ABSTRACT: A significant fraction of the particulate organic material entering the Columbia River estuary (USA) is metabolized or altered before it is carried out to the ocean. Estuarine turbidity maxima are part of the particle-trapping mechanism that lengthens theresidence time of river-borne organic material in an estuary, increasing that material's availability to estuarine bacteria and the estuarine food web. In May and June, 1992, water samples in and around the Columbia River estuarine turbidity maxima wereanalyzed to determine rates of bacterial carbon production, and bacterial and putative bacterivore abundances. Salinity, turbidity, and tidal data were used to interpret bacterial activity patterns, and to identify distributions of bacterial predators.Bacterial carbon production, based on the rate of ³H-thymidine uptake, correlated with turbidity, and varied from 0.3 to 5.6 ug l^-1 h^-1. Sharp peaks in bacterial production were found in the estuarine turbidity maxima, andwere determined to be due to particle-attached bacteria by measuring bacterial production directly on particles. Variation in bacterial production outside the estuarine turbidity maxima seemed to be related to the tidal cycle, supporting hypotheses onparticle cycling in the estuary. Nanoflagellates, small 'oligotrich' ciliates and rotifers were the most numerous grazers in the estuary. Correlation analysis between grazer and bacterial abundances and production suggested that rotifers and smallciliates may be the primary consumers of bacteria outside the estuarine turbidity maxima. Rotifers were enhanced in the estuarine turbidity maxima and therefore may be key consumers of particle-attached bacteria.