Long-term annual and seasonal trends in surface salinity of San Francisco Bay

Abstract

Trends in surface salinity at seven stations in San Francisco Bay were studied. We found that annual surface salinity over the period 1920–1986 at the ocean boundary had slightly increased and at the river boundary, had slightly decreased. None of these trends were statistically different from zero even though upstream water use has nearly doubled. However, statistically significant seasonal trends were found. In most areas, salinity had increased from February through June and decreased at other times. Salinity in the bay is affected primarily by freshwater inflow and oceanic conditions. Seasonal salinity trends at stations near the freshwater boundary are probably largely owing to the operation of upstream water projects, which have redistributed freshwater between the months. At stations near the ocean, seasonal salinity trends appear to be strongly influenced by offshore conditions, including a local rise in sea level and increased upwelling.