Live individuals of the Chinese clam,
Potamocorbula amurensis, were first
collected in northern San Francisco Bay in
1986. Within two years it spread
throughout the Bay, reaching densities of
up to 10,000/m
. It now literally
carpets the bottom, displacing much of the
former benthic community. Moreover,
because P. amurensis is a suspension
feeder, compared with the deposit-feeding
benthic fauna that dominated prior to
1986, the Chinese clam has the potential
to completely alter food-web dynamics in
the Bay [ Nichols et al., 1990]. It
so efficiently filters the water column
that summer phytoplankton blooms have been
virtually eliminated [ Alpine and
Cloern, 1992], perhaps to the detriment
of zooplankton which now must compete with
the Chinese clam for phytoplankton
resources. A decrease in the zooplankton
standing stock may then alter biodiversity
at higher trophic levels, akin to the
situation in the Great Lakes following the
introduction of the suspension-feeding
zebra mussel [ Nalepa and Schloesser,
1993].
Interestingly, both natural and human factors led to the initial establishment and widespread colonization of the Chinese clam in San Francisco Bay [ Nichols et al., 1990]. The source of P. amurensis larvae was undoubtedly ship's ballast water [ Carlton, 1985; Carlton and Geller, 1993]. Initial colonization was facilitated, however, by a major flood in central California in 1986, which decimated much of the fauna in the northern reaches of San Francisco Bay. The Chinese clam was able to spread because of the ensuing drought, beginning in 1987, which resulted in very high water salinities in the shallower, more estuarine portions of the Bay. These high salinities prevented the pre-flood fauna from returning before the more salinity-tolerant Chinese clam could become firmly established. Moreover, it has been able persist even after normal salinities returned to the Bay.