Modelling of nutrient impacts in Port Phillip Bay - a semi-enclosed marine Australian ecosystem

This paper overviews the ecosystem model developed for the Port Phillip Bay Environmental Study. The model simulates the cycling of N, P and Si in the water-column, epibenthos and sediments, and is driven by a physical transport model. The integrated model is forced by exchanges with Bass Strait and by nutrient inputs from the Western Treatment Plant, rivers, and the atmosphere. The model has been calibrated and tested by using data from process studies, flux measurements and spatial surveys at a range of scales. It provides a picture of nitrogen cycling through the water and sediments on bay-wide annual scales, which emphasizes the role of denitrification. It also reproduces well the observed spatial and temporal variation under varying nutrient load regimes, and provides insights into the factors controlling regional phytoplankton blooms, including the interaction of nitrogen and silica in the western bay. The model is used to investigate the bay's responses to changed nutrient loads, changes in sediment biogeochemistry, and marine pest invasion. The bay's assimilative capacity for nitrogen is largely controlled by its sediment denitrification capacity. If nitrogen loads approach the limits of this capacity, a rapid onset of eutrophication is predicted.