Fresh water alone can kill coral, but the cloudy and polluted river water could also smother deepwater sections of the Great Barrier Reef system. Credit: Photolibrary

The floods that have devastated swathes of southern Queensland are beginning to perturb one of the world's largest World Heritage Sites, the Great Barrier Reef, scientists in Australia say.

Southern parts of the reef, which extends more than 2,000 kilometres along the Queensland coast in northeastern Australia, are already being affected by the huge plume of polluted water that is gushing from many Queensland rivers. The full impact of the floods will take several years to play out, and they could eventually affect the entire reef system.

"The amount of water entering the reef, with rivers along southern Queensland all in flood — we haven't seen that before," says Michelle Devlin, a water-quality researcher from James Cook University in Townsville, Queensland, who is leading efforts to monitor the flood plume. Outflows from two catchments alone, the Burnett Mary and the Fitzroy River regions of central Queensland, already cover 11% of the ocean surface of the Great Barrier Reef, Devlin and her colleagues have found. And corals there are at risk of the direct, acute effects of the polluted floodwater.

The immediate impact of the floodwater is simply vast amounts of fresh water running into the sea. Fresh water kills coral, so shallow inshore reefs in the path of the plume are threatened. As the pulse of cloudy, nutrient-rich, pesticide-polluted water spreads, it also smothers coral in deeper waters, blocking out light and limiting photosynthesis while boosting the growth of coral competitors such as macroalgae.

Expanding plume

Devlin is now mapping the spread of the floodwater through the southern Great Barrier Reef. Satellite images show that the prevailing southeasterly winds have so far confined the plume to within about 65 kilometres of the shore. These winds are forecast to drop later this week, which could see the plume spread much farther out into the reef.

The plume could continue to spread for several weeks. "On land, the floods are beginning to recede — but in the marine environment the impact will continue to worsen," says Britta Schaffelke, who researches water quality and coral health at the Australian Institute of Marine Science in Townsville. More flooding remains a possibility. "We're still at the start of the rainy season," Schaffelke says. "February is usually the wettest month."

Satellite image of floodwaters from the Fitzroy River, central Queensland, shows a substantial plume in the reef on 11 January (click for larger image). Credit: Pia Harkness and Michelle Devlin, James Cook University

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Even after the flood plume disperses, the stressed corals will remain vulnerable. "After flooding we see an increased incidence of disease — so even if the coral initially survive, over the next few months they might still die, or not reproduce, or reduce their growth rate," says Schaffelke. They would also be less resilient to any subsequent bleaching events caused by periods of very warm weather, an event that has already struck the reef twice in the past decade and is predicted to become more common due to climate change. "The coral might not have the energy to recover," Devlin says.

Post-flood nightmare

However, the biggest threat to the reef could still be three years away, says Katharina Fabricius, also based at the Australian Institute of Marine Science, who studies the long-term effects of river run-off on the reef. Three years is the typical delay between a flood and the emergence of coral's biggest single threat, the crown-of-thorns starfish (Acanthaster planci)1.

"Nutrients in the floodwater plume cause algae blooms, and these microalgae are perfect food for the larvae of crown-of-thorns starfish — and those starfish eat coral," says Fabricius. "Each female produces around 60 million eggs per year, so if they hit a flood plume like this very big one, they basically explode in population."

It takes three years for the starfish to reach maturity, and they could eventually invade the entire reef. "Once you've got a primary outbreak, there's nothing stopping secondary outbreaks even in clean water," Fabricius says. "The larvae get transported from one reef to the next, like a wave along the reef matrix, killing corals everywhere."

The timing of the floods, just two years after the 2008–09 floods that were Queensland's third worst on record, makes a starfish boom even more likely, Fabricius says. "What we are really concerned about is that these prolonged good conditions for larvae are leading to another outbreak."

Coral reefs typically take up to 25 years to recover from each starfish event, and the Great Barrier Reef is being hit by a starfish population boom on average every 15 years.