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) from the Amazon plume. We found that the rank order of TM stoichiometry within the DOM fraction was underpinned by the chemical periodicity of the TM. Furthermore, the removal of the TM pool at low salinity was related to the chemical hardness of the TM ion. Thus, the biogeochemistry of TMs bound to the DOM component in the Amazon plume was determined by the chemical nature of the TM and not by that of the DOM.