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Toxic Runoff Poses Risk To Reef


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Toxic runoff poses risk to Reef

SATELLITE images of flooding in the Top End have revealed that runoff from the land is a greater threat to the Great Barrier Reef than experts believed.

Until now, most scientists thought the sediment and pollutants that washed into coastal waters after torrential rain slowly dispersed along the coastline, affecting only coral living on the inner reef.

However, a group headed by remote sensing expert Arnold Dekker of CSIRO Land and Water claimed the images clearly showed that potentially polluting plumes of water quickly travel to the outer regions of the reef, putting coral at risk.

"I was surprised and impressed," Dr Dekker said, recalling his first look at the pictures.

"I knew we had something that needed to be known broadly."

"What these images are showing is that significant rainfall is causing substantial flows into the rivers, estuaries and into the lagoon (between the coast and the Great Barrier Reef).

"The material is going - within days - right out to the outer reef and beyond."

The telltale images were taken between the 9th and 13th of February by the Moderate Resolution Imaging Spectroradiometer.

Known as MODIS, the instrument is flown aboard two paired satellites operated by NASA and the US National Oceanic and Atmospheric Administration.

Because MODIS collected data in the visible portion of the spectrum every two hours, Dr Dekker was able to track the movement of the olive-green plumes as they swept into the lagoon and on to the light blue-green reef beyond.

Flooding into the lagoon is a natural phenomenon, but increased human activities have changed the composition of the material the floodwater carries.

Floods flush sediment, fertilisers, pesticides, herbicides and other human-produced run-off into the lagoon. As well, cattle grazing along the rivers break down the banks, causing increased amounts of soil to wash into the water and sediment.

Pollutants bind to the sediment and are transported to the reef where they are released and disrupt the ecosystem, directly and indirectly, said Dr Dekker's colleague, CSIRO and Griffith University ecologist Andy Steven.

"It settles on the coral and the zooxanthellae and smothers them," he said.

Zooxanthellae are a type of algae that live with the coral, turning sunlight into energy for its neighbour.

Now that the team have demonstrated that satellite imagery can monitor plumes, they hope to develop it into a web-based monitoring and educational tool.

"A picture's worth a thousand words," said Associate Professor Steven.

"Photos like these are a compelling reason for managers to go to farmers and say we've got to make a concerted effort to reduce pollutants and sediments from getting into the water."


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