In this paper we consider simplifying a model of the nitrogen cycle in Port Phillip Bay, Victoria, Australia. The approach taken is to aggregate state variables that are linearly related using a projection in state space. The technique involved is a modification of proper orthogonal decomposition and was developed so that a resulting simplified model retains an ecological interpretation. It can be applied automatically, and enables insights into the system to be gained that were not obvious beforehand. In the case of the Port Phillip Bay model, we find that the variables representing water and sand are unaffected by the remaining variables, while only variables on the same trophic level can be grouped together. The validity of the aggregation under several nutrient loads is also discussed.
