Dissolved and particulate iron redox speciation during the LOHAFEX fertilization experiment

• Published in: Marine pollution bulletin 2022-11, Vol.184, p.114161-114161, Article 114161
• Main Authors: Laglera, Luis M., Uskaikar, Hema, Klaas, Christine, Naqvi, S. Wajih A., Wolf-Gladrow, Dieter A., Tovar-Sánchez, Antonio
• Format: Article
• Language: English
• Subjects: Copepod grazing; Iron ocean cycling; Iron redox speciation; Southern Ocean; Top-down trace element control
• ISSN: 0025-326X; 1879-3363
• DOI: 10.1016/j.marpolbul.2022.114161

The redox speciation of iron was determined during the iron fertilization LOHAFEX and for the first time, the chemiluminescence assay of filtered and unfiltered samples was systematically compared. We hypothesize that higher chemiluminescence in unfiltered samples was caused by Fe(II) adsorbed onto biological particles. Dissolved and particulate Fe(II) increased in the mixed layer steadily 6-fold during the first two weeks and decreased back to initial levels by the end of LOHAFEX. Both Fe(II) forms did not show diel cycles downplaying the role of photoreduction. The chemiluminescence of unfiltered samples across the patch boundaries showed strong gradients, correlated significantly to biomass and the photosynthetic efficiency and were higher at night, indicative of a biological control. At 150 m deep, a secondary maximum of dissolved Fe(II) was associated with maxima of nitrite and ammonium despite high oxygen concentrations. We hypothesize that during LOHAFEX, iron redox speciation was mostly regulated by trophic interactions. [Display omitted] •6-Fold increment and decay of dissolved and particulate Fe(II) during LOHAFEX•Fe(II) surface concentrations not only controlled by solar radiation•Trophic processes modulated the spatial and temporal distribution of Fe(II).•Fe(II) surface concentration gradients across fertilized patch boundaries•Presence of deep Fe(II), nitrite and ammonia maxima under oxygen saturation