3. Ecosystem health

The condition of the Region depends on a range of chemical, physical and ecological processes, the health of connected coastal ecosystems, and impacts of disease and pest outbreaks. Thirty-one ecosystem health components are assessed across those 5 areas, and around 60 per cent remain in good to very good condition; the rest are in poor to very poor condition. Five of the 31 components have deteriorated (but not changed grade) since 2019, including four components across physical processes (sea temperature and sea level), chemical processes (ocean pH) and ecological processes (reef building). These continued declines in ecosystem condition are largely driven by the ongoing chronic effects of climate change, in particular ocean warming. 

Sea temperatures have continued to increase over the past 5 years, and this component remains very poor. Record high sea temperatures were recorded in February 2020, and marine heatwaves triggered mass coral bleaching (although not significant mortality) in both 2020 and 2022. Notably, the 2022 event was the first mass coral bleaching event to occur during a La Niña year. Since 2019, the La Niña climatic phase has dominated. This phase is usually associated with cooler weather in the Region so it may have moderated the effect of extreme temperatures. Sea temperature and other physical and chemical processes, such as sea level and ocean pH, will continue to deteriorate as the effects of climate change accelerate.

Sea level is assessed as good but deteriorated. Other physical processes, such as currents, cyclones and wind, sediment exposure, freshwater inflow and light have remained stable since 2019. In the longer term, alterations in many of these processes are projected due to climate change and land-based runoff, and this will have broad implications for the Region.

With the exception of ocean pH, which is assessed as good but deteriorated as a result of increasing atmospheric carbon dioxide concentrations, chemical processes have remained relatively stable since 2019. Ocean salinity is unchanged, while nutrient cycling, which continues to be affected by land-based runoff, remains in a poor but stable condition. Ecological processes have generally remained stable since 2019, although reef building continues to deteriorate due to ocean acidification. Trends in other processes, including particle feeding, recruitment and predation, vary between different taxa. Declines in recruitment have been reported in some populations of marine turtles and dugongs, for example. The condition (and trend in condition) of some ecological processes, such as microbial processes and competition, remain poorly understood. 

Coastal ecosystems that support the Reef remain in poor condition overall, though trends in most components have stabilised. Woodlands and forests is the only coastal ecosystem type that continues to deteriorate, which is caused by the ongoing impacts from land clearing (although annual rates have reduced in recent years) and forest fires. Continued modification of these ecosystems will increase sediment inflow, reduce connectivity to the Reef, and reduce the capacity of these habitats to support the Region’s ecosystems and species.

Outbreaks of crown-of-thorns starfish is graded as poor compared to the rating of very poor in 2019. While persistent crown-of-thorns outbreaks at some southern reefs and a new outbreak in the northern region continue to alter ecosystem function on affected reefs, increased efforts in starfish control since the last report have led to both reef- and regional-scale improvements. Outbreaks of disease and occurrences of introduced species are localised and patchy across the Region and considered stable since 2019. Programs to eradicate introduced pests have been successful on some islands. A long‑term increase in Trichodesmium abundance has occurred, but data on a Region‑wide scale are limited.