Full Text | Download - Grzybowski Foundation - The ...

Alegret, Ortiz & Kaminski (eds.), 2012. Ninth International Workshop on Agglutinated Foraminifera, Abstract Volume
Dominance of the invasive agglutinated foraminifera Trochammina hadai Uchio
in San Francisco Bay, USA, less than two decades after its introduction
Mary MCGANN
U.S. Geological Survey, Pacific Coastal and Marine Science Center, 345 Middlefield Road, M/S 999,
Menlo Park, CA, 94025, USA.
e-mail: mmcgann@usgs.gov
San Francisco Bay is the largest estuary on the west coast of the United States and consists of
several subembayments, including informally named north bay (San Pablo, Suisun, Grizzly,
and Honker Bays), central bay (with Richardson Bay), and south bay. A common agglutinated
estuarine foraminifera in Japan, Trochammina hadai Uchio, was introduced into the estuary
between 1981 and 1983; the species' identification has since been confirmed by molecular
techniques (Kitazato et al., 1997; Fay et al., 2007). Trochammina hadai was probably
transported in ballast sediment, anchor mud, or in sediments associated with oysters imported
for mariculture (McGann et al., 2000). Following the species’ arrival, T. hadai proliferated,
becoming one of the dominant foraminifera in the estuary today, as well as in 14 other ports
and estuaries along the western U.S. from California to Alaska (McGann et al., 2000;
McGann, unpublished data). In Japan and along the eastern Pacific seaboard, the species
thrives in shallow estuarine waters. Here we present a summary of the arrival and expansion
of T. hadai in San Francisco Bay, illustrating how quickly and significantly this invasive
species has impacted the foraminiferal faunas throughout the estuary.
Starting in the 1960s, the first detailed investigations of the distribution of
foraminifera in surface sediments of San Francisco Bay were undertaken by Slater (1965) in
Suisun Bay, Means (1965) in Richardson Bay, and Quinterno (1968) in south bay. Similar
studies continued in the 1970s with the works of Locke (1971) in San Pablo Bay, Conner
(1975) in Richardson Bay, and Wagner (1978) whose study locations were situated in parts of
south bay, central bay, and north bay. A few years after Wagner's study, Arnal et al. (1980)
published a slightly expanded version of Quinterno's (1968) south bay study, and Sloan
(unpublished data, 1980-1981) acquired surface sediments from north, central and south bay
to investigated foraminiferal abundances estuary-wide. In all, these studies analyzed 350
surface samples at 340 sites throughout the estuary over a period of about 16 years.
These samples provide a baseline of the spatial distribution of benthic foraminifers in
San Francisco Bay before the introduction of T. hadai. Ammonia beccarii was the most
abundant and geographically widespread species in the estuary; second in abundance was
Elphidium excavatum. Haynesina germanica was most abundant in the shallower regions of
south bay and Elphidiella hannai dominated the deepest sites of Richardson, central, and
north bays. The agglutinated taxa Trochammina inflata, T. macrescens, Miliammina fusca,
and Haplophragmoides subinvolutum are ubiquitous in the marshes of San Pablo Bay and
Richardson Bay, but are mostly rare to absent in south bay except for a few localized
occurrences of T. inflata. No foraminifera were found east of Honker Bay or in the channel or
southern portion of Suisun Bay.
The earliest sample in the estuary in which T. hadai was found was in one of four
samples collected in 1983 from south bay where the species constituted 3% of the fauna. By
1986-87, T. hadai was present at 46 stations and dominating the foraminiferal assemblage,
comprising up to 89% of the fauna. In nine samples collected in central and south bays
61