute of Marine Science (2023)161

: A bar chart each for the three regions: Northern, Central, and Southern region, shows annual mean hard coral cover in percentage since 1986. These coral cover results and condition more broadly are explored in the main text.

Northern Great Barrier Reef — from Cape York to Cooktown | Coral cover: Reef-building coral cover in the northern region showed strong recovery after significant coral losses due to the cumulative effects of severe cyclones in 2014 and 2015, crown-of-thorns starfish outbreaks, and the 2016 mass bleaching event 163 (Figures 2.5 and 2.6). During summer 2022 hard coral cover reached record highs across the region; there was much variation between individual reefs.182 Although this region experienced two cyclones that made landfall since 2019 (cyclones Trevor and Tiffany), impacts to coral reefs were generally local. Crown-of-thorns starfish density has been generally low, but recent evidence reported a buildup in numbers around Cape Grenville and Lizard Island.183,184 The effects of exposure to moderate heat stress from the 2020 and 2022 bleaching events may have contributed to a flattening of recovery trajectories in 2023.162,163 

Broader condition: As elsewhere, the impacts of moderate temperature stress on coral reef biodiversity and measures of coral condition other than coral cover are poorly understood. The northern region includes many remote and poorly studied reefs. Little is known about the condition of its inshore, fringing or deeper reefs, and remote reefs have potentially been under-sampled.

From 2019 to 2022, coral cover on offshore reefs recovered strongly from low levels of coral cover, although a pause in that trajectory occurred in 2023. Trends are highly variable among reefs and the condition of most coral reef habitat components is unknown.

Figure 2.6
Changes to coral communities since 2018

Time‑series photographs showing recovery in coral cover at monitoring sites within the Region.
First row: Changes in coral cover at North Reef, Lizard Island in the northern region.
Left to right: images taken in 2018, 2021 and 2023 showing recovery of coral cover over this period.
Images © Coral Sea Foundation 2018, 2021 and 2023.
Second row: Coral recruitment in Manta Ray Bay within the central inshore region. In 2017, waves
generated by cyclone Debbie displaced a number of massive Porites corals (or bommies) in Manta
Ray Bay. Three months after impact, bommies were manually repositioned back to subtidal areas
of the reef flat. Since being moved back into place, natural recruitment of Acropora corals has been
observed on the bommies, although no noticeable increase in Porites coral tissue has occurred. Left
to right: images taken in December 2019, March 2022, and May 2023 showing an increase in the
number and size of Acropora recruits with each survey. Images © Maya Srinivasan 2019, 2022 and 2023.

Three photos in the top row show the same section of reef at North Reef, in 2018, 2021 and 2023. Over the course of the time series, the visible coral cover at that site has noticeably increased. The three photos in the second row show the coral recruitment in Manta Ray Bay. Left to right: images taken in December 2019, March 2022, and May 2023 showing an increase in the number and size of Acropora recruits with each survey.

Central Great Barrier Reef — from Cooktown to Proserpine | The central region includes areas where mid- and outer-shelf reefs are relatively close to key population centres that provide access for research and visitation, as well as popular inshore fringing reefs, such as those in the Whitsunday islands. As a result, the central region has the best studied coral reefs at a region-wide scale. The region is also influenced by freshwater inflow from the Wet Tropics NRM region catchments, which receive average rainfall of more than 1500 millimetres per year. Reefs (inshore and mid‑shelf) in the central region are affected by cumulative impacts, including coral bleaching, nutrients and sediments from land-based runoff, and pressure from fishing activities. Some inshore reefs are known to support a high diversity of coral species but they tend to have low coral cover 185 compared to their offshore counterparts. The outer continental shelf slopes more gently in this region than it does further north and is dominated by submerged reefs along the shelf edge 186 (Box 2.4). 

Coral cover: The central region has seen large and rapid fluctuations in coral cover over the past decade. Record low coral cover in 2012 (11 per cent), primarily due to the impacts of cyclone Yasi, was followed by a period of recovery that peaked in 2016 (29 percent).160 This peak was followed by significant mortality resulting from mass coral bleaching in 2016 and 2017 and crown-of-thorns starfish outbreaks on many reefs. However, since 2019, the abatement of crown-of-thorns starfish outbreaks has been hastened by active culling of substantial numbers of starfish in the region.187 Between 2019 and 2022, the period of low cyclone activity, low mortality following the 2020 and 2022 coral bleaching events and improved crown-of-thorns starfish control enabled coral cover to increase from 12 to 33 per cent (Figure 2.5), and it has remained stable since.161 This recovery has been observed mainly on upper reef slopes and there is very limited information on the condition of deeper areas. In the Whitsundays, coral cover has gradually increased since the impacts of cyclone Debbie in 2017 on inshore reefs 185 (Figure 2.6).

