Invariable nickel dynamics in the Peru, Benguela and Mauritania oxygen minimum zones

Nickel (Ni) is a bio-essential trace metal used in urease, hydrogenase and superoxide dismutase enzymes. The concentration of dissolved (<0.2 μm) Ni (dNi) in the surface ocean is rarely depleted below ∼2 nM globally. This is in stark contrast to the concentrations of many other bio-essential trace metals such as iron (Fe), cobalt (Co) and manganese (Mn). Both complexation with strong ligands, potentially rendering dNi unavailable to marine organisms, and slow depletion of dNi because of low cellular demand compared to other trace metals have been invoked to explain this peculiar elemental distribution. Here we report new measurements of pelagic dNi concentrations (n = 1094) from four cruises (M135-M138) in the Eastern Tropical South Pacific (ETSP) to evaluate dNi dynamics under variable biogeochemical conditions in a highly productive shelf region. We additionally report total and labile particulate Ni concentrations along two cross-shelf sections at 16° S and 17° S. The ETSP features a productive Eastern Boundary Upwelling System along the Peruvian-Chilean shelf. This coincides with an extensive underlying Oxygen Minimum Zone which leads to persistently high phosphate and intermittently high Fe conditions.

The vast majority of dNi concentrations measured across the Peruvian shelf were within the range 2.2–10.8 nM dNi with highly reproducible depth profiles compared to prior cruises in different seasons, years, and El Niño–Southern Oscillation phases. Evidence of lower dNi concentrations was only found at one inshore location with dNi as low as 1.3 nM. Such low dNi concentrations appear to be rare. Whilst biogenic Ni associated with diatom detritus is buried in Peruvian shelf sediments, biological dNi demand on the Peruvian shelf is <30 % of the annual upwelled dNi flux and burial in shelf sediments accounts for approximately 1–3 % of the upwelled dNi flux. Total particulate Ni concentrations measured on two cross-shelf sections ranged from below detection to 1.2 nM and were consistently low relative to dNi concentrations. On average, total particulate Ni (TpNi) was 2.6 % of dNi + TpNi with labile particulate Ni (LpNi) and TpNi remaining tightly coupled (R² = 0.99). As is the case elsewhere in the global ocean, dNi remained tightly correlated with phosphate and silicic acid concentrations. Limited biological demand for Ni relative to the upwelled dNi supply explains why dNi profiles in general were quite homogenous across the Peruvian coastal, shelf and offshore regions. Finally, profiles of dNi suggested a slightly lower than average dNi:phosphorous ratio (0.673 mmol (mol P)⁻¹) in the upper water column. Similar distributions of dNi and dNi:phosphorous ratios were found for the Benguela and Mauritania shelves reinforcing the relatively invariable nature of dNi profiles even in extremely productive shelf environments.