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

Abstract

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.

Cost-effective water quality improvement in linked terrestrial and marine ecosystems: a spatial environmental - economic modelling approach

“…Eutrophication of freshwater ecosystems in tropical Australia is, however, widespread (Brodie and Mitchell 2005) and potentially costly (Davis and Koop 2001). Consequently, freshwater quality should not be ignored when managing coastal catchment land and water resources to achieve water quality improvement in linked terrestrial and marine ecosystems.…”

Australia's tropical river systems: current scientific understanding and critical knowledge gaps for sustainable management

“…The natural land cover of these catchments is most commonly seasonally dry forest, savanna woodland or monsoon forest, although there is also rainforest in northern Queensland catchments bordering the Coral Sea. Rangeland beef grazing and intensive row-crop agriculture have transformed upland and floodplain land cover in some catchments (Brodie and Mitchell 2005), particularly in Queensland (Furnas 2003), and seasonal burning is practiced throughout the region (Dyer et al 2001;Russell-Smith et al 2003).…”

“…The monsoonal climate, as well as the interannual variability in rainfall related to the El Niño Southern Oscillation and the erratic behaviour of tropical cyclones, all contribute to this variability in river-flow regimes. Consequently, the movement of water, sediments and nutrients is seasonal and often episodic (Furnas 2003;Brodie and Mitchell 2005). Catchments in the wet tropics region in north-east Queensland retain this strong seasonal pattern, but also receive significant rainfall from June through October and have relatively low interannual variability in discharge.…”

“…Rivers draining undisturbed catchments generally carry water low in nutrients (Brodie and Mitchell 2005), and aquatic primary production is nutrient-limited (Furnas 2003;Douglas et al 2005;Townsend and Padovan 2005). Eutrophication of estuaries and coastal waters has been documented in tropical Australia and is attributed primarily to increased nutrient loading from rivers, often linked to soil erosion from agricultural activities and consequent transport of suspended particulate matter (Harris 2001;Brodie and Mitchell 2005).…”

“…Eutrophication of estuaries and coastal waters has been documented in tropical Australia and is attributed primarily to increased nutrient loading from rivers, often linked to soil erosion from agricultural activities and consequent transport of suspended particulate matter (Harris 2001;Brodie and Mitchell 2005). During floods, most rivers are very turbid, carrying fine clays in suspension that are readily transported to coastal waters (Brodie and Mitchell 2005).…”