This book reports on the findings of, and swnmarizes the conclusions from, the Port Hacking Estuary Project, a model-guided, multidisciplinary study of an estuarine ecosystem. The Project began in 1973, at a time when it was thought that environmental problems could be solved readily by assembling a multidisciplinary team of research scientists and having them co-ordinate their research around the construction of an ecosystem model. But a decade has passed and time has not been easy on this approach. The anticipated predictive dynamic models have not been produced and bitter argument has often marred the course of such studies. Yet the need to anticipate the flow of various chemical species (carbon, oxygen, nitrogen, phosphorus, toxicants) through the environment remains: the evidence is everywhere, from fertilization of urban lakes to acid rain. The magnitude of the problem ensures that funds will continue to be made available - although with short-term variations as perceptions swing. It is thus clear that although the difficulties are great, so is the need. It is from this background that we present this book. The Port Hacking Estuary Project involved some 15 - 20 research scientists over a period of 5 years. The goal was to research the flow of carbon into, within, and out of a small unpolluted estuary chosen for convenience rather than for its social significance. The idea was to use the information obtained from these studies to build a predictive dynamic model.