Limnol. Oceanogr., 45(3), 2000, 695–702

We collected samples monthly, from April to August 1998, to measure the abundance of autotrophic picoplankton in San Francisco Bay. Samples taken along a 160-km transect showed that picocyanobacteria (Synechococcus sp.) was a persistent component of the San Francisco Bay phytoplankton in all the estuarine habitats, from freshwater to seawater and during all months of the spring–summer transition. Abundance ranged from 4.6 3 106 to 5.2 3 108 cells L21, with peak abundance during the spring bloom (April and May) and during July with a persistent spatial pattern of smallest abundance near the coastal ocean and highest abundance in the landward domains of the estuary. The picocyanobacterial component (as estimated percentage of chlorophyll a concentration) was, on average, 15% of total phytoplankton biomass during the summer–autumn nonbloom periods and only 2% of chlorophyll biomass during the spring bloom. This result is consistent with the emerging concept of a gradient of increasing importance of picocyanobacteria along the gradient of decreasing nutrient concentrations from estuaries to the open ocean. For two decades now biological oceanographers and limnologists have explicitly recognized the importance of micron-sized phytoplankton (picoplankton) as components of the autotrophic communities of pelagic systems. The picoplankton, predominantly coccoid cyanobacteria (Synechococcus sp., Johnson and Sieburth 1979; Waterbury et al. 1979), can be major contributors of phytoplankton biomass and production in the oceans (Joint 1986; Olson et al. 1990) and lakes (Stockner 1988). The size distribution of the phytoplankton, and in particular the partitioning between picoplankton and larger cells, is a fundamental aspect of pelagic systems that (a) reflects the source and cycling of nutrients, and (b) influences the pathways through which production is transferred to consumers. In general, we associate the picoplankton with low-nutrient conditions where primary production is sustained by regenerated nutrients (Chisholm 1992); picoplankton production is first transferred to consumers by protozoan grazing since most metazoans cannot effectively capture micron-sized algal cells (Tamigneaux et al. 1995; Vaquer et al. 1996). On the other hand, we associate the larger phytoplankton (especially fast-growing diatoms) with high-nutrient conditions where primary production is sustained by inputs of new nutrients; trophic transfer of large-cell production begins with metazoan grazing, and some fraction of this production is exported by sinking. The distinction between picoplankton regenerating systems and large-cell new-production systems results, in part, from the competitive advantage of small cell size under conditions of resource limitation (Raven 1986; Riegman et al. 1993). This competitive advantage disappears under highnutrient conditions because the picoplankton population growth is tightly regulated by the fast-growing protozoan consumers (Ning and Vaulot 1992), whereas the larger cells have (at least temporary) refuge from predation by the slower-growing metazoan grazers (Malone 1992; Riegman et al. 1993). Therefore, inputs of new nutrients tend to promote net population growth and biomass accumulation of larger cells (Malone 1992). As a result of these differences in sizerelated growth and grazing rates, the picoplankton component of production is highest in the oligotrophic regions of the ocean (Joint 1986; Chisholm 1992). The picoplankton component also increases in regions (Joint 1986; Ning et al. 1996), and during seasons (Malone 1992; Li 1998) of high water temperature because the picoplankton have a stronger growth response to temperature variability than the larger eucaryotic cells (Andersson et al. 1994). So, the size-related aspects of pelagic primary production and trophic transfer seem to be determined largely by the nutrient-temperature regime (Malone 1992). This principle would suggest that estuaries, which have continual inputs of exogenous nutrients from their watersheds, might act as new-production systems that tend to favor production of large cells (Riegman et al. 1993). In fact, Iriarte and Purdie (1994) have proposed that phytoplankton size distribution changes along the eutrophication gradient from the land margin to the open ocean, with the picoplankton contribution . 50% offshore, ;20% in the coastal ocean, and ,10% in estuaries. The few studies of estuarine picoplankton ecology are generally consistent with this hypothesis, although there are exceptions such as the Thau Lagoon (France) where the picoplankton contribute nearly 40% of primary production (Vaquer et al. 1996). This special case might be explained by the unusual intensity of (size-selective) suspension feeding by oysters reared in this lagoon. Therefore, the balance between picoplankton and larger-cell production in estuaries might be determined by a combination of nutrient/temperature-driven differences in growth rate and the strength of grazing by benthic/epibenthic suspension feeders that typically select larger cells. San Francisco Bay as a gradient of estuarine habitats— Here, we present results of a study designed to measure the abundance of the picocyanobacteria (Synechococcus) in San Francisco Bay as an example of a nutrient-rich, temperatezone estuary in which phytoplankton dynamics are influenced by the benthic suspension feeders. San Francisco Bay has been a site of sustained estuarine research for three de-