Water, salt, and nutrient exchanges in San Francisco Bay

We constructed water, salt, and nutrient budgets for San Francisco Bay and used them to analyze the net biogeochemical performance of the bay. The bay was subdivided into three sectors, North, Central, and South Bay, with the Central Bay serving as a proxy for the oceanic end member. Separate budgets were constructed for the wet (October-March) and dry (April-October) seasons of each year for 6 yr (1990-1995). This period of record contained 2 yr of above normal runoff (1993, 1995) and 4 yr of below average runoff. Effluent from sewage treatment plants accounts for approximately 50% of the nutrient loading to the bay in winter and 80% of the summer loading. Both arms of the bay were apparently net heterotrophic during the winter, with this signal being strongest during the wet winters of 1993 and 1995. We conclude that overall the bay is slightly net autotrophic (production of new organic matter in the bay by plant growth exceeds respiratory demands); however, this varies seasonally (strongest in summer) and is complicated by the possibility of significant abiotic P adsorption in the North Bay.