2. Biodiversity

The condition of islands varies considerably across the Region and between island types. Some islands have experienced localised damage from severe weather and the accumulation of plastic pollution. Improvements have occurred on others due, for example, to pest eradication. Monitoring of island condition is increasing, and work is underway to assess the response of coral cays to sea-level rise.

Non-rocky coastlines remain dynamic. Observed changes over recent years are likely to predominantly reflect natural variation in climatic and oceanic influences, but sea-level rise is expected to increase rates of erosion, recession and inundation in coming decades.

Little change has occurred to the overall extent of mangrove forests. Gains and losses are occurring due to natural and anthropogenic processes. Evidence regarding the Reef-wide distribution and individual causes of these changes is currently limited. Acute cyclone impacts were relatively low from 2019 to 2023, but the impacts of successive cyclones during the 2023–24 wet season have not yet been assessed. Mangrove forests have expanded on some islands in the northern Great Barrier Reef.

Seagrasses are affected by increasing temperatures and localised poor water quality. Recent shifts away from colonising species in northern regions are indicative of post-disturbance recovery. Due to impacts from flooding, seagrasses in the southern Fitzroy and Burnett Mary regions have not shown the same recovery. Information on condition of deep-water seagrass meadows is limited. Acknowledging regional variation, overall condition across the Region is assessed as good (borderline poor). In some parts of the Region, summer 2023–24 brought multiple stress events, the effects of which are not yet known.

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. The culling of crown-of-thorns starfish has facilitated recovery at many locations. 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. Little is known about the condition of deeper coral communities across the Reef due to the inaccessibility of these areas, but they may provide critical refuges during periods of thermal stress. (The full extent and impact of the 2023–24 summer cyclones and mass bleaching event is not known at time of writing and is not considered in this assessment).

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.

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.  

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.  

The lagoon floor encompasses a wide diversity of habitats and species with varying sensitivity and exposure to anthropogenic impacts. Although not systematically monitored, much of the lagoon floor is exposed to increasing sea temperatures, with potential impacts to heat-sensitive species. Some areas also continue to be affected by fishing, anchoring and changes in water quality. Trend in condition has been assessed as stable since 2019 but the threats posed by climate change have not abated.

The extent and condition of other banks and shoals is not well documented due to their unique locations and often inaccessible nature. Although not systematically monitored, condition is inferred to be good and trend stable based on a relative lack of intense disturbances (for example, from cyclones) since 2019 and unchanged anthropogenic pressures.

Knowledge of the extent and structure of the northern Halimeda bioherms has improved since 2019, alongside advances in understanding their functions and biodiversity. However, information on the condition (and trend in condition) of this habitat remains lacking. Since 2019, impacts from acute disturbances are inferred to be negligible due to the depth and isolation of these habitats.

While knowledge of the detailed topography of the continental slope is advancing, and isolated biological surveys have occurred since 2019, confidence in the condition (and trend in condition) of this habitat remains low. Much of this habitat is thought to remain relatively undisturbed.

Important knowledge gaps remain regarding the condition (and trend in condition) of the water column. Increasing temperatures and changing ocean chemistry are directly affecting this habitat. Understanding of complex climate change impacts on diverse water column assemblages is improving but remains limited across the Region. Excess nutrients and fine sediments from land-based run-off also affect water column condition in many inshore areas.