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APRIL 1995 NEWSLETTERCRC RESEARCHERS DEVELOP TURBIDITY TESTERScientists in north Queensland have developed a new instument to measure turbidity in nutrient-rich rivers flooding into the Great Barrier Reef region. Until recently, little was known about the export of nutrients and sediment from north Queensland rivers. Much of it occurs during cyclones or floods, when it is difficult for scientists to reach their usual sampling sites. So before the 1995 summer wet season, two CRC research scientists from the Australian Institute of Marine Science (AIMS), Dr Miles Furnas and Dr Alan Mitchell, built and installed turbidity-testing `transmissometers' in four rivers, the Tully, Herbert, Burdekin and Fitzroy. These instruments measure water level and turbidity every half hour, and need not be attended to for periods of more than 60 days. They are completely self contained in pressure cases and will operate fully submerged during large and small floods. Turbidity is measured by shining light beams through the river water and measuring its absorbance by particulate matter. During floods, rivers carry high concentrations of suspended solids, but these transmissometers have motorised brushes which wipe the sensor optics clean before taking each reading. The measurements of turbidity are converted to estimates of suspended sediment concentrations in river water. A stainless steel cage protects the instrument and its battery pack from logs and other debris washing down, and it is programmed via laptop computers to set the sampling interval, the frequency of cleaning, and the depth of the water required to activate it. It is typically attached to a bridge pylon in the middle of a river, just above the dry season level, and will automatically switch on during any rise in the water. The information, being collected over several wet seasons, will be combined with data from other partner organisations in the CRC Reef Research Centre Ltd, including the Department of Primary Industries, to produce a comprehensive estimate of the amount of river nutrients and fine sediments which reach the central region of the Great Barrier Reef. Ultimately the project aims to resolve the annual variability of sediments flowing to the Reef, caused by yearly difference in water flow. It will also reveal how sediment flows differ between wet-tropical (the Tully and Herbert) and dry-tropical (the Burdekin and Fitzroy) river systems. This information will assist in the conservation and sustainable use of industries which impact on the Reef, and shed greater light on the relationships between land use and soil loss from wet and dry tropical watersheds. The transmissometers also have potential use in harbours where their capacity to self-clean will solve problems of algae fouling up the instruments.
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