Bacterioplankton growth associated with physical fronts during a cyanobacterial bloom

The main compartments of the microbial food web were studied in a hydrodynamically complicated area to determine the response of bacterioplankton to spatio-temporal discontinuities in the water column structure. The samples were divided according to water masses into those representing frontal areas and those representing low-saline areas. In the upper mixed water layer (UML) bacterial production was higher in the frontal water (average 4.5 mg C m⁻³ d⁻¹) than in the low-saline water (3.7 mg C m⁻³ d⁻¹). However, the proportion of bacterial production of the primary production was about the same in the frontal water (15%) and in the low-saline (16%) water. The data implied that the recorded frontal upwelling event did not drastically change the mode of production from regenerated to new production. Furthermore, the data indicated that heterotrophic flagellates did not respond to increased bacterial abundance during the intervals between hydrodynamic events. Below the thermocline, the turnover time of bacterial numbers was less than in the UML, as was thymidine incorporation (TdR) per cell, but leucine incorporation (Leu) per cell was highest in the UML. The average molar ratio of Leu to TdR was 7.7 in the UML and 3 below the thermocline. The molar ratio showed an increase in the growth rate during a storm event. Leu and TdR methods did not give equivalent rates of bacterial production over the daily timescale, although they gave quite similar estimates when averaged over the whole study period (11 d). Our data indicated that one should be very cautious in using conversion factors, which are derived from surface water, to calculate bacterial production throughout the water column, and that sometimes even higher conversion factors should be used below the thermocline than in the UML.