Nutrients in Australian tropical rivers: changes with agricultural development and implications for receiving environments

In tropical Australia, intensive studies of river suspended sediment ( SS) and nutrient dynamics have been restricted to streams on the north-east coast between the Fitzroy and Normanby Rivers ( Queensland), Magela Creek/East Alligator River ( Northern Territory) and the Ord River ( Western Australia). Historical conditions in these rivers were probably characterised by low-moderate SS concentrations and low concentrations of dissolved inorganic nitrogen and phosphorus in flow events. Introduction of agriculture has transformed SS and nutrient dynamics. Grazing has led to soil erosion and increased SS and particulate nutrient concentrations and fluxes in event flows. Fertilised cropping has increased nutrient inputs to catchments, where it forms a substantial proportion of the catchment area. Consequently, both particulate and dissolved inorganic nutrient concentrations and fluxes have increased. Australian tropical rivers have episodic flows, with most material transport occurring during large flow events. The restricted period of these highly energetic flows means little trapping of materials in waterways occurs. Loads are transported efficiently downstream and processes such as denitrification and in-channel sedimentation may be of limited importance. Owing to excessive nutrient inputs associated with agriculture, a number of northern freshwater, estuarine and coastal ecosystems are now eutrophic. Continued development, especially fertilised cropping, without adequate management of nutrient losses is likely to exacerbate these problems.