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May 2009 |
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Nutrient loads to the Great Barrier Reef – higher than first thoughtNew research shows that estimates of nutrient loads delivered from the catchments of Australia’s World Heritage-listed Great Barrier Reef to the Reef Lagoon have been significantly underestimated. In the first study of its kind undertaken in Australia, Water for a Healthy Country Flagship scientists have found that phosphorus and nitrogen loads may be 30 to 60 per cent higher than previously thought. This outcome is based on measurements of nine flood events on the Tully and Murray floodplain (North Queensland) between 2006 and 2008. “Currently, nutrient loads are estimated using monitoring equipment which measures concentrations and flow volumes in the river channel,” project leader Professor Jim Wallace explains. ”This is fine during regular river flow, however, when the river bursts its banks during a flood, which is not an uncommon event in North Queensland, large volumes of water flow out to the ocean outside the river channel. We wanted to see if the amount of nutrients carried in these flood waters was enough to significantly affect the current load estimates.” It turns out they were.
Although absolute concentrations of nutrients were found to be quite low in floodwaters, the sheer volume of water being discharged during these events meant that they made a highly significant contribution to the total marine load. The team also found that floodwaters carry different concentrations of materials to waters measured during in-river flows. Nitrogen is one of the main nutrients entering the reef and floodwaters carry more dissolved organic nitrogen (DON) than dissolved inorganic nitrogen (DIN), which is opposite to in-river waters. Consequently, DON loads to the ocean may be nearly twice those previously estimated from in-river data. “The real significance of these findings is their impact on the development and implementation of Reef Water Quality Improvement Plans,” Professsor Wallace says. “Without accurate load estimations we don’t know what the current end-of-catchment water quality is and therefore where we are starting from. Nor can we tell what improvements we might realistically hope to make or whether we’re moving towards or away from these targets.” “Our findings regarding DON and DIN loads are also important for water quality improvement planning as management actions to reduce DON may differ from actions to reduce DIN levels as the sources are likely to be very different.”
Water Quality Improvement Plans are developed as part of the Australian and Queensland governments’ Reef Water Quality Protection Plan (Reef Plan) which aims to 'halt and reverse the decline in water quality entering the Reef within ten years.’ Water quality has significantly declined since European settlement and the extensive introduction of grazing and cropping lands throughout the Reef catchments. “By improving the quality of the water we hope that the Great Barrier Reef will be more resilient to other pressures such as climate change,” Professor Wallace says. This Water for a Healthy Country Flagship project represents a collaborative effort between CSIRO, the Australian Department of the Environment, Water, Heritage and the Arts, the Marine and Tropical Science Research Facility, and Terrain Natural Resource Management Ltd, with scientific and data contributions from the Australian Centre for Tropical Freshwater Research, the Queensland Department of Natural Resources and Water, and the Australian Bureau of Meteorology. Contact: |
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