Distribution and size fractionation of nickel and cobalt species along the Amazon estuary and mixing plume

Abstract

The Amazon River has the largest drainage basin in the world, making it a major source of trace elements and dissolved organic matter (DOM) to the Atlantic Ocean. However, despite the increasing anthropogenic impacts to the Amazon basin, few recent studies exist quantifying trace element data in this region. The aim of the study was to analyze the input and removal processes that influence the transport of Ni and Co species in the Amazon and Pará River estuaries and mixing zone. Toward this goal, this work provides a comprehensive mixing and speciation study for the trace elements Ni and Co. Samples were collected during a period of high river discharge on the RV Meteor cruise M147 (Amazon – GEOTRACES process study GApr11) in the Amazon and Pará River outflow regions, as well as the aging mixing plume to the north, a mangrove belt to the southeast and the North Brazil Current (NBC) seawater endmember. Here we present the results for labile particulate (>0.2 μm), labile and total dissolved (<0.2 μm), large colloidal (0.015–0.2 μm), soluble (<0.015 μm) and ultrafiltered (<1 and < 10 kDa) fractions of Ni and Co in surface waters (towed-fish) and along the water column at different depths (CTD) using comparative approaches by adsorptive cathodic stripping voltammetry (AdCSV) and inductively coupled plasma-mass spectrometry (ICP-MS). We observed good agreement between AdCSV and ICP-MS measurements for Ni, and to a lesser extent Co. In general, dissolved and soluble Ni and Co decreased with increasing salinity, however additional non-conservative removal was also observed and attributed to possible biological uptake and colloidal flocculation. Shipboard AdCSV measurements showed that dissolved Ni was present mostly in the “reactive” form as weak complexes, suggesting high bioavailability, while reactive dissolved Co was absent, indicating the presence of strong organic Co complexes. In both Ni and Co, an elevated colloidal fraction was observed at low salinity, suggesting removal of dissolved Ni and Co via colloidal flocculation upon seawater mixing, while the soluble species were transported to the Atlantic Ocean. At depth, the soluble phase dominated, and we observed concentration maxima at 500–1000 m, indicating the presence of Antarctic Intermediate Water (AIW) and possible biological regeneration. We also observed unique source signatures in dissolved and labile particulate Ni and Co species from the Amazon and Pará River outflow regions, in addition to a contribution from mangrove belt-associated groundwater.