2-m water depth salinities were extracted from CTD casts.

Sample was filtered through pre-combusted (450degC for 5 hr) Whatman GF/F filters using a sterilized syringe. Filtrate was collected in acid-cleaned polyethylene tubes after three rinses with the filtrate, and stored at -20degC until analysis within 6 months using a Bran-Luebbe 3 autoanalyzer (adapted from (Grasshoff et al. 1999)). Samples were analyzed for nitrate+nitrite, phosphate and silicic acid. Samples for Si(OH)4 determination were thawed for at least 24 hr to minimize the issue of silicate polymerization when samples have been stored by freezing (Macdonald et al. 1986).

Bulk ice nutrients - samples were from ice cores melted without filtered seawater addition.

Water Column - 2 m water depth

Intracellular Nutrients - The method used to extract the intracellular nutrient pool was adapted from (Dortch 1982). Within 3 hr of collection, a subsample from the scrape sample was filtered onto a pre-combusted (450°C for 5 h) Whatman GF/F filter within an acid-cleaned filter head mounted on a large Erlenmeyer flask. Once enough material was concentrated on the filter (visible confirmation), vacuum pressure was released and a 60-mL acid-cleaned polyethylene tube, rinsed with boiling reverse osmosis water, was suspended below the filtration head within the Erlenmeyer flask. Then, 40 mL of boiling reverse osmosis water was poured directly into the filter funnel. The water was left for 10 minutes and then vacuum pressure restored and the filtrate was collected in the suspended tube. Following collection of the filtrate, the tube was sealed and placed immediately into the -20degC freezer. Following the above-mentioned protocol, a subsample of the boiling reverse osmosis water was also collected as a blank for every sample day.

References

1. Q. Dortch, Effect of growth conditions on accumulation of internal nitrate, ammonium, amino acids, and protein in three marine diatoms. Journal of Experimental Marine Biology and Ecology 61, 243–264 (1982).

2. K. Grasssshoff, K. Kremling, M. Ehrhardt, “Frontmatter” in Methods of Seawater Analysis, (John Wiley & Sons, Ltd, 1999), pp. i–xxxii.

3. R. W. Macdonald, F. A. McLaughlin, C. S. Wong, The storage of reactive silicate samples by freezing. Limnology and Oceanography 31, 1139–1142 (1986).

Data were collected every 4 days between 19 May and 8 June. Snow depths were measured at every core extraction location, with targeted sampling of three different sites to capture the available range of snow depth conditions, including thin (17 cm) snow covers. Bottom-ice samples were collected from each of these extraction locations using a Kovacs Mark II coring system (9-cm inner diameter) and processed for analysis of i) bottom-ice chlorophyll a concentration (chl a) and community composition, ii) intracellular nutrients and, iii) bottom-ice bulk nutrients.

For quantitative measurements of bottom-ice chl a and community composition, up to three ice cores were extracted from each site and the bottom 3 cm were pooled into isothermal containers before melt in 0.2-m filtered seawater (FSW) to limit osmotic shock to the algae during melt processing. The FSW-diluted core solution was melted in the dark over a 15 to 20-hr period. For intracellular nutrient measurements, a bottom-ice scrape sample was collected from 1-3 cores per sampling site depending on visible algal coloration. The scrape procedure used a stainless-steel knife to scrape off the soft skeletal bottom-ice layer, which contained the strongest coloration of algal matter (<0.5 cm), directly into 500 mL of FSW at a temperature near freezing. This technique minimizes stress on algal cells during ice melt processing by: i) maintaining sample salinities similar to growth conditions at the ice-ocean interface, and ii) reducing time of exposure to potentially stressful melt conditions, as all scrape samples were processed within 3 hr of collection. For bulk ice nutrient measurements, the bottom 3 cm of an ice core was collected and placed immediately into a sterile bag (Nasco Whirl-Pak) and then melted over a 15 to 20-hr period in the dark.

