Algal blooms in rivers and estuarine waters in south west Western Australia are a symptomatic response to excess nutrient input. Whilst a range of Best Management Practices (BMPs) are available to reduce the causes of phosphorus (P) and nitrogen (N) pollution, most investment has been directed towards symptoms. In order to treat nutrient pollution causes effectively, possible nutrient reductions and the likely adoption costs of a range of BMPs require evaluation. Catchment-scale evaluation of implementation scenarios offers insights not possible through long term on-ground implementation and performance monitoring, and assists community groups and government to respond to pollution issues through adhoc funding or programs. The Support System for Phosphorus and Nitrogen Decisions (SSPND) is a risk based tool used in south west Western Australia to estimate costs and benefits of implementing conventional BMPs. It is an adaptation of a P indicators approach which combines source factors, transfer factors, and delivery factors. Model estimates for the Geographe Bay and Peel- Harvey catchments indicate that the net effect of catchment nutrient management to date has been to reduce P loss by 5-10%. SSPND indicates that a further 50% reduction is possible, with approximately half coming from P fixing soil amendments applied to sandy soils in these catchments. For N the picture is similar, but the major management options are riparian fencing and planting, with reduced applications of fertiliser and the use of non-legume species such as perennial pastures being significant also. For the Geographe Bay catchments, a 20yr plan of targeted investment could see significant reductions of nutrient in the Geographe Bay catchment, over 40% for P and 30% for N. However even over a 20-yr timeframe, and with an investment of over $20M, the resulting nutrient load reductions are unlikely to meet water quality targets in most catchments. SSPND provides a range of outputs which assist in the development of management plans for nutrient reduction, and can be used to target nutrient BMP implementation on the basis of water quality, cost/benefit or nutrient reduction. It is currently providing direction in the development of the Geographe Bay Water Quality Improvement Plan
