The Great Barrier Reef is one of the world’s most beautiful natural wonders, and is recognised as such by UNESCO’s World Heritage Committee. Containing more than 2,500 individual reefs spread over an area larger than Victoria, its natural values add $5.2B annually to the Australian economy in tourism.
But the long-term persistence of the Reef is under threat. Coral cover on the Reef has declined by half since 1985 and has probably been in steep decline since the 1950’s. Climate change and ocean acidification have only played minor roles thus far.
Right now, the Reef faces more immediate dangers; chief among them is poor water quality.
Poor water quality is strongly linked to human activities, such as agriculture and coastal development. Declines in coral cover have been greatest near the coast in the southern and central parts of the Reef, where there are more people and more development. Indeed, the average coral cover for the Reef is now just 14% compared to a coral cover often in excess of 70% for the more pristine reefs in the north.
Suspended sediments are a leading reason for poor water quality. Corals are heavily dependent on symbiotic algae for their nutrition. By reducing light levels in the water column, the sediment inhibits the ability of the algae to photosynthesise and can thereby severely limit the energy available to the coral.
In turn, this makes the coral less resilient and slower to recover from disturbances such as tropical cyclones and outbreaks of the crown of thorns starfish.
The crown of thorns starfish continues to be a significant threat to the Reef. Although the starfish have probably always been residents of the Reef, devastating outbreaks are increasingly common. And this has also been linked to water quality issues.
A single starfish can produce tens of millions of larvae, with only a tiny fraction of these surviving to become coral-eating starfish. However, just a small change in the survival rate of their larvae can lead to orders of magnitude of more starfish arriving on reefs.
Starvation is a prominent source of mortality for the crown of thorns larvae. By allowing nutrient-rich sediment to run-off from the land, we fuel blooms of the microscopic algae that provide food for the starfish larvae, increasing their chances of survival.
Although there has been some slow progress towards modifying agricultural practices to improve water quality on the Great Barrier Reef, another practice that produces a large amount of sediment continues unchecked.
A number of ports line the coast neighbouring the Reef. Many are being expanded. Port expansions require massive dredging operations releasing more or less sediment depending on the location that the dredge spoil is dumped.
The recently approved Abbot Point port expansion will make it one of the largest coal export ports in the world. Three million cubic meters of sediment, about 5.4 million tonnes, will be dredged and dumped within the Great Barrier Reef Marine Park over 5 years. In comparison, agricultural run-off is estimated to be 6 million tonnes of sediment annually for the whole of Queensland. And there are bigger port dredging projects on the table.
The World Heritage Committee has growing concerns about the health of the Great Barrier Reef World Heritage Area. When its members meet in June this year, many believe they will downgrade the status of the Reef to ‘World Heritage In Danger’. It would be an international embarrassment for Australia and is likely to have significant impacts on the tourist industry.
To keep the Great Barrier Reef on the World Heritage list and increase its resilience to other impacts, the federal and Queensland governments need to do more to address water quality on the Reef.