Carbon dioxide (CO₂) is the most important anthropogenic greenhouse gas in terms of radiative forcing. The marine carbon cycle is a key component of the climate system, since the ocean constitutes one of the major sinks for anthropogenic CO₂ emitted to the atmosphere. Conversely, the uptake of CO₂ during the past 200 years has led to the acidification of the surface ocean and could have major impacts on the ocean biogeochemical carbon cycling and ecosystem dynamics. The resulting positive and negative feedbacks on climate change are presently poorly understood.

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), which is only produced by marine ecosystems. 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 related processes such as marine snow formation and sinking. In addition, owning to its mineral ballast effect, biogenic calcium carbonate (CaCO₃) contributes to the export of organic carbon from surface ocean to deep waters. Coccolithophores play thus key roles in the global carbon, carbonate and sulphur cycles, and, in turn, in climate regulation. Nevertheless, little is known about the potential changes of these communities in response to increasing CO₂ concentration and the feedbacks on climate change.

The overall objective of the present project is to evaluate the role in climate regulation of calcification, primary production and export processes during coccolithophorid blooms. We will use a transdisciplinary approach that combines process-oriented field investigations with laboratory experiments and modelling tools.

Specific objectives are:
1) to study the net ecosystem dynamics during coccolithophorid blooms,
2) to unravel the link between the bacterial community, grazing, TEP dynamics, carbon export and DMS cycling during coccolithophorid blooms,
3) to assess the effects of ocean acidification on coccolithophorid metabolism and TEP production, and
4) to model coccolithophorid dynamics and their impact on ocean dissolved inorganic carbon (DIC) chemistry.

The project will be coordinated by the Université Libre de Bruxelles (ULB), Laboratoire d'Océanographie Chimique et Géochimie des Eaux, who has the expertise in biogeochemical cycles of organic carbon, carbonate and nutrients in freshwater, estuarine and marine environments. ULB also has a long experience in coordinating interdisciplinary national and EU projects. The network includes three other partners: 1) Université de Liège (ULg), Unité d'Océanographie Chimique, who has a broad expertise in the inorganic carbon cycle and related air-sea CO₂ fluxes in the coastal zone and open ocean and in biogeochemical modelling, 2) Universiteit Gent (UGent), Protistologie & Aquatische Ecologie, who has a long-standing experience in ecophysiological, molecular and genetic investigations of protist groups, and 3) Alfred Wegener Institute for Polar and Marine Research (AWI), Global change and the future marine carbon cycle group, who has an extensive experience in marine carbon cycle and particularly in marine aggregate studies.

Field investigations, supported by remote sensing data, will be conducted in the Northern Gulf of Biscay (one of the main coastal European marine areas) where coccolithophorid blooms are frequently and recurrently observed. This region has been visited by the Belgian biogeochemistry community since the late 1980s within the framework of the PPS Science Policy "Global Change" and SPSD-II "Climate" programmes, and the EU OMEX I and II projects. Long-term series of physical, biological and chemical variables are available for model validation. A suit of fundamental physico-chemical variables (temperature, salinity, pH, total alkalinity, dissolved organic and inorganic carbon, oxygen, nutrients) will be measured in the water column. In addition, during both field and laboratory studies, attention will be paid to determine key parameters of calcification and associated processes such as algal characterization and bacterial community structure and diversity, rate of organic and inorganic carbon production, degradation and export, and air-sea exchange of CO₂ and DMS. The role of TEP in CO₂ sequestration during coccolithophorid blooms will be evaluated as well.

Synthesis of the acquired data and future projections in relation to increasing pCO₂ and ocean acidification will be achieved through a biogeochemical model that will explicitly describe the DIC and coccolithophorid dynamics (primary production, calcification, CaCO₃ and organic carbon export). The model will be specifically tuned with the newly and previously acquired field and laboratory data (mesocosm and field experiments) and will be coupled with a hydrodynamic model of the region.