No silver bullet for the Great Barrier Reef

But new research reveals that even if water quality is improved, it won’t be enough on its own to save the Great Barrier Reef.

Because of the flooding, rivers have dumped millions of litres of polluted water onto the Reef, but until now the impact of these events on reef corals and marine life has been difficult to assess.

Using a combination of advanced satellite imaging and over 20 years of coral monitoring across the Reef, a team of researchers from ARC Centre of Excellence for Coral Reef Studies at James Cook University (Coral CoE),  Dalhousie University, and the University of Adelaide has found that chronic exposure to poor water quality is limiting the recovery rates of corals across wide swathes of the GBR.

“We found the Great Barrier Reef is an ecosystem dominated by runoff pollution, which has greatly reduced the resilience of corals to multiple disturbances, particularly among inshore areas,” said lead author Dr. Aaron MacNeil of Dalhousie University.

“These effects far outweigh other chronic disturbances, such as fishing, and exacerbate the damage done by crown-of-thorns starfish and coral disease. Perhaps most critically, poor water quality reduced the rates at which coral cover recovers after disturbances by up to 25 percent. This shows that, by improving water quality, the rates of reef recovery can be enhanced.”

“Our results provide strong support for government policies aimed at reducing nutrient pollution to help increase the resilience of the Great Barrier Reef, in recovering from damage due to tropical cyclones, crown-of-thorns outbreaks and coral bleaching,” said co-author PhD candidate Sam Matthews of Coral CoE.

Yet the effects of water quality only go so far. Using a series of scenarios modelling future changes in climate and the likelihood of coral bleaching, the team found that no level of water quality improvement was able to maintain current levels of coral cover among the most scenic and valuable outer-shelf reefs that sustain much of the reef tourism industry.

Dr. Camille Mellin of the University of Adelaide noted that: “Coral reefs, including the Great Barrier Reef, are subject to an increasing frequency of major coral die-off events associated with climate change driven by coral bleaching. With these increasingly common disturbances becoming the new normal, the rate of coral recovery between disturbances has become incredibly important.”

“While the effects of improved water quality on recovery rates of inshore reefs were encouraging, our analysis puts into perspective the limits of how much reducing pollution in river runoff can do to improve the state of the outer Great Barrier Reef.

“No level of water quality improvement will be sufficient to ensure maintenance of the clear water reefs on the outer shelf, the very reefs that tourists come to Australia to see.”

“What these results emphasise is that there is no silver bullet for addressing the threats facing the Great Barrier Reef,” said Dr MacNeil.

“Clearly reducing pollution in river runoff can have widespread, beneficial effects on reef corals and should continue to be supported. But for areas of the reef not impacted by water quality, our emphasis must be on mitigating carbon emissions to slow down climate change.

“We must give our reefs the time and conditions to recover. Without that, the most stunning and iconic parts of the reef will soon decline and be unrecognisable from their current form.”

Paper: MacNeil M, Mellin C, Matthews S, Wolff NH, McClanahan TR, Devlin M, Drovandi C, Mengersen K, & Graham NAJ. (2019) Water quality mediates resilience Great Barrier Reef. Nature Ecology & Evolution.

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