Distribution, speciation and composition of humic substances in a macro-tidal temperate estuary

• Published in: Estuarine, coastal and shelf science Vol. 255; p. 107360
• Main Authors: Riso, Ricardo, Mastin, Manon, Aschehoug, Arthur, Davy, Romain, Devesa, Jeremy, Laës-Huon, Agathe, Waeles, Matthieu, Dulaquais, Gabriel
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 05.07.2021; Elsevier
• Subjects: Biogeochemistry; Estuary; Humic substances; Sciences of the Universe; Suspended particulate matter; Biogeochemistry; Suspended particulate matter; Humic substances; Estuary
• ISSN: 0272-7714; 1096-0015
• DOI: 10.1016/j.ecss.2021.107360

In aquatic environments, the term humic substances (HS) encompasses terrestrial and autochthonous refractory organic matter. HS are one of the main fractions of natural organic matter and are important chelators of trace elements. In environmental studies, the determination of HS is often restricted to the dissolved fraction, and the content of HS in the suspended particles remains unknown. In this work, we present the composition and dynamics of HS along the mixing gradient of a macrotidal estuary in both the dissolved and particulate fraction. After the isolation of particulate HS using a solid-liquid alkali extraction method, HS were characterized by size exclusion chromatography (SEC) and electrochemical methods. The method, validated using a certified reference material, demonstrated a low detection limit (μg-C L−1), a good repeatability (7.7%) an excellent reproducibility (1.3%) and was poorly prone to contamination (filter blank < 1 μg-C). Analyses of environmental samples showed a particulate fraction contributing significantly to the total humic pool in the estuary (3–20%) and representing up to 35% of electroactive HS. Phase transfers from the dissolved to the particulate fraction were observed and the electroactive HS were strongly affected in the maximum turbidity zone. Multi-detection SEC analyses showed differences in the molecular composition between the dissolved and particulate fractions. Particulate HS were more nitrogen-rich and electroactive than dissolved HS. The non-conservative changes in the composition of HS along the land-sea continuum evidence the existence of abiotic and biotic processes that altered HS during their transit from river to marine waters. [Display omitted] •A simple method for the determination of humic substances in suspended particulate matter is developed and validated.•The method allows the quantification and the characterization of particulate humic substances along the entire land-sea continuum of a temperate estuarine system.•Internal abiotic and biotic estuarine processes alter the chemical composition of humic substances during estuarine mixing.