Optical and electrochemical measurements characterize different functional fractions of aquatic humic substances

Abstract. The extent to which the metal-binding functionality of dissolved organic matter (DOM) can be inferred from its conventional optical properties remains largely unknown. Optical measurements of dissolved organic matter (DOM), including chromophoric (CDOM) and fluorescent (FDOM) properties, are commonly used as proxies for humic substances, yet it remains unclear whether these approaches also reflect the same metal-binding fraction quantified by recently developed electrochemical techniques. In this study, electroactive humic substances (eHS), CDOM, FDOM, and dissolved trace metals were measured across a humic-rich tropical terrestrial-marine continuum, and the results were compared with a previous dataset from a less humified estuarine system. Principal component analysis revealed that eHS was decoupled from conventional optical indices, including the absorption coefficient at 254 nm (a254) and the humification index (HIX), but was closely associated with dissolved manganese and cobalt. Despite the contrasting environmental settings, a similar separation between eHS and optical DOM properties was observed, indicating this decoupling is a general feature rather than a site-specific phenomenon. These findings show that optical proxies alone do not adequately characterize the fraction of humic substances responsible for trace metal complexation and transport, whereas electrochemical measurements selectively quantify a chemically distinct, metal-reactive fraction of the DOM pool. Integrating optical and electrochemical approaches therefore provides complementary perspectives on DOM composition and functionality, offering a more complete framework for improving our understanding of trace metal-DOM interactions in aquatic environments.