Abstract
We report depth‐resolved in situ time series of the partial pressure of CO2 (pCO2) and other carbon‐related parameters spanning the development and decline of a high‐latitude phytoplankton bloom. A suite of sensors was deployed on a mooring in the Labrador Sea from June to August 2004. The study became quasi‐Lagrangian when the mooring broke free in late June. Measured parameters included pCO2, chlorophyll a fluorescence, beam c, optical backscatter, and photosynthetically active radiation. During the bloom, the pCO2 was drawn down from 330 to 260 μatm, corresponding to a 70 μmol kg−1 decrease of dissolved inorganic carbon (DIC). One‐dimensional model results suggest that the observed drawdown was primarily driven by local processes and contributions from horizontal advection were minimal. A mass balance using the DIC and particulate organic carbon found that 47 mmol C m−2 d−1 of DIC was assimilated into biomass. The bloom biomass was not remineralized in the mixed layer but was rapidly exported below 35 m within 15 days of the bloom. As a consequence, the large air‐sea pCO2 gradient persisted, and approximately 30% of the DIC was regained through air‐sea exchange by the end of the study. It is likely that all of the exported organic matter, corresponding to 5.4 ± 1.9 Tg of carbon, was replaced by atmospheric CO2 prior to the onset of deep convective mixing.
