This data was collected during the Polarstern cruise PS138 (ArcWatch1) from August 3 to September 30, 2023 to the Central Arctic Ocean. At five stations during the Polarstern cruise PS138 (ArcWatch1) we conducted bioassays on board in order to assess the interactive effects of light and dSi on the uptake rates of carbon (C), nitrogen (N), and silicon (Si). For each bioassay we collected water samples from the chlorophyll maximum (Chl a max) using a CTD rosette, prefiltered the water with a 200 µm mesh and added different concentrations of dSi (0, 2, 5, 10, and 20 µmol L⁻¹ above ambient concentrations) to establish a dSi gradient. Samples were spiked with 13C labelled sodium bicarbonate, 15N labelled potassium nitrate and 30Si-sodium metasilicate at a 10% enrichment level. All samples were subsequently incubated for 24 hours under a light gradient (0, 10, 40, 80, 320 µmol photons m-2 s-1) creating a 5x5 gradient of light and dSi. After incubation, a subsample for microscopy was taken from the bottle incubated without added dSi at 10 µmol photons m-2 s-1 and preserved with hexamine-buffered formaldehyde (1-2% final concentration). 50 mL of sample were settled in an Utermöhl chamber (Utermöhl, 1958) and the different species were counted with an inverted light microscope (Zeiss IM 4717, Oberkochen, Germany). Identification to the lowest possible taxonomic level was based on Hoppenrath et al., (2009), Kraberg et al., (2010), Throndsen et al., (2007) and Tomas et al., (1997).