Trace metal stoichiometry of dissolved organic matter in the Amazon plume

Dissolved organic matter (DOM) is a distinct component of Earth’s hydrosphere and provides a link between the biogeochemical cycles of carbon, nutrients, and trace metals (TMs). Binding of TMs to DOM is thought to result in a TM pool with DOM-like biogeochemistry. Here, we determined elemental stoichiometries of aluminum, iron, copper, nickel, zinc, cobalt, and manganese associated with a fraction of the DOM pool isolated by solid-phase extraction at ambient pH (DOM(SPE-amb)) from the Amazon plume. We found that the rank order of TM stoichiometry within the DOM(SPE-amb) fraction was underpinned by the chemical periodicity of the TM. Furthermore, the removal of the TM(SPE-amb) pool at low salinity was related to the chemical hardness of the TM ion. Thus, the biogeochemistry of TMs bound to the DOM(SPE-amb) component in the Amazon plume was determined by the chemical nature of the TM and not by that of the DOM(SPE-amb).