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Abstract:
Data collected from sea ice cores and ice-ocean interface water in two Svalbard fjords, Tempelfjorden and Van Mijenfjorden, include sea ice algal gross primary productivity (GPP), photophysiology, chlorophyll a (Chl a), community composition as well as general sea ice characteristics (snow depth, ice thickness, draft, freeboard), bulk-ice salinity, temperature, inorganic bulk and brine nutrients concentrations (nitrite + nitrate (NO2 + NO3), phosphate (PO4), silicic acid (Si(OH)4)) and molar ratios (N:P, N:Si), particulate organic carbon-nitrogen (POC/N) and molar ratios (POC:PON), dissolved inorganic carbon (DIC). The sea ice cores, and ice-ocean interface water were collected at three sampling sites on three different days. First, at one site at Tempelfjorden (TF1, 78.41 ºN, 17.08 ºE) on 12 April, 2021, being freshwater influenced by the tidewater glacier Tunabreen, and then at two sites at Van Mijenfjorden, site 1 (VM1, 77.80 ºN, 15.76 ºE) on 17 April, 2021, and site 2 (VM2, 77.82 ºN, 15.71 ºE) on 14 April, 2021.
The data collection (zip file) contains a README file that describes all the files in the data set. Further details of the methods are in the manuscript “Photophysiological responses of Arctic bottom sea-ice algae to freshening” that has been submitted for publication. For detailed Methods see also: Parsons, T.R., Maita, Y., and Lalli, C.M. (1984). A manual of chemical & biological methods for seawater analysis. First ed (New York, NY: Pergamon Press, Elsevier), 184. and Platt, T., Gallegos, C., and Harrison, W.G. (1981). Photoinhibition of photosynthesis in natural assemblages of marine phytoplankton. Journal of Marine Research 38, 687-701.
Quality
Method for field-based data (Svalbard): Sea ice cores were collected using a 9 cm ice core barrel (Kovacs Enterprise Mark II) at TF1 (total n = 13), VM1 (n = 7) and VM2 (n = 12) and ice-ocean interface water samples (i.e., within 10 cm of surface seawater) were collected through an ice core hole using a peristaltic pump (Masterflex® L/S® Portable Sampling Pump). Snow depth, ice thickness, draft and freeboard were measured for each sampling site, just prior to the collection of each ice core. Ice cores from TF1, VM1 and VM2 were pooled and melted, respectively, with interface-filtered seawater (FSW) (0.2 μm) added at an approximate ratio of three parts FSW to one part ice (FSW3:1) for measuring primary productivity, photophysiology, POC/N and Chl a. Besides, individual ice cores were collected at each sampling site, and were melted without FSW to measure bulk-ice salinity profiles, inorganic nutrients and DIC. Ice temperature was measured on the same ice core as the bulk-ice salinity, prior to ice melt. Moreover, one individual ice core was collected at VM1 and VM2 sites and was melted with FSW for obtaining sea ice Chl a profiles. All ice samples were melted at room temperature in darkness over a period of 24 h. Temperature measurements were collected at the ice-ocean interface at all sites using a CTD (SonTek Castaway®). Sea ice temperature and bulk-ice salinity were measured for the entire ice core at a vertical resolution of 5 cm. Sea ice temperature was measured on an ice core, at 2.5 cm distance from the ice-ocean interface and at 5 cm intervals over the entire core length using a thermometer probe. The same core was then immediately sectioned into bottom 2.5 cm and above 5 cm thick sections and melted back at laboratory facilities. Following a 24 h melt period and complete melt, bulk-ice salinity was measured using a conductivity meter (ProfiLine Cond 3110-WTW). Brine volume fraction (Vb) was calculated from bulk-ice salinity and sea ice temperature measurements. Measurements of NO2 + NO3, PO4, Si(OH)4, DIC, POC/N were collected on bottom-ice sections (0-3 cm) following from undiluted sample melt (i.e., individual section melt without FSW). Other measurements of DIC, Chl a, ice algal community composition, and gross