Michael Bell1, Britta Schaffelke2, Philip Moody3, David Waters4 and Mark Silburn4
1 School of Agriculture and Food Science, University of Queensland, Gatton, Qld 4343, Australia. [email protected]
2Australian Institute of Marine Science, Townsville Qld 4810, Australia, [email protected]
3 Landscape Sciences, Department of Science, Information Technology and Innovation, Dutton Park Qld 4102, Australia, [email protected]
The water discharged from rivers draining into the Great Barrier Reef (GBR) lagoon carries land-derived suspended sediments, nutrients and pesticides. Total nitrogen (N) loads have more than doubled since development, with the extensive grazing and intensive sugarcane industries the largest contributors. Runoff and soil erosion are the main sources of riverine particulate nitrogen (PN) while fertilizer application has contributed to the increase in dissolved inorganic nitrogen (DIN). Dissolved organic N (DON) loads have increased less and it is unclear if DON has changed with development. DIN is rapidly taken up by marine plants and cycled through the marine food web, resulting in typically low concentrations of DIN in GBR waters, and an N-pool dominated by DON and PN of marine origin. The productivity of marine plants is sustained by rapid recycling of organic nutrients. Additional available N, as occurs from land runoff, can result in adverse effects on coral reefs by increasing coral vulnerability to temperature stress, and by benefitting coral competitors and predators.
Ambitious targets to reduce N loads from key catchments have been set and a combination of changes to land use and nutrient management practices will be required to achieve the necessary water quality improvement. Prioritization of actions that maximize water quality benefits will require new fertilizer technology for cropping industries and greater certainty around underlying processes contributing to bioavailable N loads from all land uses. Uncertainties include the relative importance of DIN compared to total bioavailable N, and of the contribution of runoff compared to other N loss pathways like subsurface lateral flow and deep drainage.