Ancient Blue Oaks Reveal Human Impact on San Francisco Bay Salinity

San Francisco Bay is one of the most important estuaries on the west coast of the Americas. Its water quality is controlled primarily by strearnflow from the Sacramento and San Joaquin rivers. In fact, freshwater inflow from the Sacramento-San Joaquin Delta explains 86% of the salinity variability at the mouth of the San Francisco Bay estuary [klerson et al., 19891. The massive diversion of strearnflow by the California State Water Project and the Central Valley Project, part of the largest nianmade water control system on Earth [Reislrer, 19881, has raised salinity in the estuary on daily seasonal,and annual timescales [~Viclrob et al.. 1986; Pelerson et al.. 19891. Reduced freshwater inflow and increased salinity are part of a larger syndrome of anthropogenic impacts that imperil water quality and ecosystem function in this impor[ant estuary Regional drought conditions not only lead to high salinib but also to increased concentrations of contaminants and nutrients in Sari Francisco Bay Biologically available metal concentrations and dissolved nutrients reached record levels during the 1976-1977 drought ancl had serious consequences on the pelagic food web and fisheries in the Bay [Nichols et al., 19861. Salinity variations are also strongly linked to total biological productivity i n the estuary especially in response to changes in the geographic position of the null zone, the region of convergence between fresh surface and saline bottom currents in the northern reach of the Bay [Nichols et al.. 1986;Jassby et al., 19951. A nea~bottom salinity threshold has been used since 1995 as a sensitive indicator of the ecological response to changing fresliwater inflow into the estuary (US. Environmental Protection Agency standards listed i l l the Federal Registcv Part 11:4463-4709, I!%). During the post-World War I1 era of rising salinity, many populations of aquatic organisms in the Bay have declined,due in part to decreased freshwater inflow higher salinityand changes in the geography and geometry of the null zone. Because salinity and freshwater inflow are so tightly coupled, high salinity conditions are also synonymous with low inllow,lower freshwater flushing, higher concentrations of pollutants in the Bay, and salt water intrusion into the agricultural complex of the Sacrarnentcr San Joaquin Delta. The degree to which freshwater diversion has altered the natulal variability of San Francisco Bay salinity has been difficult to quantify given the short pel-iod of salinity measurement and the large natural variation in strearnflow and estuarine salinity A new tree ring reconstruction of surface salinity for Fort hint on the south shore of the Golden Gate,using extreme moisture-stressed blue oak trees (Quercus do~rglusii; Figure I), indicates that the appropriation of freshwater by state and federal water projects has led to unnatural salinity extremes and long-term trends that are unprecedented in the Bay for over 400 years.