Dissolved Concentrations and Organic Speciation of Cu in the Amazon River Estuary and Mixing Plume

The Amazon River basin has the largest water output by volume of any river on Earth, and is therefore a substantial source of trace metals and organic matter to the Atlantic Ocean. The trace metal copper (Cu) can act as both an essential nutrient and a toxin to phytoplankton and is highly organically complexed (>99%) in the marine environment. Copper-bound organic ligands dramatically decrease the concentration of free Cu²⁺, thereby decreasing its toxicity. Furthermore, large colloidal organic ligands (i.e., humic substances) and inorganic colloids, both of which are abundant in river systems such as the Amazon, can be expected to alter the size distribution of dissolved Cu. In order to better understand the role of the Amazon River in the cycling of Cu and associated dissolved organic matter, samples were collected on the GApr11 Amazon-GEOTRACES (M147) cruise during a period of high river discharge in April-May 2018. Here we present results for size-fractionated (<0.2 µm,<0.015 µm, 1 KDa and 10 KDa) dissolved Cu and Cu speciation in surface samples collected over the full salinity gradient (~0 to >35 ppt) along Amazon River estuary and mixing plume, in addition to several trace metal clean CTD depth profiles. In both surface and depth samples, dissolved Cu displayed highly conservative behavior with respect to salinity. Strong L₁ ligand classes (logK ~12.5-14) were present throughout the range of salinity gradients, with a notable river influence, and a second weaker L₂ class was also sometimes observed in the Amazon plume. Here we discuss how organic ligands govern Cu biogeochemistry and fluxes in the Amazon River estuary and mixing plume.