Document Type

Article

Publication Date

2001

Conference Title

In proceedings of the Third Australian Stream Management Conference, eds Rutherfurd, I., Sheldon, E., Brierley, G. and Kenyon, C. August 27 - 29, 2001, Brisbane, pp 647-652

Place of Publication

Brisbane

Keywords

Nutrient, scale, water quality, eutrophication, phosphorus

Abstract

Seagrass, which once dominated the habitat of Oyster Harbour on the south coast of Western Australia has been replaced by macroalgae because of increased nutrient and sediment discharge from the rural dominated catchment. Total Phosphorus (TP), Total Nitrogen (TN), Suspended Sediment (SS) and Electrical Conductivity (EC) concentrations from a catchment – wide (168 sites), event-driven snapshot, water quality monitoring program conducted from 1994 to 1996, were analysed in relation to stream order and published survey data on riparian zone condition. This analysis was performed to examine relationships between stream order, riparian zone condition and water quality, and implications for the allocation of limited resources for stream fencing, rehabilitation and stock exclusion towards the moderation of nutrient loss for the benefit of the harbour at the catchment exit. Eighty percent of the stream length was in low order streams (stream order 1 and 2) and the remainder in order 3 and above. Riparian zone condition worsened as stream order decreased. Total Phosphorus, and to a lesser degree TN and SS, decreased with increasing stream order, whilst EC increased with increasing stream order. Amongst many factors, one factor influencing the change in water quality is that low order streams exhibit the poorest riparian zone condition and therefore have little capacity to moderate paddock nutrient runoff. The systematic change in EC implies that low order streams are dominated proportionally more by surface runoff than groundwater, and hence represent a greater relative opportunity to moderate surface derived nutrients and sediment. In summary, low order streams in this case study represent the greatest length, have the poorest condition, show the highest nutrient and sediment concentrations, have greater surface runoff, and therefore are priority candidates for the purpose of minimising the downstream impacts of nutrients when limited funds are available.