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 so no profile was collected. Stns 4 and 7 were rafted floes with a flooded internal layer, with Stn 7 > 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 4-7), ice cores were taken with a 7.5 cm Kovacs corer separated by at least 1 meter (m) horizontally. At Stns 5-7, "physical cores" were taken and temperature was measured with a digital thermometer with a probe at 5 cm intervals along physical cores, which were then cut into 5 cm sections and placed in separate Whirlpak bags for conductivity measurements measured with both a refractometer and conductivity probe after melting. Brine volumes and salinity were calculated from bulk salinity and temperature (Frankenstein and Garner 1967; Cox and Weeks 1986).