5 sampling campaigns were carried out on the Côtes de la Manche vessel (CNRS/INSU) between January 2015 and April 2016. These campaigns covered the different parts of the estuary: upstream (Rouen / Caudebec), in the MTZ (zone of maximum turbidity, between Fatouville and Tancarville) and downstream of the latter (Honfleur or La Carosse). Water samples (about 500 for all the campaigns) and sediment samples (8 cores of 10 cm) were taken in each part of the estuary. The characteristics of OM in its different forms (dissolved - DOM; particulate - POM; sedimentary) were determined at global and molecular level. The principal component analysis carried out from these data showed a strong variation of the OM signature between the compartments, the composition of the DOM differing significantly from that of the POM and sedimentary OM. The 14C dating of the samples also revealed the recent age (> 1950) of the DOM and the POM, unlike the sedimentary MO, whose age (up to 1000 years B.P. -Before Present-) depends strongly the season and the associated hydroclimatic conditions. The spatial variability of OM characteristics along the estuary has also been highlighted. Thus, the mixing of fresh and marine waters appears to be the main factor controlling the composition of the OM in the estuary, as revealed by (i) the linear increase in delta13C of the POM from upstream to downstream and (ii) the concomitant decrease in the molecular diversity of DOM (ca. 5000 molecules upstream vs. 2700 downstream). Furthermore, the non-linear dilution of the colored and/or fluorescent DOM along the estuary showed the highly dynamic nature of the DOM, which does not only result from a mixture of fresh and marine waters, but undergoes transformations from upstream to downstream and is subject to various inputs. The combination of isotopic and molecular analyzes of the OM also indicates a mainly autochthonous origin of the latter in the estuary, even if compounds of terrigenous origin could be detected in the different samples. Finally, seasonal variations in the composition of OM have, to a lesser extent, been observed, with spectroscopic analyzes showing for example that DOM is more aromatic and has a higher mass in winter than in summer.

In parallel, our results also showed that the nutrients and a large part of the trace elements had a conservative behavior in the estuary. Different biogeochemical processes transform nutrients by degrading OM. We sought to inventory the activities and microorganisms associated with biogeochemical cycles in the water and sediments of the estuary, with a particular focus on the nitrogen cycle. The water column and in particular the MTZ shows a disappearance of nitrate, a production of N2O and a very high abundance of denitrifying bacteria. This indicates denitrifying activity in this overall oxic environment and suggests the presence of anoxic microniches in the suspended solids of the water column to allow this process to take place. The rate of nitrate reduction is much higher (factor 1000) in sediments due to more favorable conditions, namely the presence of organic matter and the absence of oxygen. It follows that the sediments are composed as sinks of NO3-. The degradation of OM in the sediments leads to a flux of ammonium emitted from the sediments.

The analyses of organic contaminants showed that the light PAHs were preferentially present in the dissolved phase and the heavier ones in the sediment and the particular phase, and that their concentration in the water was strongly linked to the tide (decrease by a factor 2 to 3 between high tide and low tide). Conversely, the PAH content in the particular phase is generally constant over the periods studied. In parallel, a screening of pharmaceutical compounds was carried out on a panel of samples, making it possible to identify 8 "tracer" molecules, triggered as representatives of all the compounds. The latter are weakly concentrated in the sediments and in the particular phase, and were mostly (99%) detected in the dissolved phase. The concentrations of pharmaceutical compounds are generally stable upstream and in the mud plug, and decrease only at the mouth. Moreover, they are highly dependent on the tide (low tide vs high tide) and hydrological conditions (flow and water level). Thus, in Fatouville and Honfleur, the concentrations of pharmaceutical compounds can vary by up to 80% in a few hours during periods of low flow (measured at Poses) and high tidal coefficient.

Finally, the dynamics of OM, contaminants and nutrients were jointly examined. We have shown that both qualitative and quantitative variations of DOM directly influence the concentrations of pharmaceutical compounds and PAHs in the dissolved phase. Certain constituents of DOM interact more specifically with organic contaminants. OM-organic contaminant interactions depend on the nature of the OM and the contaminants. The dynamics of DOM and organic contaminants, interdependent, show a strong seasonal variability. Furthermore, the PAH concentration, normalized to the organic carbon content of the OM, decreases along the estuary in the particulate phase and increases in parallel in the sediment. An accumulation of PAHs is therefore observed in the sedimentary compartment at the mouth of the Seine estuary. Similarly, the dynamics of OM and that of nitrogen compounds are closely linked. The estuary thus has a strong potential to purify the high loads of nitrogen it contains, as consistently shown by the numbers of denitrifying microorganisms, the nitrate reduction rates and the high levels of OM metabolizable by the microorganisms. organisms.

In conclusion, we showed, through distinct and complementary approaches, the close interactions between the dynamics of OM, contaminants and nutrients in the water column and the sediment. The numerous results , grouped together in a database available to the scientific community via the GIP Seine-Aval, can be used to feed the models of biogeochemical dynamics of the Seine estuary.

The main available results are presented in the MOSAIC project report: https://www.seine-aval.fr/wp-content/uploads/2017/01/RR-SA5-MOSAIC.pdf

Synthesis diagram illustrating the dynamics of organic matter in its different forms (dissolved, particulate and sedimentary) in the Seine estuary and variation of the main associated global descriptors. Derived from the report of the GIP Seine-Aval MOSAIC project.