Terrestrial-marine continuum of sedimentary natural organic matter in a mid-latitude estuarine system

Purpose: Humic acids (HA) have several environmental roles, but are particularly important in aquatic environments, being recognized as redox active natural organic matter (NOM) components. We examined NOM in recent sediments of a low-energy coastal environment which is free from inputs of dissolved terrestrial HA as their solubility is suppressed by bonding with Ca2+ ions. Our aim is to investigate the contribution of autochthonous versus terrestrial C sources to HA and their fractions along a river-coastal lagoon transect. Materials and methods: Surface sediments were collected along the Aussa River (R), in the central basin of the Marano and Grado Lagoon (L) and within a secluded lagoon fish farm (FF). Extractable NOM components were obtained by extracting sediments first with 0.5 M NaOH (free NOM) and then with 0.1 M NaOH plus 0.1 M Na4P2O7 (bound NOM). Extracts were separated into non-humic and humic fractions by solid phase chromatography. Organic carbon (Corg), total nitrogen (Ntot), δ13C, and δ15N were determined with an Isotope Ratio Mass Spectrometer (Thermo Scientific Delta V Advantage) coupled with an Elemental Analyzer (Costech Instruments Elemental Combustion System). Fourier-transform infrared (FTIR) spectra were recorded with a FT-IR100 PerkinElmer Spectrometer. UV-vis spectra were recorded at pH 7 by a Varian Cary Spectrophotometer. Results and discussion: Both NOM and HA display typical traits of terrestrial origin in river sediments and of a more marine (algal) origin in lagoon and fish farm sediments. This trend is evident in free HA, whereas bound HA seem more influenced by terrestrial inputs. A larger proportion (60–70%) of non-humic C was extracted by NaOH in all samples. Bound HA differ from free HA for their C/N ratios, which are higher and vary within a much narrower range. The changes in HA’s 13C isotopic composition, FTIR spectra, and spectroscopic parameters (SUVA254, SR, and aromaticity) highlight a progressive mixing of terrestrial and marine substrates that either undergo in situ humification or are transported as materials sorbed onto suspended mineral particles. Conclusions: Our results highlight the existence of a complex, but continuous pattern of terrestrial and marine contributions to C sequestration and humification even in transitional environments where allochthonous humic C inputs are restricted due to insolubilization of humic substances by Ca2+. Along the examined transect, the NOM and free and bound HA appear well differentiated. Terrestrial inputs contribute to the bound HA fraction via transported mineral particles in all the samples, no matter the environment encountered.