The diversity of bacterioplankton or specific components of bacterioplankton assemblages and their distribution in space in time, and response to environmental change, remain incompletely understood. Composition of bacterioplankton assemblages traditionally studied with cultivation-based methods, is now being studied with molecular methods. The approach allows direct characterization of microbial community structure, and has revealed extensive phylogenetic diversity. This dissertation explored bacterioplankton assemblage variation over depth, spatial, and temporal gradients in temperate and polar environments. Specific attention was directed to the planktonic archaea. This group has recently been recognized as a significant member of bacterioplankton assemblages. Relative abundance of planktonic constituents (at the domain level and group specific level) was determined using small subunit ribosomal RNA-targeted hybridization. Ribotype diversity of bacterial and some archaeal assemblages was assayed using denaturing gradient gel electrophoresis. The results suggest that bacterial assemblage composition and relative rRNA abundance of planktonic archaea can vary substantially when environmental conditions show intense seasonality, as seen in Antarctica. Vertically stratified bacterioplankton assemblages were detected within and between the photic and aphotic zones in coastal and open ocean environments in all depth profiles examined. Bacterial assemblage composition and archaeal rRNA abundance were surprisingly stable over short spatial scales in coastal Antarctic Peninsula regions. Larger scale latitudinal transects across Drake Passage indicated that several bacterial ribotypes had distributions coincident with the Polar Frontal boundary. Conversely, approximately 50% of the ribotypes were distributed uniformly across Drake Passage. A 32 month study of Santa Barbara Channel bacterioplankton assemblages showed that bacterial composition was moderately stable, while the relative archaeal rRNA abundance, though quite variable, appeared to be generally uncoupled from the environmental parameters measured. These observations contrast with those for the phytoplankton, which respond directly to upwelling conditions, or increases in daylength. Two phylogenetically distinct archaeal subgroups were found to consistently inhabit distinct zones in the water column where they responded uniquely to environmental conditions. These results suggest that marine bacterioplankton assemblages can respond to biological, physical, and chemical changes in the water column by shifting either (sub)species composition or relative abundance of assemblage.