Influence of Hydrologic Processes on Reproduction of the Introduced Bivalve Potamocorbula amurensis in Northern San Francisco Bay, California1 Francis Parchaso and Janet K. Thompson2 Abstract: Monthly censusing of reproductive condition of the Asian clam Potamocorbula amurensis at four sites in northern San Francisco Bay over a 9-yr period revealed year-to-year differences in local reproductive activity that are associated with patterns of hydrologic variability. Between 1989 and 1992, Northern California experienced a drought, whereas the period between 1993 and 1998 was marked by a mix of wet and dry years. We took advantage of the extreme year-to-year differences to examine reproductive responses to river inflow patterns. Populations of P. amurensis at the upstream sites in Suisun Bay and Carquinez Strait were more reproductively active during wet years than dry years. Conversely, at the downstream site in San Pablo Bay, the population was more reproductively active during dry years than wet years. We suggest that the different reproductive patterns observed reflect the clam’s response to different sources of food. During wet years, organic matter from the rivers augments food supplies in Suisun Bay. During dry years, when inflow into the San Francisco Bay Estuary from the rivers is reduced, water transported from the adjacent ocean into the estuary as far as San Pablo Bay provides a supplemental food supply for the local production. The populations take advantage of these spatially distinct food supplies by initiating and maintaining local reproductive activity. We conclude that the ability of P. amurensis to consume and use various types of food to regulate its reproductive activity is part of the reason for its success as an invasive species. The Asian clam Potamocorbula amurensis, a native of East Asia, Khabarovsk (a republic of the former Soviet Union), and Korea, was introduced into San Francisco Bay in 1986. The success with which P. amurensis colonized and established itself in the benthic community of San Francisco Bay has been well documented (Thompson et al. 1988, Carlton et al. 1990, Nichols et al. 1990), although the reasons for its success have not been discussed. In this period of accelerating rates of exotic species introductions in estuaries, and in San Francisco Bay in particular (Cohen and Carlton 1998), the mechanisms of the successful spread of a species following inoculation is of growing interest. In a 2-yr period (1986–1987) P. amurensis spread throughout San Francisco Bay, occupying a variety of intertidal and subtidal substrates (sand, clay, mud, peat, and combinations thereof ) in a wide range of salinities (1 to >30 psu) (Carlton et al. 1990). The spatial success of P. amurensis is acutely relevant in this estuary because P. amurensis is an efficient filter feeder, able to filter food ranging in size from bacteria to copepod larvae (Werner and Hollibaugh 1993, Kimmerer et al. 1994). Several authors (Cloern 1982, Nichols 1985) have suggested mechanisms by which the filterfeeding benthos, primarily bivalves, can control phytoplankton biomass in both South and North San Francisco Bay, and Alpine and Cloern (1992) hypothesized that P. amurensis 1 Support for this project was provided by the San Francisco Bay Toxic Substances Hydrology Program and the San Francisco Bay Place-Based Ecosystem Program. Manuscript accepted 28 November 2001. 2 U.S. Geological Survey, Water Resources Division, Menlo Park, California 94025. Pacific Science (2002), vol. 56, no. 3:329–345 : 2002 by University of Hawai‘i Press All rights reserved 329 is responsible for the disappearance of the annual phytoplankton bloom in the northern bay. The continuing dominance of P. amurensis in San Francisco Bay has also resulted in changes in the San Francisco Bay ecosystem by altering both benthic community structure and interspecific dynamics (Nichols et al. 1990). An important component in the spatially extensive success of P. amurensis is its ability to reproduce and recruit into established benthic communities under highly variable conditions characteristic of the estuarine environment. In this study, we examined connections between patterns of change in the...