This data was collected from a mesocosm experiment to assess the influence of silicic acid on Arctic phytoplankton, incubating the spring bloom community of Kongsfjorden, Svalbard, for 3 weeks (May 4 - May 26) under a gradient of Si concentrations. Every 4 days we conducted stable isotope (13C, 15N, 30Si) primary productivity bioassays. The water sampled from each mesocosms was transferred into duplicate 2L clear polycarbonate bottles. Stable isotopes were added to one set of replicates. The other set was used as a control. 13C-sodium bicarbonate was added at a 10 % enrichment level, 15N-sodium nitrate was added as a fixed quantity of 0.5 µmol/L. 30Si-sodium metasilicate was added relative to the intended Si concentration in the mesocosms aiming for a 15% enrichment. The bottles were incubated for 24 h under simulated in situ conditions in the lab at 2°C with 80-90 µmol photons m-2 s-1 of light. On day 6, 14 and 22) subsamples for single cell uptake analysis were taken from the bottles after the 24 h incubation.The samples were fixed with 2 % EM-grade paraformaldehyde (PFA; EMS, USA) for 24 h in the dark at 4 °C, then filtered onto 2 μm pore size polycarbonate filters (ø 25 mm; Merck Millipore Ltd., Cork, Ireland) and washed three times with ~2-3 mL of phosphate-buffered saline (Merck Millipore Ltd., Cork, Ireland). Single-cell 15N and assimilation rates were determined by high resolution secondary ion mass spectrometry (HR-SIMS; IMS 1280, CAMECA, Gennevilliers, France) following (Halbach et al., 2025).