Nutrient transport to the Swan - Canning Estuary, Western Australia

Abstract

Catchment nutrient availability in Western Australia is primarily controlled by the disposal of animal waste and the type and rate of fertilizer application, particularly on the relatively narrow (~25 km wide), sandy coastal plain. Nitrogen (N) and phosphorous (P) concentrations and fluxes during the wet season of 15 tributaries, including four urban drains to the Swan-Canning Estuary, were evaluated from 1986 to 1992 and additionally concentrations only were evaluated throughout the year from 1993 to 1996. Concentrations of filtered reactive P (FRP) and total P (TP) were generally low, with the volume-weighted means for all sites being 0.06 mg 1-1 and 0.12 mg 1-1 respectively. The urban drains had higher TP concentrations (volume-weighted mean of 0.21 mg 1-1) than the streams (0.12 mg 1-1), with the high concentrations associated with particulate matter. Total inorganic N (TIN, NH4N plus NO3N) and total N (TN), which is of interest to eutrophic status of the N-limited estuary, were likewise low, compared with other developed areas having a similar climate. Both TIN and TN were higher in the urban drains (0.76 mg 1-1 and 1.5 mg 1-1 respectively) than the streams (0.31 mg 1-1 and 1.2 mg 1-1 respectively). The Avon River, which drains 98.5% of the 121 000 km2 catchment area, contributes most of the N (0.03 kg ha-1 year-1 or 65%) and a high percentage of the P (<0.01 kg ha-1 year-1 or 32%) to the estuaries. The Avon River nutrient fluxes are much less than other tributaries closer to the estuary. The coastal plain receives significantly higher rainfall (1,200 mm year-1) and has more intense horticulture and animal production than inland areas (<300 mm year-1). Annual rainfall is seasonal, occuring primarily from May through December. The surficial aquifers on the coastal plain generally are sandy with a low nutrient retention capacity, and rapidly transmit soluble and colloidal material in subsurface flow. Ellen Brook, on the coastal plain, drains pastures treated with superphosphate and has the highest FRP (0.51 mg 1-1), TP (0.7 mg 1-1) and TN (2.1 mg 1-1) of any tributary to the estuary. The coastal plain is also undergoing urbanization, particularly in areas adjacent to the estuary. Nutrients are subsequently available for transport during the onset of seasonal wet weather. Perennial baseflow from urban areas is an important source of nutrients. Water yield from the urban areas was high, being as much as 50% of annual rainfall. The timing of the nutrients delivered by the tributaries may be an important control on estuarine ecology. Copyright ?? 2001 John Wiley and Sons, Ltd.