Response of central Arctic primary productivity to co-limitation by silicic acid, nitrate, and light

Arctic Ocean primary productivity is largely projected to increase with increasing light availability due to sea ice loss, yet nutrient constraints—particularly of silicic acid which affects diatom productivity—remain poorly quantified. To date, no studies have directly assessed the role of silicic acid in central Arctic productivity. During an expedition in August–September 2023, we conducted multi-parameter experiments manipulating light and silicic acid availability at five central Arctic Ocean stations. We combined stable isotope (13C, 15N, 30Si) incubations with single-cell Secondary Ion Mass Spectrometry analysis to resolve phytoplankton productivity at both community and single-cell levels. Overall, productivity and biomass were low with active nutrient assimilation confined to a small fraction of cells, suggesting community-level metabolic dormancy. At most of the stations, silicic acid kinetics revealed strong limitation, with uptake rates at 20 ± 18% (n = 3) of maximum capacity (Vmax) under ambient conditions. Light availability regulated carbon and nitrogen uptake demonstrating community adaptation to low irradiances, while silicic acid uptake remained light-independent. Nitrogen uptake was constrained by in situ nitrate availability, suggesting co-limitation by nitrate and silicic acid across the wider area. Near the North Pole, however, we observed silicic acid-replete conditions, likely via advection with the Transpolar Drift. Here, single-cell analyses implied a dominant contribution of diatoms to primary productivity (57% of carbon uptake), highlighting diatoms as locally relevant primary producers. Our results identify silicic acid as a previously unrecognised control on central Arctic productivity, potentially limiting diatom growth and likely influencing bloom persistence and taxonomic succession. This study reveals silicic acid as another key regulator of productivity in the central Arctic Ocean and highlights the value of single-cell analysis to resolve group-specific primary productivity in mixed communities.