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.