Trends in inorganic and organic carbon in a bloom of Emiliania huxleyi in the North Sea

Abstract

During a survey of the end phase of an Emiliania huxleyi bloom in the northern part of the North Sea we measured total inorganic carbon (TIC) and the fugacity of CO2 (fCO2), as well as standing stocks of CaCO3 and particulate organic carbon (POC). Production of CaCO3 by E. huxleyi resulted in an immediate increase of fCO2, but led to a long-term decrease in fCO2. Observations during a surface survey and at 24 h stations showed a large increase at fCO2 with the standing stock of CaCO3. The immediate increase of fCO2 is caused by a shift in the chemical equilibria in the inorganic carbon system when alkalinity decreases relative to dissolved inorganic carbon (DIC). Average fCO2 in the high reflectance area (with high numbers of detached coccoliths) was lower than average fCO2 in the reference areas, located outside the E. huxleyi bloom. The long-term decrease in fCO2 is due to an enhanced sedimentation of both organic and inorganic carbon in faecal pellets containing heavy calcite. This enhanced sedimentation is reflected in the vertical gradient of TIC between the surface mixed layer and the aphotic zone, which increased from the POC-rich zone to the CaCO3 maximum. The overall effect el production, air-sea exchange, mineralisation and sedimentation was a decrease of fCO2 due to a net transport of carbon to below the pycnocline. We tentatively calculate an atmospheric carbon sink of l.3 mol m-2 for this bloom of E. huxleyi.