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 of 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 of 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 1.3 mol m^-2 for this bloom of E. huxleyi.

KEY WORDS: TIC · fCO₂ · CaCO₃ · POC · Algal bloom · Coccolithophorid

Published in MEPS Vol. 143 (1996) on November 14
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996