Interactions of trace metals with dissolved organic matter and colloids in the Amazon and Pará River estuaries and associated plume as key processes for trace metal fluxes into the Atlantic

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The Amazon and Pará River systems discharge the largest volume of freshwater into the ocean and form an important interface for trace metal and dissolved organic matter (DOM) input from land into the ocean. In addition to the impact of the Amazon River on the global ocean trace metal budget, its micronutrient input also has a large influence on the biological productivity within the coastal and shelf region and beyond. The main goal of the proposed project is to understand the role of chemical speciation and physico-chemical size fractionation of trace metals in the mixing continuum of the Amazon River and the neighboring Pará River to the Atlantic Ocean. We will study the interactions of trace metals with DOM and colloids in the water column and surface sediments of the Amazon and Pará estuaries and the associated plume as well as the mangrove belt with groundwater discharge southeast of the Pará River. This will help to better understand the contribution of large DOM-rich tropical rivers to the trace metal budget in the ocean. Based on water and sediment including pore water samples taken during research cruise M147 in the high-discharge period of 2018 and preliminary data produced in our lab, we will investigate changes of trace metal distributions and speciation in the larger Amazon outflow region along the salinity gradients. To assess what controls the chemical and physical speciation and transport of trace metals in the estuary and plume we will focus on three different processes: • Trace metal size fractionation, sorption and removal: sorption of trace metals on riverine particulate matter and removal due to coagulation of colloids, and size fractionation; how do trace metal associations with different soluble, colloidal and particulate fractions change along the salinity gradient, and what does this mean for the source-sink balance?• Solution complexation: formation of soluble metal-organic complexes; how does this process enhance the metal transport by competition with the sorption on colloids and removal in an DOM-rich estuarine system? • Trace metals in sediments: how do sediments and pore water act as a sink and source of trace metals and contribute to the trace metal fluxes in the region? In addition to voltametric and ICP-MS analyses of the M147 samples, we will complete a systematic investigation of the mixing behavior of diverse groups of elements (conservative, particle-reactive and organically-complexed) by conducting laboratory mixing experiments with seawater and river water endmembers collected during the upcoming cruise M174 in the Amazon region. With this, we expect to obtain a holistic picture of the complex processes controlling trace metal biogeochemistry and elemental fluxes in this largest estuarine system of the world. This knowledge will also be important to foresee potential effects in this area due to ongoing anthropogenic impact in this region and changing climatic conditions.