Conservative behavior of terrestrial trace elements associated with humic substances in the coastal ocean

The transport of terrestrial sources of trace elements to the ocean has proven to be crucial in constraining not only their own biogeochemical behaviors but also regulating global carbon cycling and ecosystem changes. Most terrestrial dissolved trace elements are known to be largely scavenged in the estuarine mixing zone or coastal waters before reaching the open ocean, owing to their particle-reactive nature. In this study, we investigated the behavior of trace elements (Mn, Fe, Ni, Cu, and Zn) and rare earth elements (REEs), together with other biogeochemical parameters, in a coastal embayment, which features a long water residence time. Our results showed intense depletion of inorganic nutrients throughout the bay relative to the fresh and open-ocean water mixing curve. Moreover, REE fractionation also revealed notable particle scavenging with a significant removal of Ce. However, most trace elements (Mn, Fe, Ni, Cu, and Zn) showed higher concentrations in lower salinity waters and significant positive correlations with humic substances. Thus, we hypothesize that terrestrial trace elements (Mn, Fe, Ni, Cu, and Zn), associated with terrestrial humic substances, can survive particle scavenging and biological consumption in the coastal mixing zone, and may be important sources of trace elements to the open ocean globally.