Dissolved inorganic carbon dynamics during coccolithophorid blooms in the northeast European continental margin (northern Bay of Biscay), University of Liège

Since, the onset of the Industrial Revolution, anthropogenic emissions have increased the atmospheric CO2 concentration at a rate three times faster than observed during glacial interglacial cycles. The oceans play a critical role in the global carbon cycle as they are by far the largest reservoir of carbon on Earth that regulates atmospheric CO2 concentrations, and has already absorbed 48 % of the anthropogenic CO2 emitted, mainly by fossil fuel burning and cement manufacturing (Sabine et al., 2004). The CO2 that dissolves in the surface waters and exchanges with the atmosphere is regulated by three pumps (physical, chemical and biological) which maintain the vertical gradient in CO2 between the surface ocean and the deep ocean layers. Global simulations predict important changes in the carbon budget due to alterations of the marine organic and inorganic carbon pumps as a result of ocean acidification. Coccolithophores are one of the most important contributors to the inorganic carbon pump. Nevertheless, little is known about the potential changes of these communities in response to increasing CO2 concentration and the feedbacks on climate change. The aims of this thesis are to evaluate the effect of net community calcification (NCC) and net community production (NCP) during coccolithophorid bloom development in the northern Bay of Biscay on seawater carbonate chemistry and in particular on the flux of CO2 across the air-sea interface; to evaluate the diagenetic organic carbon degradation and the potential CaCO3 dissolution in sediments. These results will contribute to a better understanding of the response of marine calcifying ecosystems to ocean acidification and climate change, as well as the associated feedback mechanisms. It was observed that the effect of NCC in decreasing the CO2 sink during the cruises was low (on average ~12 % of the total air-sea CO2 flux). If this is a general feature in naturally occurring phytoplankton blooms in the northern North Atlantic and in the global ocean, then the potential feedback in decreasing or reversing the air-sea CO2 flux from climate change related feedbacks on calcification is probably minor compared to feedbacks related to changes in primary production. At the sediment surface, low organic matter degradation was observed in predominant sandy sediments that were mainly mixed through physical processes. Dissolution of CaCO3 in the sediments was coupled to benthic organic matter degradation (metabolic driven CaCO3 dissolution). However the measured rates were very low compared to those reported by other continental slope studies and represented ~1 % of the pelagic calcification rates due to coccolithophores.