This project is a one year continuation of the PI's long term study of Tomales Bay, California. It will complete his ongoing hydrographic and nutrient survey. Tomales Bay is unique because of its hydrgraphic simplicity and because of the long residence time of its water. These factors greatly facilitate mass and flux calculations for the water, salt, and nutrients within the Bay, itself. In addition, the Bay is optimum for studying the exchange of material between the land, the estuary and the coastal ocean. It is a "net heterotrophic system," meaning that it consumes more organic matter than it consumes. It is also a denitrifying system where nitrogen gas is released to the atmosphere as the microbes decompose organic matter. The PI seeks to answer the following question. Is the heterotrophy of this and similar systems fueled by material from the land or by material from the coastal ocean? A great deal of organic matter is delivered to the Bay from the coastal ocean, but when upwelling is weak terrestrial runoff is strong and may contribute even more organic matter. With this additional year of sampling the PI will resolve the relative importance of these potential sources. The results of this study may force a recalculation of the global carbon and nitrogen budget because other estuarine systems may also be net heterotrophic rather than net autotrophic as previously thought.