Tracing chemical diversity in dissolved organic matter from freshwaters to marine environments

Abstract

Dissolved organic matter (DOM) shapes carbon cycling, nutrient availability, and contaminant transport in aquatic systems through its proton and metal binding properties. Here we investigate seasonal variation in DOM heterogeneity and proton binding affinity along the Ebro River, from its headwaters to the Mediterranean Sea, revealing clear spatial and temporal shifts in composition. River samples contained higher concentrations of low-affinity binding groups (94.9 ± 0.4 to 133.6 ± 0.4 mmol·mol C–1) compared with the marine endmember (84.9 ± 0.2 mmol·mol C–1). They also showed a broader range of high-affinity groups, though at lower concentrations (14.1 ± 0.2 to 33 ± 2 mmol·mol C–1), encompassing marine values (23 ± 2 mmol·mol C–1). Organic contributions to total alkalinity ranged from 2.5 to 15.6 μmol of binding groups per L of water sample. Seasonal patterns were linked to greater heterogeneity of proton binding sites in headwaters, while downstream processes, including groundwater inflows, terrestrial inputs, and degradation, reduced both the abundance and diversity of oxygen-rich groups. Overall, the findings are consistent with the River Continuum Concept in this watershed, capturing the downstream loss of acidic functional groups and the associated reduction in binding site diversity.

We gratefully acknowledge Ana Hernández, Arne Bratkič, and Andrea B. Roselló for their valuable support with riverine sampling. We also extend our thanks to the team of the Observatorio Submarino Cableado (OBSEA), led by Joaquín del Río, for their assistance with marine sampling operations. Special thanks are due to Gustavo S. Arrial for producing the land cover map of the Ebro River Basin. This research was supported by funding from AGAUR FI 2021 (expedient number: 2022 FI_B 00172). We thank Agencia Española de Investigación for the financial support through the research project MCIN/AEI/10.13039/501100011033 (grant numbers PID2020-117910GB-C21, PID2022-140312NB-C21, and PID2024-156219NB-C21). P.L. also acknowledges support by the Ministerio de Ciencia, Innovación y Universidades of Spain, and the University of Lleida (Beatriz Galindo Senior award number BG20/00104).