Heterotrophic bacterioplankton production (H-3-thymidine incorporation rate) and abundance in the surface water of the Schelde estuary (The Netherlands) were studied during an annual cycle in 1991 along the salinity gradient (0.8 to 33 psu). Bacterial production and numbers increased from the lower to the upper estuary. This spatial variability of bacterial production was positively correlated with concentrations of dissolved and particulate organic carbon and negatively correlated with the oxygen concentration. On an annual scale the ratio gross bacterial production:net primary production was larger than 1 in the whole estuary and increased from the lower to the upper estuary. This indicates that bacteria processed more carbon than was put into the system by phytoplankton primary production. A comparable situation is found in other estuaries and shows the high degree of heterotrophy of these systems. Despite the high degree of heterotrophy, temporal variation in bacterial production in the lower Schelde estuary was characterised by a spring and summer bloom and was correlated with phytoplankton primary production. This apparent contradiction is explained by rapid uptake of labile algal exudates by bacteria and a subsequent increase of bacterial production rate during and after the phytoplankton bloom. This idea is corroborated by the absence of an increase in dissolved organic carbon during phytoplankton blooms in the lower estuary. Due to an excess of allochthonous organic carbon and nutrients in the upper estuary, temperature was the most important factor in explaining the variability of bacterial production in this region. [KEYWORDS: heterotrophic bacterial production; organic carbon; primary production; estuary Phytoplankton primary production; ecosystem model moses; fresh-water bacteria; westerschelde estuary; growth-rate; bacterioplankton abundance; thymidine incorporation; organic-carbon; chesapeake bay; biomass] ; Heterotrophic bacterioplankton production (H-3-thymidine incorporation rate) and abundance ...