[en] Coccolithophores, one of the most productive calcifying phytoplanktonic groups, often form massive blooms in the temperate and sub-polar oceans, and in particular at continental margins and in shelf seas. Export of organic carbon and calcification are the main drivers of the biological CO2 pump and are expected to change with oceanic acidification. Coccolithophores are also a major producer of dimethyl sulphide (DMS), whose oxidation products of DMS affect the number and size distribution of tropospheric cloud condensation nuclei, with possible consequences for cloud albedo and heat balance. Coccolithophores are further known to produce transparent exopolymer particles (TEP) that promote particle aggregation and sinking. Coccolithophores play thus key roles in the global carbon, carbonate and sulphur cycles, and, in turn, in climate regulation.
The objectives of the Belgian CCCC project (Role of Oceanic Production and Dissolution of Calcium Carbonate in Climate Change) was to evaluate the role in climate regulation of calcification, primary production and export processes during coccolithophorid blooms. Field investigations, supported by remote sensing data, were conducted in the Northern Gulf of Biscay on the continental shelf and slope region (47°- 50°30'N, 5°-11°W) where coccolithophorid blooms are frequently and recurrently observed. During the cruises in May 2002 and 2003 on board the r/v Belgica, fundamental variables (temperature, salinity, primary production, Chl. a, particulate organic carbon) were measured in the water column. To estimate the spatial variability of particle dynamics in surface waters in relation with the coccolithophorid bloom development, we had employed the natural radionuclide 234Th. The preferential scavenging of the particle-reactive daughter 234Th (t1/2= 24.1 days) while its soluble parent, 238U, remains nearly constant, provides an appropriate tool for assessing temporal variations of the removal of particles from surface waters, at a time scale of weeks.
The two cruises have permitted to sample two different situations; in particular in May 2002, it was possible to sample a well-developed bloom. As a result, 234Th present contrasted profiles in the upper 0-80 m during the two investigations. In May 2003, 234Th was nearly in equilibrium with 238U (its radioactive parent) along with high particulate activities: this seems to indicate an early bloom situation with low grazing fluxes. On the opposite, in May 2002, deficits of 234Th toward 238U were indicating more efficient particle export from upper waters. Synthesis of the acquired data will be discussed to compare particle dynamics and the magnitude of particulate carbon export using 234Th and POC data at different states of coccolithophorid bloom in the Northern Gulf of Biscay.
