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 (f CO2), as well as standing stocks of CaC03 and particulate organic carbon (POC). Production of CaC03 by E. huxleyi resulted in an immediate increase of f C 0 2 , but led to a long-term decrease in fCO, Observations during a surface survey and at 24 h stations showed a large increase of fCOz with the standing stock of CaCO,. The immediate increase of f C 0 2 is caused by a shift in the chemical equilibria In the inorganic carbon system when alkal~nity decreases relative to dissolved inorganic carbon (DIC). Average f C 0 2 in the high reflectance area (with high numbers of detached coccoliths) was lower than average fCOz In the reference areas, located outside the E. huxleyi bloom. The long-term decrease in fCO, is due to an enhdnced 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 surf~lce mixed layer and the aphotic zone, which increased from the POC-rich zone to the CaCO, maximum. The overall effect of production, air-sea exchange, mineralisation and sedimentation was a decreabr of f C 0 2 due to a net transport of carbon to below the pycnocline. We tentatively calculate an atmospheric carbon sink of 1.3 m01 m-2 for this bloom of E. huxleyi.