LATE 20th CENTURY FORAMINIFERAL DISTRIBUTION IN CENTRAL SAN FRANCISCO BAY, CALIFORNIA

One of the strongest tidal flows in the world travels through the Golden Gate into San Francisco Bay. As a result, sediment is in constant motion as currents exchange water between the outer coast and the three subembayments (central, north, and south bays) that make up San Francisco Bay. Although the distribution of foraminifera is well documented in the north and south bays, that is not the case for the central bay. The extremely high-energy tidal currents in this region create a challenging environment for benthic foraminifera, resulting in depauperate faunas as low as 0.1 specimen/g dry sediment.

Fifty-five grab samples from surface sediments in the central bay obtained in 1998 were analyzed to characterize their foraminiferal fauna. Thirty-five species were identified, including the invasive Japanese species Trochammina hadai that was introduced into the estuary in the early 1980s. A cluster analysis grouped the samples from the central bay into three clusters (assemblages) and one outlier. The Shallow Subtidal Assemblage is characterized by a marsh to shallow-subtidal arenaceous fauna, dominated by T. hadai, but also including T. inflata, T. macrescens, Haplophragmoides subinvolutum, and Miliammina fusca. The Intermediate Subtidal Assemblage, the Intermediate Subtidal Outlier, and the Deep Subtidal Assemblage are dominated by calcareous species, most noteably Ammonia beccarii, Elphidium excavatum, and Elphidiella hannai. Ammonia beccarii is most abundant in the warmer, intermediate depths of eastern central bay, abundances of E. excavatum peak in the cooler estuarine water near Alcatraz Island, and E. hannai thrives in the cold water west of Angel Island in a transitional setting between the deep subtidal estuarine and the nearshore marine environments. The presence of oceanic species as far east as Angel Island indicate that the western central bay is the most marine-influenced region of San Francisco Bay.

Samples collected between 1965 and 1998 were also used to investigate how the distribution of benthic foraminifera in the central bay has changed over the latter half of the 20th Century, especially since the invasion by T. hadai in the 1980s. A cluster analysis clearly separated these samples into pre- and post-invasion assemblages, thus illustrating how dominant T. hadai has become in the central bay.