Bacterioplankton productivity in Antarctic waters of the eastern South Pacific Ocean and Drake Passage was estimated by direct counts and frequency of dividing cells (FDC). Total bacterioplankton assemblages were enumerated by epifluorescent microscopy. The experimentally determined relationship between in situ FDC and the potential instantaneous growth rate constant (μ) is best described by the regression equation ln μ = 0.081 FDC − 3.73. In the eastern South Pacific Ocean, bacterioplankton abundance (2 × 105 to 3.5 × 105 cells per ml) and FDC (11%) were highest at the Polar Front (Antarctic Convergence). North of the Subantarctic Front, abundance and FDC were between 1 × 105 to 2 × 105 cells per ml and 3 to 5%, respectively, and were vertically homogeneous to a depth of 600 m. In Drake Passage, abundance (10 × 105 cells per ml) and FDC (16%) were highest in waters south of the Polar Front and near the sea ice. Subantarctic waters in Drake Passage contained 4 × 105 cells per ml with 4 to 5% FDC. Instantaneous growth rate constants ranged between 0.029 and 0.088 h−1. Using estimates of potential μ and measured standing stocks, we estimated productivity to range from 0.62 μg of C per liter · day in the eastern South Pacific Ocean to 17.1 μg of C per liter · day in the Drake Passage near the sea ice
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