The speciation of copper across active gradients in nitrogen-cycle processes in the eastern tropical South Pacific

Copper (Cu) complexation and distribution were characterized using competitive ligand exchange adsorptive cathodic stripping voltammetry and isotope dilution inductively coupled plasma mass spectrometry along two transects (20°S and 10°S) in the eastern tropical South Pacific. In the southern and westernmost stations, Cu showed upper water column depletion to values as low as ∼ 0.26 nmol L−1, the lowest concentrations ever reported. However, Cu levels were much higher within the secondary nitrite maxima of the oxygen minimum zone (OMZ) in the northern (10°S) transect. The enrichment of Cu in the reducing conditions of the OMZ has not been reported before and probably reflects remineralization and offshore transport from the shelf. Free [Cu2+] was typically low throughout the water column, ranging from about 3.15 × 10−15 mol L−1 to 1.34 × 10−13 mol L−1, and depth profiles exhibited similar features to those for dissolved Cu, though they showed more variability near the surface. Offshore and beyond the influence of the OMZ, the lowest dissolved and free [Cu2+] was within the primary nitrite maxima (PNM), where ammonia oxidation and nitrate reduction rates are important. This finding is of interest because the two competing explanations for the PNM—iron limitation of diatoms and high rates of ammonia oxidation relative to nitrite oxidation—have high Cu requirements. The low concentrations of free Cu2+ measured here could impose significant constraints on the rates of these processes.