10 pages, 6 figures, 1 tableThe number and relative strengths of trophic linkages in the microbial community of the oligotrophic subtropical North Pacific were studied in 12 experiments from August 1998 to April 1999 at Stn ALOHA (100 km north of Oahu, Hawaii). Collected seawater was manipulated by sequential size-fractionation to truncate the food web at different organism sizes (1, 2, 5, 10 and 20 μm), and the response variable, net bacterial growth rate, was assessed from flow cytometric analyses of the changes in cell abundance (combined heterotrophic bacteria and Prochlorococcus) after 24 h incubations. The corresponding size structure of the protistan grazer assemblage was measured microscopically. With a coefficient of variability of 7% and a 2-fold range overall, total bacterial abundance displayed relatively low temporal variability. Despite the relative constancy of standing stock, however, microbial community interactions varied markedly among the experiments. For experiments conducted at higher levels of bacterial biomass, the bacteria showed little growth response to the removal of predators and may have been resource limited. In contrast, the growth response was highest when conditions were defined by relatively low bacterial biomass and high heterotrophic flagellate biomass. Trophic cascades were evident only at intermediate to high levels of bacterial biomass, and may appear in transitions between high and low levels of bacterial biomass. These results suggest that resources and predators oscillate in importance in regulating open-ocean microbial populations. In such oscillations, the indirect influences of a protistan predatory chain may determine the balance between resource limitation and strong predatory control.This work was supported by National Scienceud Foundation Grant OCE-9218152 to M.R.L. and a post-doctoral fellowship (EX96-46651369) from the Ministerio de Educación y Cultura (Spain) to A.C. Contributions 5344 from the School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, and 659 from the US JGOFS Program.Peer reviewe