Bacterial activity during early winter mixing (Gulf of St. Lawrence, Canada)

Bacterial dynamics were examined in early winter in the Gulf of St. Lawrence (Canada) at a time of rapid changes in water column properties. At all stations, bacterial production as measured by [methyl-H-3]-thymidine (H-3-TdR) was significantly correlated with chl a and with the number of Actively Respiring Cells (ARCs). ARCs were defined as metabolically active bacteria which reduced the tetrazolium salt 5-cyano-2, 3-ditolyl tetrazolium chloride (CTC) to red fluorescent, water-insoluble formazan crystals that were detected by epifluorescence microscopy. There was little change in the total bacterial concentrations determined with DAPI (4',6-diamidino-2-phenylindole) and no significant correlation between the thymidine production values and DARI counts. ARCs represented ca 1 to 2% of total bacterial cell counts in the surface mixed layer when the water column was stratified, falling to less than 0.1% when there was deep mixing. These observations suggest that only a small fraction of the total bacterial biomass is metabolically active at any one time. These findings support the view that the relatively constant total bacterial cell concentrations in the sea are determined by loss rather than growth processes.