Queensland flood-mud and silt in the ocean (from ABC Scienceshow)

Silt flowing from the Fitzroy River into Keppel Bay, east of Rockhampton (NASA)

The recent Queensland floods are carrying tonnes of fresh water, nutrients and pesticides to the ocean, placing enormous stress on the Great Barrier Reef.

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 just released NASA satellite photographs.

Floods damage corals in a number of ways. Corals cannot survive in freshwater because their physiology is adapted to salt water. Silt is also clouding the water and blocking 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, however researchers from Central Queensland University in Rockhampton and the Australian Institute of Marine Science 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,” said 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.

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 Moreton 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.

Silt and mud from the floods that swept through Brisbane and Ipswich spreads out over Moreton Bay (ABC News: Sarah Clarke)