Iron and Manganese Cycling in the Atlantifying Barents Sea: Concentrated Inputs and Emerging Limitations

Dissolved iron (dFe) and manganese (dMn) are essential micronutrients required in marine primary production; however, their low availability limits productivity and impacts the efficiency of the biological carbon pump. Therefore, it is crucial to elucidate their sources, sinks, and internal cycling. We present high-resolution dFe and dMn measurements from the Barents Sea during summer 2023, to provide new insights into their spatial distribution and biogeochemical cycling. Localized surface enrichment of dFe and dMn indicate inputs from sea ice (Fe: 0.23–1.31 and Mn: 3.55–7.33 nmol/kg) and glacial melt (Fe: 1.17–3.19 and Mn: 22.44–102 nmol/kg). In the subsurface waters, dFe and dMn maxima (Fe: 4.95–10.70 and Mn: 9.05–33.61 nmol/kg) are a result of sediment resuspension, indicated by low beam transmission. Using the spatial distribution of dFe and dMn and their correlation with salinity and seawater isotopic composition, these inputs were found to be spatially constrained, with limited vertical and lateral transport from the source regions. We observed high rates of primary productivity in the upper 51 m of the southwestern Barents Sea (2,200 ± 222 mg C m−2 d−1), corresponding to depleted dFe and dMn in the surface waters (Fe: 0.08–0.18 and Mn: 0.18–3.15 nmol/kg). Using nutrient limitation ratios, we found potential Fe-limitation in the southern Barents Sea, resulting from limited vertical resupply of Fe and high biological uptake. Our results suggest that Atlantification is causing the southwestern Barents sea to transition to an Atlantic-like seasonally Fe-limited system.