El Niño‐Driven Oxygenation Impacts Peruvian Shelf Iron Supply to the South Pacific Ocean

Abstract

Upwelling ocean currents associated with oxygen minimum zones (OMZs) supply nutrients fuelling intense marine productivity. Perturbations in the extent and intensity of OMZs are projected in the future, but it is currently uncertain how this will impact fluxes of redox-sensitive trace metal micronutrients to the surface ocean. Here we report seawater concentrations of Fe, Mn, Co, Cd, and Ni alongside the redox indicator iodide/iodate in the Peruvian OMZ during the 2015 El Niño event. The El Niño drove atypical upwelling of oxygen-enriched water over the Peruvian Shelf, resulting in oxidized iodine and strongly depleted Fe (II), total dissolved Fe, and reactive particulate Fe concentrations relative to non-El Niño conditions. Observations of Fe were matched by the redox-sensitive micronutrients Co and Mn, but not by non-redox-sensitive Cd and Ni. These observations demonstrate that oxygenation of OMZs significantly reduces water column inventories of redox-sensitive micronutrients, with potential impacts on ocean productivity.Plain Language Summary Some trace metals, including iron, are essential micronutrients for phytoplankton growth. However, the solubility of iron is very low under oxygenated conditions. Consequently, restricted iron availability in oxygen-rich seawater can limit phytoplankton growth in the ocean, including in the Eastern Tropical South Pacific. Under typical conditions, depleted oxygen on the South American continental shelf is generally thought to enhance iron supply to the ocean, fuelling phytoplankton productivity in overlying waters. However, the impacts of changes in oxygenation, which are predicted to occur in the future, are not known. The 2015 El Niño event led to unusually high oxygen on the Peruvian shelf, offering a system-scale test on how oxygen influences seawater iron concentrations. We show that El Niño-driven oxygenation resulted in marked decreases in iron and other metals sensitive to oxygen (cobalt and manganese), while metals not sensitive to oxygen (cadmium and nickel) were unaffected. The measured reductions in iron may have led to decreased phytoplankton productivity.

Toward resolving disparate accounts of the extent and magnitude of nitrogen fixation in the Eastern Tropical South Pacific oxygen deficient zone

“…3). Rapp et al (2020) observed that Fe concentrations were significantly reduced during the 2015-2016 El Niño event (concurrent with this study) along onshore-offshore transects crossing the Peruvian continental shelf, except where the shelf was thinnest (corresponding to Sta. 18 in this study).…”

“…12, 14, 15, and 18), and decreased to < 4 and < 0.7 μ M, respectively, at most offshore stations. In addition to transporting macronutrients, upwelling can supply surface waters with Fe and other trace elements liberated from shelf sediments, provided that upwelled waters remain relatively reducing (Rapp et al 2020).…”

“…Both seasonal and interannual variability affecting upwelling (e.g., the El Niño‐Southern Oscillation) can thus alter the availability of organic substrates above and within the O 2 deficient zone (Pennington et al 2006). Moreover, benthic Fe supply, likely the major source of Fe both on‐ and offshore (Cutter et al 2018), is diminished during periods when upwelling is relaxed and upwelled waters are less reducing (Rapp et al 2020). As N 2 fixation in the Eastern Tropical South Pacific is thought to be largely Fe‐limited (e.g., Dekaezemacker et al 2013), changes in the Fe inventory may directly affect diazotrophs.…”

“…This likely reflects the degree to which methodological choices influence the detection and calculation of low rates ( see discussion below), at least in part. However, spatial heterogeneity in the effects of El Niño/La Niña (e.g., Rapp et al 2020) may help explain discrepancies among these studies as well, given their far‐ranging distribution—from the continental shelf to the edge of the South Pacific gyre (Fig. 3).…”

Iron colloids dominate sedimentary supply to the ocean interior

“…RD of Fe is coupled to OC oxidation and is widely observed in shallow porewater in sediments with high-oxygen consumption rates, under productive shelf seas, near zones of upwelling, and overlain by oxygen-depleted seawater (10). Low seawater oxygen content serves to enhance the efflux of reduced and soluble Fe (sFe, filtered <0.02 μm) from ferruginous porewaters and enables sFe(II) to propagate further in the water column (11)(12)(13)(14).…”