Trace Metal Behavior and Size Fractionation Along the River Water – Seawater Mixing Gradient in the Amazon River Estuary and Plume

The Amazon River – Atlantic Ocean continuum represents one of the largest environmental gradients in the world, with far-reaching impact in the Atlantic including element budgets and bioproductivity. During research cruise M147 (GEOTRACES cruise GApr11) we studied the distribution of trace elements along the salinity gradients in the mixing zone of the Amazon estuary and its freshwater plume, in order to quantify their biogeochemical cycling, interaction with dissolved organic matter and particulate matter and the riverine contribution to ocean metal budgets. Our data show a highly diverse system with partly variable behavior of individual metals in the southern and northern outflow zones of the Amazon River as well as the Rio Pará which forms part of the large estuarine system. Metals with mostly conservative behavior, such as Mo, V, and U, showed some deviation from the conservative mixing line and other elements such as Ti, Fe and Pb are strongly adsorbed onto particulate matter at low salinities with some desorption at mid-salinities. Both loss and gain of the dissolved fraction of Ni, Co and Cd during mixing and correlation with phosphate may indicate biological uptake and desorption. Size fractionation (0.2 µm, 0.015 µm, 10 kDa and 1 kDa) investigations of the water samples indicated a dominance of the truly dissolved fraction for quasi-conservative elements and changing colloidal fractions of most other elements along the mixing line, indicative of adsorption and colloidal aggregation. Considering the numerous environmental challenges of the Amazon River system, we anticipate that the observed processes in the Amazon estuary and fluxes of metals to the Atlantic may significantly be affected by these changes in the near future.