Optical properties of dissolved organic matter play pivotal roles in mediating trace metal forms in a mangrove-dominated tidal estuary.

Dissolved organic matter (DOM) could alleviate aquatic metal toxicity by the altering metal speciation. The origin and compositions of DOM are sensitive to coastal vegetation, which may alter the DOM-metal interactions and thus the metal speciation in aquatic environment. This study aims to assess if coastal land covered by mangrove forests mediate dissolved fraction of trace metals by altering the DOM optical characteristics in a tropical estuarine lagoon ecosystem. In situ water quality sondes and discrete field samplings were conducted to inspect the linkage between the optical DOM properties and dissolved metal fractions between sites. DOM properties were analyzed using ultraviolet-visible absorbance and fluorescence spectroscopy with excitation emission matrix analysis. Comparing to the offshore site, trace metals concentrations were 17.4 % to 54.0 % higher in the mangrove site; however, dissolved fractions of all metals showed 3.8 % to 13.8 % lower. Results show that the DOM pool characterized by a mixture of lower molecular weight (i.e., higher SR values, 5.62 ± 1.47), higher aromaticity (SUVA254, 4.39 ± 1.01) and humification (HIX, 0.73 ± 0.05) organic matters near the mangrove site. Meanwhile, the higher degree of recently produced autochthonous DOM (i.e., BIX, 1.19 ± 0.18 and β/α, 1.12 ± 0.14) in the offshore site. These site-specific optical properties were compelling proxy that links the lagoon DOM properties to the dissolved fraction of trace metals between the sites, with/without the effect of mangroves. This study reveals that coastal vegetation cover, such as mangrove forests might play a pivotal role in ruling the trace metal forms in coastal waters, as reflected by the DOM origin and quality entering and forming in the coastal ecosystems. Results highlights the necessity of assessing aquatic metal toxicology at landscape-scale by considering the nearby coastal vegetations. This is particularly true when assessing site-specific exposure risks and when considering the circumstances of environmental and climate changes. CONTRIBUTION TO THE FIELD: Metal contaminants are hazardous, toxic, and ubiquitously distributed environmental pollutants. Issues on metals remain a global concern because they are ubiquitously distributed throughout the environment and can biomagnify through the food chain to harmful levels. The variations in metal bioavailability and toxicity are primarily affected by the chemical forms of metals, which is primarily mediated by the organic matters in oceans. Changes in oceanic DOM concentration and quality are suggested to be connected to the nearby terrestrial land cover and use; however, knowledge about whether and how the metal forms in estuarine ecosystems dominated by mangrove is affected by changes in DOM characteristics is limited. This is the first study to demonstrate that regional coastal land covered by mangrove forests play a key role in mediating dissolved metal fraction by altering water-borne DOM quality and quantity. This study highlights the necessity of assessing aquatic metal toxicology at landscape-scale by considering coastal vegetation covers. This is particularly true when assessing site-specific exposure risk to metals under the circumstances of environmental and climate changes.