Estuaries are key ecosystems from ecological and economical points of view, making their protection a real societal concern. The quality and amount of organic matter (OM) exert a key role on water quality and biogeochemical processes in estuaries. It is thus essential to investigate the nature and characteristics of OM in these ecosystems, which condition the transfer of both natural and anthropogenic substances from continents to oceans. Nevertheless, to date, the elucidation of the OM dynamics in estuaries remains a huge challenge due to (i) the heterogeneous and complex nature of this material, constituted of a large variety of compounds of different size and physico-chemical properties and (ii) the high variability of environmental (e.g. light, salinity) and physical (river discharge, tidal currents, resuspension, …) processes in estuaries. Aquatic OM consists of particulate (POM) and dissolved (DOM) organic matter, which are commonly separated by filtration. The low concentration of DOM in estuaries (few mg/L) coupled to the high salinities observed in marine environments (ca. 35 g/L) makes the characterization of the estuarine DOM especially challenging.
This presentation will present some of the methods which can be used to constrain the sources and fate of estuarine OM, by taking the example of the Seine Estuary (France), which flows through Paris, one of the largest European megalopolis. 5 sampling campaigns were performed between January 2015 and April 2016, allowing the investigation of OM spatiotemporal variability. Water samples and sediment cores were collected along the estuary. The dynamics of the OM along the Seine Estuary was investigated by comparing the bulk and molecular characteristics of the OM in the different compartments – dissolved (DOM) and particulate (POM) organic matter and sediment. Such characterization was performed by combining (i) elemental and isotopic analyses, (ii) optical characterisation of DOM and (iii) state-of-the-art molecular analyses (13C solid state NMR; pyrolysis -GC-MS; HPLC- and GC-MS lipid analyses). Principal component analysis performed on bulk and molecular data showed that DOM composition significantly differs from that of sediment OM and POM. 14C dating revealed the recent age of DOM and POM, in contrast with sedimentary OM, whose age, in the surface layer of sediment cores, strongly depends on the season and related hydroclimatic conditions. The mixing of riverine and marine water masses seems to be the main factor controlling the OM composition in the estuary. Moreover, the non-linear dilution of the chromophoric DOM along the estuary confirmed the mixing of marine and freshwater end-members, but also highlighted other sources or transformation processes of DOM. Both isotopic and molecular analyses pointed to a predominant autochthonous origin of the OM in the estuary, even though terrestrial-derived compounds were also detected.
In conclusion, the OM pool (DOM vs. POM and sedimentary OM) and the OM origin (marine vs. fresh waters) are the main factors controlling the OM composition in the Seine Estuary. Our work emphasizes the necessity to combine complementary bulk and molecular techniques to better understand the complex estuarine OM dynamics.
Dr. Arnaud Huguet is a CNRS research associate from Sorbonne Université (Paris, France) with an habilitation to supervise research (HDR). He started his scientific education by an Engineer Diploma in Chemistry in Montpellier (France) and MSc in Chemistry at the National University of Singapore. He then made his PhD in Analytical Chemistry and Environment at the University of Bordeaux (France), where he worked on the development of new methods of isolation and characterization of dissolved organic matter in coastal estuarine and marine environments. He then moved to Paris for a postodoc and was interested in the analysis of microbial membrane lipids in various environmental settings. He obtained a permanent position in Paris in 2010, developing his expertise in organic biogeochemistry, and more precisely the characterization of natural organic matter in aquatic and terrestrial environments.