Beschreibung: 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) samples 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.

Beschreibung: The Amazon is Earth's largest river by volume output, making it an important source of trace metals and dissolved organic matter (DOM) to the Atlantic Ocean. Despite major recent anthropogenic disruptions to the Amazon catchment area, data for trace metals such as copper (Cu) in the Amazon River estuary and associated mixing plume are still rare. Furthermore, there is currently no existing data in this region for Cu-binding ligands, which govern the amount of bioavailable Cu. To understand trace metal mixing and transport processes, the GEOTRACES process study GApr11 (cruise M147 with RV Meteor) was conducted in 2018 in the Amazon and Pará River estuaries and mixing plume in the tropical North Atlantic Ocean during high river discharge. Size-fractionated surface samples were collected along the full salinity gradient for concentrations of Cu, apparent Cu-binding organic ligands (LCu) and corresponding conditional stability constants (K′CuL, Cu2+cond), electroactive humic substances (eHS), solid phase extractable organic Cu (SPE[sbnd]Cu), dissolved organic carbon (DOC), chlorophyll a (Chl a) and macronutrients. Dissolved (〈0.2 μm) and soluble (〈0.015 μm) Cu correlated negatively with salinity and largely followed values expected from conservative mixing. Cu was primarily in the soluble fraction, with the exception of a minor fraction of large colloidal Cu at low salinity (S ≤ 10). Organic ligands (log K′CuL, Cu2+cond = 12.6–15.6) were present in excess of Cu and likely played a role in solubilizing Cu and preventing Cu being affected by colloidal flocculation. Cu-associated DOM (measured as LCu, eHS and SPE[sbnd]Cu) correlated negatively with salinity and appeared to be primarily governed by river input and mixing with seawater. However, an increase in the colloidal fraction for LCu and eHS observed at S ~ 6–10 was attributed to possible additional autochthonous (phytoplankton) ligand production. In all dissolved samples, organic complexation kept free Cu below levels potentially toxic for phytoplankton (〈1 pmol L−1). Despite increasing anthropogenic activity over the past century, we find Cu concentrations remained similar to the 1970s, suggesting that the large overall river flow may so far minimize the impact of Cu pollution.