A Niskin sampler was lowered through an ice hole to collect water at a 2-m depth.

Instrument: Cond 330i, WTW

Melt ice core without filtered seawater dilution and measure salinity at room temperature.

• Salinity

Instrument: 10-005R Turner Designs fluorometer

Melted ice core samples were filtered onto Whatman GF/F glass fiber filters (nominal pore size of 0.7 µm) for analysis of bottom-ice chl a. Filters were placed in 90% acetone for 18 to 24 hr, and the extracted chl a was measured before and after acidification with 5% HCl using a 10-005R Turner Designs fluorometer. All measurements were made with ice core melt using 3:1 filtered seawater dilution and corrected for the dilution.

• Chl a concentration

Instrument: Inverted Microscope

Melted ice core samples were preserved with acidic Lugol’s solution (Parsons et al. 1984) and stored in the dark at 4°C for later analysis of cell identification and enumeration. Cells > 4 µm were identified to the lowest possible taxonomic rank using inverted microscopy according to (Lund et al. 1958); however, information is only presented on total autotrophic cell abundance and percent contribution of pennate diatoms. All measurements were made with ice core melt using 3:1 filtered seawater dilution and corrected for the dilution.

References:

1. T. R. Parsons, Y. Maita, C. M. Lalli, A Manual of Chemical and Biological Methods for Seawater Analysis. (Pergamon Press, 1984) https:/doi.org/10.25607/OBP-1830 (March 4, 2024).

2. J. W. G. Lund, C. Kipling, E. D. Le Cren, The inverted microscope method of estimating algal numbers and the statistical basis of estimations by counting. Hydrobiologia 11, 143–170 (1958).

• Percent contribution of main algal taxa

Sample was filtered through pre-combusted (450degC for 5 hr) Whatman GF/F filters using a sterilized syringe. Filtrate was collected in acid-cleaned polyethylene tubes after three rinses with the filtrate, and stored at -20degC until analysis within 6 months using a Bran-Luebbe 3 autoanalyzer (adapted from (Grasshoff et al. 1999)). Samples were analyzed for nitrate+nitrite, phosphate and silicic acid. Samples for Si(OH)4 determination were thawed for at least 24 hr to minimize the issue of silicate polymerization when samples have been stored by freezing (Macdonald et al. 1986).

Bulk ice nutrients - samples were from ice cores melted without filtered seawater addition

Water Column - 2 m water depth

Intracellular Nutrients - The method used to extract the intracellular nutrient pool was adapted from (Dortch 1982). Within 3 hr of collection, a subsample from the scrape sample was filtered onto a pre-combusted (450°C for 5 h) Whatman GF/F filter within an acid-cleaned filter head mounted on a large Erlenmeyer flask. Once enough material was concentrated on the filter (visible confirmation), vacuum pressure was released and a 60-mL acid-cleaned polyethylene tube, rinsed with boiling reverse osmosis water, was suspended below the filtration head within the Erlenmeyer flask. Then, 40 mL of boiling reverse osmosis water was poured directly into the filter funnel. The water was left for 10 minutes and then vacuum pressure restored and the filtrate was collected in the suspended tube. Following collection of the filtrate, the tube was sealed and placed immediately into the -20degC freezer. Following the abovementioned protocol, a subsample of the boiling reverse osmosis water was also collected as a blank for every sample day.

Q. Dortch, Effect of growth conditions on accumulation of internal nitrate, ammonium, amino acids, and protein in three marine diatoms. Journal of Experimental Marine Biology and Ecology 61, 243–264 (1982).

K. Grasssshoff, K. Kremling, M. Ehrhardt, “Frontmatter” in Methods of Seawater Analysis, (John Wiley & Sons, Ltd, 1999), pp. i–xxxii.

R. W. Macdonald, F. A. McLaughlin, C. S. Wong, The storage of reactive silicate samples by freezing. Limnology and Oceanography 31, 1139–1142 (1986).

• Macronutrient concentrations