The jury will be composed of :
Dr Goulven Gildas Laruelle, Université Libre de Bruxelles, Reviewer
Dr Patrick Raimbault, MIO, Reviewer
Pr Eric Thiébaut, SBR, Examiner
Dr Matthieu Waeles, UBO, Examiner
Dr Yann Bozec, SBR, PhD director
Dr Pascal Morin, SBR, PhD co-director
The anthropogenic impact of the raise of atmospheric CO2 was observed on the global oceanic scale, resulting in the Ocean Acidification (OA). OA has major effects on ecosystems, particularly on organisms using calcium carbonate (CaCO3) for their shells and skeletons. Largely present in the coastal ecosystems, a decrease of their population could have significant socio-economic consequences. However, coastal ecosystems was low documented, representing only 7% of the global ocean. Representing the third of the total primary production in the oceans, its play a key role in the global carbon cycle. They are characterized by heterogeneous conditions influenced by continental inputs, which complicates the study of the carbonate cycle and the air-sea exchanges of CO2. This PhD thesis investigated at different spatial and temporal scales the variability of the carbon cycle in megatidal environments of the North Western European Shelves (NWES). From 2015 to 2019, we installed an autonomous sensor of dissolved CO2 (Sunburst SAMI-CO2) on a cardinal buoy located on the coast of Roscoff, in the south of the English Channel. Coupled with the proximal and offshore observations of the carbon cycle and the physicochemical parameters, we were able to describe precisely this ecosystem and discuss the tidal, diurnal and interannual variability. Secondly, we followed the variability of these parameters at the decadal scale, based on regular samples from 2008 to 2018 in two coastal environments very close geographically (Brest and Roscoff, NWES), but more or less influenced by the rivers. Finally, we ascended the coast-estuary-river gradient in order to characterize the influence of river inputs to the coastal environments and to understand the consequences of the soil modification (asphalting, agricultural activities…) on the export of elements. We followed the cycle of two dissolved Climatically-Actives Gas (CAGs): the carbon cycle coupled to the methane cycle. Methane remained low studied, but playing an increasingly key role in the understanding of the coastal ecosystem functioning.
From the tidal to decadal scale, from the open sea to the river, between several coastal ecosystems and following several dissolved CAGs, this PhD thesis provides a very complete overview of the complexity of the carbon cycle in the coastal ecosystems. In addition, this thesis showed the importance of the development of observation networks at different scales in the understanding of the influence of CAGs.