The magnitude of spring bacterial production in the North Atlantic Ocean

Ducklow, H. W., D. L. Kirchman, T. R. Anderson

Limnol. Oceanogr., 47(6), 2002, 1684-1693 | DOI: 10.4319/lo.2002.47.6.1684

ABSTRACT: Dissolved organic carbon (DOC), a major reservoir in the ocean carbon cycle, is produced by a profusion of plankton sources and processes but is consumed mainly by bacterioplankton. Thus bacterial metabolism regulates the entry of DOC into the longer scale global carbon cycle. Bacterial production (BP) is the routinely measured quantity for evaluating the roles of bacteria in carbon cycling. However BP cannot be measured directly and instead is estimated from related metabolic processes requiring the use of poorly constrained conversion factors. BP, and thus the total carbon utilization, are potentially uncertain by a factor of two or more. In the North Atlantic Bloom Experiment (NABE), BP was estimated to be about 30% of the simultaneous particulate primary production (PP), with some daily estimates exceeding 50%. Here we reassess these estimates, synthesizing knowledge and understanding of plankton dynamics gained since the 1989 NABE study. Daily BP derived from six different conversion factors averaged 20% of PP but ranged from 3 to 68%. The coupling of BP to PP was not consistent with either short-term cycling of labile DOC (hours) nor with much longer term cycling of semilabile DOC (seasons). Trophodynamic processes, including release of DOC from phytoplankton, by themselves could have maintained BP at about 15% of PP. Use of decomposing POC or previously accumulated semilabile DOC could each have supported some additional increment of BP for brief periods. Both reconsideration of observations and model results indicated that higher estimates of BP exceeding 20% of PP could not be supported without extraordinary and prolonged inputs of allochthonous carbon. Recent assertions of high BP in the tropics and other oceanic regimes should be considered carefully, especially if external subsidies are not obvious.

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