ABSTRACT: The Amazon and Orinoco River low-salinity plumes extend >200 km eastwards into the west tropical Atlantic Ocean and Caribbean Sea; combined, they have the greatest discharge of fresh water and suspended sediments to oceans worldwide. However, their impacts upon bacterioplankton assemblages have not been previously described. We investigated bacterioplankton assemblage composition using a sensitive DNA fingerprinting technique—automated rRNA intergenic spacer analysis (ARISA)—over vertical profiles from surface waters to 1200 m depth. Bacterioplankton fingerprint richness and diversity were lowest immediately below the plume at 40 m, but at all other depths were statistically indistinguishable. Surface water bacterial production was significantly and negatively correlated with salinity (r = –0.805, p < 0.001, n = 22), and fingerprint Simpson’s Reciprocal Index (diversity) and the total number of operational taxonomic units (OTU) per fingerprint (richness) were significantly correlated to bacterioplankton production (r = 0.52 and 0.74, respectively, p < 0.05, n = 22), but not to other measured environmental parameters. In both plume and non-plume oceanic waters, assemblages were dissimilar at surface and 40 m, suggesting different selection for taxa occurs vertically at these depths, irrespective of productivity and irradiance. Furthermore, there was no clear relationship between surface water assemblages located within plume waters and those experiencing more oceanic conditions, despite apparently distinct occurrence of certain common (>1% of fingerprint total amplified DNA fluorescence) OTUs in plume, intermediate plume and oceanic conditions. The inconsistency of total assemblage composition within surface plume waters demonstrates that assemblage composition is not tied directly to salinity but possibly to rapid microbial cycling and mixing of assemblages within small water masses. Moreover, our results demonstrate that the Amazon and Orinoco River offshore plumes have significant effects upon surface and underlying water bacterioplankton production in adjacent tropical waters.