Thielecke, Antonia; Robertson, Elizabeth K; Stefels, Jacqueline; Shahin, Mohammad: Bioassays for C, N and Si uptake rates in a mesocosm study on silicic acid effects in Kongsfjord, Svalbard [dataset]. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.995571 (dataset in review)

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. After incubation samples were taken for 13C-labelled dissolved inorganic carbon (13C-DIC), 13C and 15N labelled POC and PON (13C/15N-POC/PON) and 30Si labelled biogenic silica (30Si-BSi). 13C-DIC was measured in duplicates using a Cavity Ring-down Spectrometer G2101-i (Picarro, Santa Clara, USA) at Groningen University (Bach et al., 2025). 13C/15N-POC/PON was measured on a isotope ratio mass spectrometer (EA-IRMS, Sercon, UK). Biogenic silica was analysed using a 30 min two-step alkaline digestion with 0.1M Na2CO3 following (Krause et al., 2023). Stable isotope composition of these subsamples was measured using a high-resolution sector field inductively coupled plasma mass spectrometer (Element XR; Thermo Fisher Scientific, Waltham, US) following (Fripiat et al., 2009). Uptake rates were calculated analoguous for C, N and Si uptake rates as described in Bach et al. (2025) for C uptake. Natural abundance 13C, 15N and 30Si percentages for the dissolved phase were obtained from the literature for the Eurasian Arctic (Bauch et al., 2015, Thibodeau et al., 2017; Liguori et al., 2020). The enrichment in all samples exceeded 3 standard deviations of the respective blank.