Remarkable invasion of San Francisco Bay (California, USA), by the Asian clam Potamocorbula amurensis. II, Displacement of a former community

Long-term macrobenthic sampling at a site in northern San Francisco Bay has provided an unusual opportunity for documenting the time course of an invasion by a recently introduced Asian clam Potamocorbula arnurensis. Between 1977, when sampling began, and 1986, when the new clam was first discovered, the benthic commun~ty varied predictably in response to river inflow. During years of normal or high river inflow, the community consisted of a few brackish or freshwater species. During prolonged periods of low river inflow, the number of species doubled as estuarine species (e.g. Mya arenana) migrated up the estuary. In June 1987, at the beginning of the longest dry period in recent decades, large numbers (> 12 000 m-') of juvenile P. arnurensis were discovered at the site. By midsummer 1988 the new clam predom~nated (> 95 %) in both total number of individuals and biomass, and the expected dry-penod estuarine species did not become re-established. The rapid rise of P. arnurensls to numerical dominance throughout the region of the original introduction was probably facilitated by the fact that this region of the bay had been rendered nearly depauperate by a major flood in early 1986. Once introduced, the clam had sufficient time (> 1 yr) to become well established before the salinity regime was appropriate for the return of the estuarine species. Subsequently, the new clam was apparently able to prevent the return of the dry-period community. Its ability to live in low salinity water (< 1 %o) suggests that P. amurensis may not be displaced with the return of normal winter river flow and, therefore, may have permanently changed benthic community dynamics in this region of San Francisco Bay.