Sea surface pC0₂ and carbon export during the Labrador Sea spring-summer bloom: An in situ mass balance approach

We report depth-resolved in situ time series of the partial pressure of C0₂ (pC0₂) 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 pC0₂, chlorophyll a fluorescence, beam c, optical backscatter, and photosynthetically active radiation. During the bloom, thepC0₂ was drawn down from 330 to 260 juatm, corresponding to a 70 /xmol 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 pC0₂ 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 C0₂ prior to the onset of deep convective mixing.