The recent Queensland floods are carrying tonnes of fresh water, nutrients and pesticides to the ocean, placing enormous stress on the Great Barrier Reef, say concerned Australian researchers.
For the past five weeks, plumes of silt-laden fresh water have been flowing onto reefs off the Queensland coast. The impact is so massive it can be seen in NASA satellite photographs.
Researchers list the Keppel Islands near Rockhampton, Moreton Bay and Fraser Island - north of Brisbane - at being most at risk.
Dr Alison Jones from Central Queensland University in Rockhampton has seen first-hand the impact of the floods on corals in Keppel Bay.
"You can't see anything at all from above," she says. "As you take the camera down, it's looks like a big brown soupy mess … deeper down the water is a bit clearer, and you can see bleached white [coral] colonies appearing out of the gloom."
Dr Jones checked five islands and found stressed coral around all of them.
"Halfway Island was much worse than North Keppel. It was just dead coral, killed by the fresh water. There wasn't really a single thing alive.
"There also seems to be some temperature bleaching, believe it or not, from the ocean being warm, which is completely unrelated to the flooding."
Dr Britta Shaffelke, a researcher at the Australian Institute of Marine Science (AIMS), says wind direction over the next few days will be crucial in determining the extent of the damage.
"At the moment the mud plume (from the Fitzroy River) is confined to the Keppel Bay area. However if the wind turns around from the south east to the north, the plume might reach much further to the outer reefs such as Heron Island," says Shaffelke.
Floods damage corals in a number of ways. Corals can't survive in freshwater because their physiology is adapted to salt water. Silt is also clouding the water and blocks out sunlight, stopping corals from photosynthesising and feeding themselves.
Nutrients such as nitrogen and phosphorous can kick start toxic algal blooms, which strip oxygen from the water and at the same time, provide food for the larvale of crown of thorns starfish. Pesticides carried in floodwater call also kill corals.
Researchers are most concerned about the impact of the sediments.
"What has changed is that the load of sediment in the rivers has increased 4- to 10-fold since pre-European times," says AIMS scientist Dr Katharina Fabricius.
"Reefs exposed to high levels of nutrients and sediments have up to five-fold higher cover of seaweeds (which can smother corals) and half the biodiversity of species of coral - these are the long term effects of these floods," she says.
Dugongs at risk
Meanwhile, further south in Moreton Bay, experts are worried about the long-term impact on dugongs. In 1996, a flood left many dugongs starving, as sediment and nutrients overwhelmed and killed the seagrass beds in the area.
"For Morteton bay, the flooding event last week was significantly bigger for sediment deposition and fresh water than the flood of 1996," says Dr Eva Abal, chief scientific officer at the Great Barrier Reef Foundation.
"My expectation about the impact on the bay is that we will experience some seagrass loss, but it also depends on how quickly water clears up."
Shaffelke also points out that there are unusually vast amounts of turbid freshwater off the coast of Brisbane.
"That hasn't happened for many, many decades in the Brisbane area, so many plants and animals will imediately die or be very stressed," she says. "I expect there to be quite serious impacts as well."
"In relation to the floods in Rockhampton … that is certainly not typical or happens very often. For both humans and the enviroment this is an extraordinary event.
"For the marine environment, the events are still unfolding. The highest rainfall is actually in February, so we are certainly not at the end of this season."