Hard data on soft mud

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Satellite image of flood plumes between Cape Tribulation and Bowen, February 2007. Imagery - NASA, processed by Slivkoff.

In February 2007, heavy rains produced large fresh water flood plumes that carried sediment, nutrients, other pollutants and debris from coastal catchments to theGreat Barrier Reef. Satellite images showed a band of floodwaters up to 25 kilometres wide extending along hundreds of kilometres of coast from major rivers between the Whitsunday Islands and Princess Charlotte Bay.Automated mud-loggers built in the AIMS workshop measured sediment loads in six major rivers simultaneously while AIMS scientists measured water currents, salinity, turbidity, light, nutrients, chlorophyll, plankton and suspended sediment levels in the GBR Lagoon, offshore from theTullyRiver. These samples and data provided much needed information to help in interpreting the spectacular satellite images of the fresh water plumes.AtDunkIsland, the daily rate of sedimentation averaged more than 250 grams per square metre within the 28-day study period, and two centimetres of mud accumulated in the sediment traps and on parts of the reef within 10 days. Concentrations of dissolved and particulate nutrients originating from the river were also high. Floodwaters blocked 99% of the light reaching corals at 4 metre depths for a period of 10 days. The observed low light would have prevented coral photosynthesis, while the sedimentation rate would have been lethal to some juvenile corals. The mud may ultimately be deposited elsewhere, however, this transport occurs at time scales much longer than the flood event. Consequently the mud is likely to affect coral physiology for significant periods after the flood has subsided. In some areas bleaching of shallow corals occurred, indicating that freshwater was another source of stress on inshore reefs. AIMS scientists also observed sunken logs and other terrestrial debris breaking fragile corals in exposed sections of the reefs.Although flood plumes are natural events, climate change could worsen their impact by increasing the frequency and possibly intensity of droughts followed by flood events, and thus the quantity of land-based runoff and pollutants making it to the reef.