The heterotrophic bacterial response during the Southern Ocean Iron Experiment (SOFeX)

Oliver, Jacques L., Richard T. Barber, Walker O. Smith, Hugh W. Ducklow

Limnol. Oceanogr., 49(6), 2004, 2129-2140 | DOI: 10.4319/lo.2004.49.6.2129

ABSTRACT: We studied heterotrophic bacterial dynamics as part of the Southern Ocean Iron Experiment (SOFeX), January-February 2002. Two phytoplankton blooms were monitored following infusions with iron sulfate (FeSO4). The first bloom was initiated north of the Antarctic Polar Front Zone (APFZ) in silica-poor waters (North Patch) and was observed sporadically over 42 d, whereas the second was south of the APFZ in silica-rich waters (South Patch) and was continuously observed for 30 d. In both experiments, iron additions resulted in increased chlorophyll a (Chl a), particulate organic matter (POC + PN), and a drawdown of inorganic nutrients. Heterotrophic bacteria responded by increasing their abundance (110% and 60% increases in bacterial abundance in the North and South Patch, respectively, relative to nonenriched waters). Thymidine (TdR) and leucine (Leu) incorporation rates in the North Patch increased by 400% and 120%, respectively, with more modest increases in the South Patch (80% and 70%, respectively). In the South Patch, bacterial production (BP) was significantly correlated with net particulate primary production (PP) and Chl a. Bacterial abundance was also significantly correlated with Chl a. Net bacterial accumulation rate in the South Patch was 0.02 d-1 over 17 d, excluding physical dilution by mixing with water outside the South Patch. The evidence suggests that bacterial growth during Southern Ocean Iron Experiment (SOFeX) was limited by labile carbon rather than by iron. Despite the close association of BP to PP, BP remained a small fraction of PP (<10%).

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