BACTERIOPLANKTON GROWTH-ASSOCIATED WITH PHYSICAL FRONTS DURING A CYANOBACTERIAL BLOOM

The main compartments of the microbial food web were studied in a hydr odynamically complicated area to determine the response of bacteriopla nkton to spatio-temporal discontinuities in the water column structure . The samples were divided according to water masses into those repres enting frontal areas and those representing low-saline areas. In the u pper mixed water layer (UML) bacterial production was higher in the fr ontal water (average 4.5 mg C m(-3) d(-1)) than in the low-saline wate r (3.7 mg C m(-3) d(-1)). However, the proportion of bacterial product ion 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 rec orded frontal upwelling event did not drastically change the mode of p roduction from regenerated to new production. Furthermore, the data in dicated that heterotrophic flagellates did not respond to increased ba cterial abundance during the intervals between hydrodynamic events. Be low the thermocline, the turnover time of bacterial numbers was less t han in the UML, as was thymidine incorporation (TdR) per cell, but leu cine 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 thermocli ne. The molar ratio showed an increase in the growth rate during a sto rm event. Leu and TdR methods did not give equivalent rates of bacteri al production over the daily timescale, although they gave quite simil ar estimates when averaged over the whole study period (11 d). Our dat a indicated that one should be Very cautious in using conversion facto rs, which are derived from surface water, to calculate bacterial produ ction throughout the water column, and that sometimes even higher conv ersion factors should be used below the thermocline than in the UML.