Iron distribution in the subtropical North Atlantic: the pivotal role of colloidal iron

The low availability of the essential micronutrient iron (Fe) in the ocean impacts the efficiency of the biological carbon pump, and hence, it is vital to elucidate its sources, sinks, and internal cycling. We present size‐fractionated dissolved Fe (dFe, <0.2 μm) measurements from 130 surface samples and 7 full‐depth profiles from the subtropical North Atlantic during summer 2017 and demonstrate the pivotal role of colloidal (cFe, 0.02 to 0.2 μm) over soluble (sFe, <0.02 μm) Fe in controlling the dFe distribution. In the surface (<5 m), a strong west‐to‐east decrease in dFe (1.53 to 0.26 nM) was driven by a dust gradient, which retained dFe predominantly as cFe (61% to 85% of dFe), while sFe remained largely constant at 0.19 ± 0.05 nM. In the euphotic zone, the attenuation of dFe resulted from the depletion of cFe (0% to 30% of dFe), with scavenging as an important driver. In the mesopelagic, cFe was released from sinking biogenic and lithogenic particles, creating a zone of elevated dFe (0.7 to 1.0 nM) between 400 to 1100 m depth. While the ocean interior, below the mesopelagic and above the seafloor boundary, exhibited a narrow range of cFe (40% to 60% of dFe), the abyssal cFe fraction varied in range from 26% to 76% due to interactions with seafloor sediments and a hydrothermal source with almost 100% cFe. Overall, our results produced an hourglass shape for the vertical cFe‐to‐dFe fraction and highlight the primary control of cFe on the dFe distribution. Plain Language Summary Phytoplankton require nutrients such as phosphorus, nitrogen, and iron. Of these, iron is particularly interesting due to the paradox of its requirement for life‐supporting mechanisms on the one hand and its low oceanic concentrations on the other. Iron is >1000‐fold lower than the “traditional” nutrients. Hence, it is important to know how much iron is introduced to the ocean (sources), how much is removed (sinks), and how it is processed during its residence in the water. This study addressed these questions by measuring the iron concentrations in the subtropical North Atlantic. Our samples were filtered through two filter sizes to investigate the distributions of iron's different size fractions, a popular tool to gain a detailed understanding of the overall iron cycle. We found that the smallest size fraction, “soluble iron,” does not vary much throughout the water column, but the slightly larger “colloidal iron” varies a lot, especially in the upper ocean and close to the seafloor, where dynamic supply and removal processes occur. The unequal behaviour of these fractions is an important finding that will improve the accuracy of biogeochemical models for iron, which in turn can improve the prediction of phytoplankton growth in the present and future ocean.