Recovery since cyclone Debbie has been slow at some inshore reef sites

Early reports suggest cyclone Jasper in December 2023 led to minor and isolated bleaching on a few reefs, likely as a result of freshwater plumes, and minor damage to some exposed reef aspects.188 Following cyclone Kirrily in January 2024, preliminary observations indicated some impacts to branching and plate corals in a limited number of inshore and mid-shelf reefs off Townsville.189 

Broader condition: Improvements in inshore coral condition, measured through the coral index, a composite of 5 indicators (coral cover, proportion of macroalgae, juvenile coral density, rate of coral cover change and hard coral community composition), have been slow since cyclone Debbie, including for the Whitsundays reefs.185 Recovery has been hindered primarily due to the persistence of macroalgal cover in some inshore reef sites.185

Since 2019, recovery of coral cover has been recorded in the central region; full community-level recovery may take much longer. Condition of individual reefs is highly variable, and recovery of some inshore reefs has been limited.

Southern Great Barrier Reef — from Proserpine to Baffle Creek | The southern region encompasses a huge area with a diversity of very different reef types. This region includes inshore reefs vital to tourism and has a greater coastal population with more industry than further north. It also includes many remote offshore reefs that are rarely visited and poorly studied. The continental shelf widens in this region, extending to the outer Pompey Reefs and the Swain Reefs.

Coral cover: The southern region has experienced the largest fluctuations in coral cover since systematic monitoring began.161,185 Cyclone Hamish in 2009 decreased coral cover to a record low in 2011 in offshore reefs. A period of recovery brought coral cover to 37 per cent in 2017, but this was reduced again in 2018 by crown-of-thorns starfish outbreaks. Average coral cover in 2022 and 2023 was around 34 per cent (Figure 2.5).161,178 The southern region was not as severely affected by the 2016 and 2017 mass bleaching events as the central and northern regions. Despite widespread bleaching of corals in 2020 and 2022, subsequent coral mortality on surveyed reefs was minimal. Since 2019, coral cover has been more variable between individual offshore reefs in the southern region than in the northern and central regions 161 (Figure 2.5). More reefs were recorded with high coral cover than with low or moderate. Outbreaks of crown-of-thorns starfish were ongoing in the Swain Reefs in early 2024,167 but culling on these reefs has likely mitigated some of the coral losses. In the inshore reefs of the Keppel Islands group, coral cover has steadily increased since 2014, but variability exists between reefs.190

Crown-of-thorns starfish control has mitigated some coral losses

Broader condition: In the inshore region, the condition of coral reefs as measured by the coral index has been stable since 2019. However, macroalgal cover has steadily increased since 2020, which is hindering recovery of coral condition in some areas.190 In mid-shelf and offshore reefs, macroalgal cover, after a decline between 2020 and 2022, macroalgae increased in 2023, though cover is still one-tenth that of inshore reefs.178 

Underwater image of a colourful coral reef with schools of small fishes.
Teeming fish on a coral reef. © Matt Curnock 2021

Overall, coral cover improved, or at least stabilised, since 2019 at most reefs in the southern Great Barrier Reef. Coral cover is highly variable among reefs. In the inshore area, coral condition has stabilised since 2019, but remained poor for most reefs. Persistence of high macroalgal cover above local thresholds is a likely driver influencing overall coral condition in the inshore region. Crown-of-thorns starfish continue to affect reefs but are generally below outbreak levels, except in the Swain Reefs.

Box 2.4

Deep coral communities

Shallow-water coral reefs, which are dominated by photosynthetic organisms, generally require high light conditions for the growth and calcification of reef‑building corals and algae (Section 2.3.5). These reefs accumulate calcium carbonate faster than it is removed.191 However, hard and soft corals also form abundant communities on deeper banks, which are fully submerged and not growing towards the surface. These banks are the remnants of earlier periods of reef growth and may be topped by drowned ‘give-up’ reefs that could not keep pace with rapidly rising sea levels following the most recent ice age.192 Formed when sea levels were much lower, some shelf-edge banks and drowned reefs are found below 100 metres depth.193 Numerous small drowned reefs have been mapped in the northern Reef since 2019.194 

In combination with other physical processes, such as water movement, sedimentation, and temperature fluctuations, assemblages of deeper coral communities vary markedly with depth along gradients of light availability.195 In deeper environments, such as the continental slope, communities are dominated by corals that can survive without light.196 These depth-related changes in benthic community composition are associated with corresponding changes in the structure and function of fish and other faunal assemblages.197,198 Fish assemblages in deeper habitats are dominated by mobile predatory species while shallower habitats support fish with a wide range of dietary preferences.195

Technological advances over the past two decades, including robotics and advanced diving technology, have seen an exponential increase in research interest on deeper coral communities.198,199 Nevertheless, despite active research efforts and the importance of these habitats as potential refugia,180 the logistical challenges of studying and monitoring deep-water habitats mean that knowledge of the ecological condition of the Region’s deeper coral communities and how they have changed over time remains lacking.198,200

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