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. Water samples were taken every second day with a Kemmerer bottle (Volume 6L, Wildco, Yulee, US). pH was measured immediately after sampling from the sampling container (826 pH mobile with a LL aquatrode plus pt1000, Metrohm) and samples were taken for DIC, microscopy, nutrients, pigments, DNA and particulate organic matter. DIC samples were taken gas-free by filtering subsamples in triplicates (0.2µm) into gas-free Zinsser vials (1.8mL) and were analysed using the flow injection system (Hall and Aller, 1992). Dissolved inorganic nutrients (nitrate, nitrite, phosphate, dissolved silicic acid and ammonium) were analysed using a continuous flow analyzer (QuAAtro Autoanalyzer, SEAL Analytical, Norderstedt, Germany) connected to a fluorometer (FP-2020, JASCO, Tokyo, Japan) to measure ammonium. From 25.05. onwards samples were frozen and measured in the home laboratory, no ammonium was measured from frozen samples. For POC/N and POP samples were filtered onto precombusted glass-fiber filters (GFF, ø25 mm, 0.7 µm nominal pore size, Whatman, UK) and for BSi analysis onto polycarbonate filters(ø47 mm, 2 µm pore size, Merck Millipore Ltd., Cork, Ireland). POC/N sampes were measured using a Euro EA3000 Elemental Analyzer (Eurovector, Pavia, Italy). POP filters were autoclaved for 30 min in 100 mL Schott Duran glass bottles using an oxidizing decomposition solution (Merck, catalogue no. 112936) to convert organic phosphate to orthophosphate. Phosphate concentrations were determined spectrophotometrically following (Hansen and Koroleff, 1999). Biogenic Silica (BSi) was measured using an alkaline time-course digestion (0.1M Na2CO3) following a modified procedure by (DeMaster, 1981) (Krause et al., 2023), followed by a 48 h hydrofluoric acid (2.0 M) digestion to dissolve the remaining lithogenic silica (LSi). For dPCR, watersamples were filtered onto precombusted GFF filters (GFF, ø25 mm, 0.7 µm nominal pore size,Whatman, UK), stored in Eppendorf tubes and stored at 20°C until further analysis. DNA was extracted using the NucleoSpin Soil extraction kit (Macherey­ Nagel GmbH, Germany) according to the manufacturer's protocol. Dye-based digital PCR (dPCR) assays for absolute quantification of Phaeocystis target-gene copies, used primer sequences (82F-(5′-GTG AAA CTG CGA ATG GCT CAT-3′)/P1np(5′-CGG GCG GAC CCG AGA TGG TT-3′) based on (Metfies et al., 2016). Assays were performed on the QIAcuity Four Digital PCR System (QIAGEN) with a 8.5k 96-well nanoplate.