Dynamics of dissolved and particulate organic matter along the land-sea continuum of the Seine Estuary (France)

Estuaries are critical zones from ecological, economical, and biogeochemical points of view and play an important role in regulating Dissolved Organic Matter (DOM) and Particulate Organic Matter (POM). To date, estuarine DOM and POM were mostly studied separately, hampering our understanding of their overall dynamics and associated estuarine biogeochemical functioning. The aim of this PhD thesis was to determine the sources, transformations and fate of estuarine POM and DOM in the Seine Estuary (North Western France). To this aim, the spatiotemporal variations of POM and DOM characteristics were determined using water samples (n=383) collected along the land-sea continuum of this estuary during 24 sampling campaigns from June 2019 to November 2022. First, the POM dynamics was investigated at the bulk and molecular scales, through elemental and isotopic analyses as well as lipid biomarkers. A novel molecular proxy (Riverine IndeX, RIX) based on bacterial membrane lipids was developed to trace the riverine POM inputs into estuaries. POM is a mixture of molecules from different sources (terrestrial, algal/microbial, anthropogenic), which showed distinct dynamics along the Seine estuary, highlighting that the complex transformations to which POM is subjected is closely dependent on its composition. Furthermore, DOM properties were investigated by optical techniques (UV–Visible absorbance and Excitation-Emission Matrix fluorescence spectroscopy) coupled with unsupervised and supervised machine learning. This led to the development of a model disentangling the complexity of DOM and capturing specific characteristics of the latter in different zones of the Seine Estuary, with varying levels of molecular weight, aromaticity, and autochthonous material. Such DOM signatures are not effectively identified using traditional approaches. Finally, this PhD work shows that DOM and POM dynamics are decoupled along the Seine land-sea continuum and that the estuary controls the transport and transformation of various constituents of organic matter, which are linked, in particular, to hydroclimatic conditions and land use.