Field sampling:
Ice samples for primary production measurements were collected mid-morning from 6 stations along the western Antarctic Peninsula in November and December of 2019 on board the R/V Nathaniel B. Palmer along a north-south transect from 64.8°S to 67.8°S. For Stations (Stns) 2 and 3, the ice was "rotten" (sufficiently melted to be disintegrating structurally, present only in small pieces) and collected as an ice-seawater slurry. Small ice chunks were collected directly from the sea surface via a crane-suspended "personnel basket". At the additional 4 stations, sea ice was collected by coring the ice. At Stns 4 and 7, algal samples were collected from internal ice-core layers. Stns 4 and 7 were rafted floes with a flooded internal layer, with Stn 7 > 2500 square meters (m²) in size. Stns 5 and 6 were on landfast sea ice, where the algae were collected from the bottom 10 centimeters (cm) of the ice. At these 4 stations (Stns 4 through 7), ice cores were taken with a 7.5 cm Kovacs corer separated by at least 1 m horizontally. Cores were shaded from direct sunlight, while 5 to 10 cm of the visible algal band at the bottom or middle was sectioned with an ethanol-cleaned saw and placed into acid-washed (10% HCl) containers. Each replicate consisted of either a single core (for 10 cm sections) or pools of two cores (for 5 cm sections).

All ice samples were stored in dark, insulated containers for a maximum of 4 hours. On the ship, all ice samples used for primary production measurements were melted in a 3:1 volumetric ratio of melting solution to ice to minimize the effect of the melt on sea-ice communities. The melting solution was a 0.2 micrometer (μm) filtered artificial salt mixture containing three main sea salts plus bicarbonate according to ESAW artificial seawater (3.63 x 10⁻¹ molar (M) NaCl, 4.71 x 10⁻² M MgCl₂.6H₂O, 2.5 x 10⁻² M Na₂SO₄, and 2 x 10⁻³ M NaHCO₃⁻, salinity 35). Additional ice cores collected for ancillary biological measurements were melted in a 1:1 volumetric ratio of melting solution to ice. Melts were conducted in the dark at approximately 20° Celsius (C). To speed the melting process, ice samples were further broken into pieces with acid-washed pickaxes with most ice completely melted within 5 hours. Volume and salinity were measured as soon as the ice was completely melted, with sample temperatures remaining below 0°C. All reported volumes were corrected to the original ice volume.

POC and PN:
All samples were collected and processed according to protocols of the Marine Chemistry Laboratory (MCL) at the University of Washington. Samples for particulate organic carbon (POC) and particulate nitrogen (PN) were filtered through combusted (450°C, 4 hours) 25-millimeter (mm) glass fiber filters (GF/F, pore size 0.7 μm pre-combustion) and frozen at -80°C until analysis. Following fuming with HCl to remove inorganic carbon, POC and PN were measured on a CEC440 Elemental Analyzer. For nutrients, ~50 milliliters (mL) of samples were filtered through surfactant-free cellulose syringe filters (25 mm, 0.45 μm, Nalgene) with filtrate collected in sample-rinsed HDPE bottles and frozen at -20°C until analyzed using a Technicon AutoAnalyzer II at MCL. Trace metals were not measured. While every effort was done to minimize trace metal contamination, the ice corer is metal and the facilities are not trace metal clean. Blanks for the collection protocol (bottles, filters) and for the melting solution were subtracted from all samples. Unless otherwise stated, all values were reported as concentrations within bulk ice (ice plus brine).

913566_v1_poc_pn.csv