Carbon uptake rates in water and ice samples during Polarstern cruise ArcWatch-1 (PS138) in 2023

Net primary production (NPP) and photosynthesis–irradiance (PI) parameters were determined from water column and sea-ice samples collected during the ArcWatch-1 (PS138) expedition aboard RV Polarstern (3 August – 30 September 2023). Water samples were collected from the surface and the Chl a maximum using a CTD-rosette (SBE 911+, Sea-Bird Electronics); sea ice was sampled with a Kovacs corer and under-ice water (UIW) and melt pond water (MPW) was sampled with a hand pump from directly beneath the ice and the meltponds, respectively. Ice cores were cut into a bottom section of 10 cm and the remaining top section. Upper core sections were mechanically broken to accelerate melting and pooled. NPP was measured via the ¹⁴C uptake method (Steemann Nielsen, 1952): samples were spiked with NaH¹⁴CO₃ (1 µCi mL⁻¹) and incubated for 8 h at 0°C across a light gradient (0–430 µmol photons m⁻² s⁻¹, spherical quantum sensor). Particulate ¹⁴C uptake was quantified by liquid scintillation counting after filtration (0.2 µm), with dark uptake subtracted. Dissolved inorganic carbon samples were collected in triplicates and were measured by flow injection analysis (Hall and Aller, 1992) to calculate isotope incorporation. Chlorophyll a was quantified by reverse-phase HPLC (modified from Barlow et al., 1997). PI curves were fitted using the Platt et al. (1980) model (nonlinear least-squares, R package minpack.lm), yielding maximum photosynthetic capacity, initial slope, and photoinhibition parameter, with derived light-response parameters calculated following Platt et al. (1982).