The Great Barrier Reef is losing its fight against climate change thanks to water pollution caused by man-made and natural phenomena hampering its ability to regenerate.
In the most rigorous review of its kind, researchers have for the first time linked the effects of land-based activities on the reef's water quality to the impacts of climate change.
Human-induced climate change remains the number one threat to the reef but water pollution from fertilisers and urban and coastal developments is hampering its ability to recover, the review found.
The latest Scientific Consensus Statement involved more than 200 experts and evidence from over 4000 publications.
Land management activities and impacts, including vegetation degradation, erosion, increased sediment, nutrient and pesticide runoff, and urban and coastal developments have been found to damage the reef's water quality.
Sediment pollution in the Great Barrier Reef was up to five times worse than pre-development times, while dissolved nitrogen levels from agriculture were at least 1.5 times higher.
Elevated levels of sediment from flood plumes and especially land grazing and clearing reduced the amount of light in the reef ecosystems, slowing the rate at which seagrass meadows and coral reefs regenerated.
The highest concentrations of nitrogen and phosphorus nutrients in the Great Barrier Reef were found in inshore areas, leading to an increase in phytoplankton blooms, the food supply for coral predators, the crown-of-thorns starfish larvae.
In the five years since the 2017 consensus statement, inshore coral reefs declined while coral cover on shallow mid and outer shelf reefs had shown clear recovery, linking water pollution to coral recovery.
Sugar cane farming remained the main source of pesticide pollution, which increased the vulnerability of local aquatic species to other events including heatwaves.
Cost-effective land management practices and remedies that improve water quality are available but scientists say authorities must target hotspots and boost the take up, through entire community consultation, to harness their full potential.
Good water quality was essential to helping the Great Barrier Reef recover from the impacts of climate change, Australian Institute of Marine Science research scientist Katharina Fabricius said.
"This is particularly important in areas close to the coast, which are more susceptible to runoff and not recovering as well as reefs further offshore," Dr Fabricius said.
The review confirmed dissolved inorganic nutrients had wide-ranging effects on all parts of the reef's ecosystems, Griffith University professor Guillermo Diaz-Pulido said.
But the impacts differed based on location and while excess nutrients affected corals, including lowering their tolerance to bleaching, the most severe consequences may be indirect.
Seaweed blooms are generally correlated with elevated nutrients in the water and can outcompete corals and reduce coral cover.
The reef had suffered its worst summer on record following cyclones, severe floods, crown-of-thorns starfish outbreaks and its fifth mass bleaching event in the past eight years, the Australian Marine Conservation Society said.
All levels of government were urged to do more on water pollution and end broadscale clearing of reef catchments.
"Managing water quality, along with carbon emissions reductions, and research to improve the recovery and resilience of corals, will help the Great Barrier Reef survive climate change," Dr Fabricius said.
