Addressing off-site nutrient pollution through conventional management actions: a modelling case study
The ecology of estuaries on the south coast of Western Australia has been disrupted by increased nutrient and sediment discharge from predominantly rural catchments. A range of conventional management actions are available to reduce nutrient loss at source, and it is important to evaluate possible reductions, and costs, so that limited funds can be targeted to realise the greatest moderation of nutrient loss. A lumped landuse nutrient generation rate model was developed for four catchments (Wilson Inlet, Oyster Harbour, Torbay Inlet and Princess Royal Harbour) near Albany Western Australia and the output compared with existing monitoring data. The nutrient moderating effects of six conventional management actions (perennial pastures; vegetated stream buffers; minimum tillage, effective fertiliser use; stock control and water management; and effluent management) and their associated costs were implemented at different levels in the model. These were used to determine the extent to which these actions could address offsite nutrient pollution, and the cost of doing so. Management actions were implemented in three major scenarios representing the status quo (current nutrient reduction efforts through volunteerism), the highest possible implementation of each action (perhaps indicating mandated change) and the most cost effective set of actions. In each catchment dominated by diffuse nutrient sources, current nutrient reduction efforts amounted to about 10%, whilst the highest possible reductions were of the order of25–30% above this. In the point source dominated catchment current nutrient reduction efforts amounted to about 40%, with an additional 40% possible. The most cost effective scenarios reduced nitrogen more than phosphorus. Under the most cost effective scenarios, it was estimated that the net cost of management actions over 10 years was budget positive, resulting in a net benefit to the land managers involved. This suggests that there are limited economic barriers to the adoption of these conventional management actions, and on this basis current nutrient reduction efforts could be significantly increased. However, predicted reductions from the implementation of conventional management actions may not be sufficient to arrest estuarine decline.