Zhelin Bay is one of the most important bays for large-scale marine culture in Guangdong Province, China. Due to the increasing human population and the expanding mariculture in the last two decades, the ecological environment has changed dramatically, e.g., a large-scale of harmful algal blooms (Phaeocystis) frequently occurred. Although degenerative and eutrophic environment has brought huge economic loss and caged-fish culture has become more and more difficult, few studies were followed. From May 2000, our group initiated an ecological investigation around Zhelin Bay, in which we monthly investigated the phytoplankton, zooplankton, bacterioplankton, nutrients, water temperature, salinity and other biochemical parameters. The present study reports the spatial and temporal distribution of bacterioplankton and their relations with water temperature and dissolved oxygen. Samples were collected monthly between September 2001 and December 2003 at nine sampling stations, and quarterly in 2004 at all stations. Each sampling was scheduled to cover high tide ( 1.5 h). Five liters of water samples were collected with HQM-1 sampling bottles 0.5 m under the surface and 0.5 m above the bottom at each station, and 100ml water from each sample were aliquoted into sterile polythylene bottles, and immediately fixed with 5% formaldehyde (final concentration). Bacterioplankton was stained with Acridine Orange (1g/L), filtered through 0.22 μm polycarbonate filters stained by Irgaian, and counted with a Zeiss Axioplan 2 plus fluorescence microscope (magnification 1000×). Temperature, salinity, and dissolved oxygen were determined in situ with a portable instrument for water quality analysis (YSI6600-02, USA). Relations between bacterioplankton abundances and environmental variables were analyzed by means of the linear fit using software Origin 7.0. The total abundance of bacterioplankton varied from 0.7×10⁴ cell/dm³ to 540.0×104 cell/dm³, with an overall average of 59.4×10⁴ cell/dm³, which is similar to values reported for other eutrophic estuaries environments across the world. There were no significant differences for their overall averages of bacterioplankton between samples collected on the surface (58.8×10⁴ cell/dm³) and at the bottom (60.4×10⁴ cell/dm³). Spatially, total abundance of bacterioplankton was greater in samples collected from the inner than outer boundaries of Zhelin Bay, and also was greater in samples collected from caged-fish culture areas than areas without mariculture. In addition, bacterioplankton abundance showed an increase trend over the years within the investigation period. The findings suggest that mariculture may effect the bacterioplankton dynamics in the long run. Temporally, total abundance of bacterioplankton ranged from 1.6×10⁴ cell/dm3 to 265.0×10⁴ cell/dm³. Total abundance had a unimodal annual pattern and was greastest in summer (from June to August) and least in winter (from the first year December to February of the second year). The abundance of bacterioplankton was positively correlated with water temperature and negatively correlated with dissolved oxygen. It was possible that the low concentration of dissolved oxygen mainly resulted from low density of phytoplankton but not respiration of bacterioplankton in caged-fish culture areas. On the contrary, low dissolved oxygen could lead to the rapid growth of some specific or compatible anaerobic bacteria in cage culture areas.