REGULATION OF ANNUAL VARIATION IN HETEROTROPHIC BACTERIAL PRODUCTION IN THE SCHELDE ESTUARY (SW NETHERLANDS)

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 sal inity gradient (0.8 to 33 psu). Bacterial production and numbers incre ased 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 wit h the oxygen concentration. On an annual scale the ratio gross bacteri al production:net primary production was larger than 1 in the whole es tuary and increased from the lower to the upper estuary. This indicate s that bacteria processed more carbon than was put into the system by phytoplankton primary production. A comparable situation is found in o ther estuaries and shows the high degree of heterotrophy of these syst ems. Despite the high degree of heterotrophy, temporal variation in ba cterial 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 bact erial production rate during and after the phytoplankton bloom. This i dea is corroborated by the absence of an increase in dissolved organic carbon during phytoplankton blooms in the lower estuary. Due to an ex cess of allochthonous organic carbon and nutrients in the upper estuar y, temperature was the most important factor in explaining the variabi lity of bacterial production in this region.