Bacterial abundance and production in epipelagic and mesopelagic waters in the Subantarctic and Polar Front zones south of Tasmania

Heterotrophic bacteria influence the carbon export and consequently the efficiency of the biological carbon pump through the remineralization of organic matter. Bacterial remineralization was investigated during the SAZ-Sense cruise (January–February 2007) in the Subantarctic (SAZ) and Polar Front Zones (PFZ) of the Southern Ocean south of Tasmania, by combining bacterial biomass (BB) and bacterial production (BP) measurements in the epipelagic (0–100 m) and mesopelagic (100–700 m) water column. Bacterial carbon demand (BCD) was assessed using different conversion factors and growth efficiencies and was confronted to primary production and carbon export flux estimates. Surface layer bacterial biomass and production were higher in SAZ waters east of Tasmania (SAZ-East) compared to SAZ waters west of Tasmania (SAZ-West), while values at the PF were similar to those for the SAZ-West. At the PF, subsurface maximum values of bacterial production were observed. Bacterial parameters followed chla and dissolved organic carbon distributions. Bacterial abundance, biomass and production drastically decreased below 100–200 m. However, depth-integrated biomass and activity rates revealed that the mesopelagic zone contributed significantly to the upper 700 m water column stocks (41–68% for BB) and rates (10–74% for BP). Highest and lowest contributions of mesopelagic BP to epi-plus mesopelagic water column BP were observed at the PF and in the SAZ-East, respectively. Results show that the SAZ-East region had a poor carbon sequestration efficiency compared to the SAZ-West and the PFZ. Despite some uncertainties in carbon flux estimations and discrepancies between methods the present study highlights the importance of studying bacterial dynamics in the twilight zone because of their significant role in shaping the carbon fluxes through the water column.