primary productivity (GPP) were obtained from bottom-ice sections (0-3 cm) following diluted pooled sample melt (i.e., 3 cm pooled core sections melting into cooler jugs filled up with FSW). Inorganic nutrients (NO2 + NO3, PO4, Si(OH)4) of the ice-ocean interface water and bulk-ice were measured in pseudo-duplicate on a nutrient autoanalyzer (QuAAtro 39, SEAL Analytical, Germany) after filtration of the melted ice samples (onto 25 mm GF/F filter (Whatman)) and storage at -20ºC. In situ brine nutrient concentrations were also calculated by calculating brine salinity (Sb), and by multiplying the nutrient concentrations with the ratio of brine salinity to bulk-ice salinity. Molar ratios of NO2 + NO3 to PO4 (N:P) and NO2 + NO3 to Si(OH)4 (N:Si) were calculated for the ice-ocean interface, bulk-ice and brine. Besides, DIC was measured in duplicate using an Apollo SciTech Inc. Infrared CO2 analyzer after spiking 20 µL of HgCl2 to the ice melted (in darkness) and storage at 4°C of the samples in the dark. Additionally, POC/N measurements were processed on a Lab-Leeman CEC 440 CHN Analyzer after filtration (onto 21 mm GF/F filters (Whatman)), previously combusted at 450ºC for 6 h) and storage at -20ºC. All samples of NO2 + NO3, PO4, Si(OH)4, DIC and POC/N were processed within six months of collection. Moreover, measurements of Chl a were obtained in pseudo-duplicate from raw fluorescence measurements taken using a Turner Designs Trilogy fluorometer before and after addition of 2N HCl, after filtration (onto a 25 mm GF/F filter (Whatman)) and pigment extraction (in 90% acetone at 4ºC in darkness for 24 h). Community composition was assessed by subsampling 100 mL of the pooled ice samples into Nalgene bottle and fixation with 10 mL of a 20% formalin solution before storage at 4°C. Cell identification was conducted under an inverted microscope (Leica DM IL LED), largely to genus level, and were counted in an Utermöhl chamber (Hydro-Bios, 2.973 mL) over three transects of 20 mm for a total of 400 cells (at minimum). Ultimately, GPP was measured at each sampling site, conducting 3 h-incubations of sea ice algal samples with a 14C radioactive solution (4 µCi mL-1) and then, by measuring the radioactivity of the samples on a scintillation counter (Tri-Carb 2900 TR) within one month of collection. From these measurements photosynthesis versus irradiance (PI) curves were modelled using the exponential function in the presence of photoinhibition (Platt et al., 1981). The Chl a-normalized photophysiological parameters such as maximum photosynthetic rate in the absence P_s^B or presence of photoinhibition〖 P〗_m^B (μg C μg Chl a-1 h-1), respectively, photosynthetic efficiency α^B (μg C μg Chl a-1 h-1 [µmol photons m-2 s-1]-1), photoacclimation index Ik (µmol photons m-2 s-1), photoinhibition rate β^B(μg C μg Chl a-1 h-1 [µmol photons m-2 s-1]-1) were determined with Matlab (R2020b (9.9.0), Mathworks Inc., USA).
Method for laboratory-based data (UiT): Data collected at UiT The Arctic University of Norway on ice algal cultures originally sampled at Van Mijenfjorden Site 2 (VM2) in 2021/2022 include Chl a, relative community composition (%), cell abundance (cells L-1), 14C-based GPP and photophysiology were collected after short-term growth (4 h- 24 h) and long-term growth (168 h) of the VM2 ice algal cultured community in two salinity treatments, one control of 33 (normal ocean surface salinity) and one lowered salinity of 10. The Chl a concentration was also measured at initial time of the experiment (0 h) to investigate the evolution of algal biomass in the treatments over time. All measurements were obtained following the methodology described previously for field samples, with the exception that samples for taxonomy and cell abundance were fixed with acidic lugol solution, and theoretical DIC concentrations assuming 100% saturation (Parsons et al., 1984) were used rather than directly measured. This experiment was conducted in triplicate over four months.