DOI: 10.5285/d7708d08-4bd4-439a-99e2-307a175977ea
title: Fatty acid composition of particulate organic matter (POM) collected from surface waters and bottom sea-ice of the Central Arctic Ocean during the MOSAiC expedition in 2019/2020

description: During the MOSAiC expedition in the Central Arctic Ocean (CAO, 2019-2020), POM was sampled weekly to fortnightly from surface waters and the Chlorophyll a maximum layer (Chl a max) via CTD casts and from bottom sea ice of the floe via ice coring (first- and second-year ice, two layers nearest to the water-ice interface). The POM was filtered onboard (GF/F filters) and deep frozen for the subsequent analysis of a suite of lipid biomarkers, including fatty acids (FA), FA-compound-specific stable isotopes (FA-CSIA), sterols, and highly-branched isoprenoids (HBI). These biomarkers can provide valuable information about the nutritional value, the taxonomic composition (e.g. diatoms vs flagellates), and the origin of the POM that represents the basis of the Central Arctic food web. This dataset comprises the results from the FA analysis only, those from other biomarkers will be submitted in due cause. The separation of the various lipid biomarkers was carried out at the University of Plymouth. After addition of internal standards for each of the 3 components, the filters were saponified with KOH. Thereafter, non-saponifiable lipids (HBI and sterols) were extracted with hexane and purified by open column chromatography (SiO2). Fatty acids were obtained by adding concentrated HCl to the saponified solution and re-extracted with hexane. Further steps of the FA analysis were carried out at the Alfred-Wegener-Institute (AWI) in Bremerhaven. Here samples were converted into fatty acid methyl esters (FAME) and analysed using an Agilent 6890N gas chromatograph. The Clarity chromatography software system (DataApex, Czech Republic) was used for chromatogram data evaluation. FAME were quantified via the internal standard, Tricosanoic acid methyl ester (23:0) (Supelco, Germany) to provide the total amount of FA (TFA) per filter. These TFA quantities per filter can be normalised to the volume of filtered seawater or melted ice core water. Additionally, we provide the mass percentage composition of the TFA, considering 48 individual FA. The FA are presented in shorthand notation, i.e., A:B(n-x), where: A indicates the number of carbon atoms in the straight fatty acid chain, B represents the number of double bonds present, n represents the terminal methyl group and x denotes the position of the first double bond from the terminal end. The biochemical nomenclature of the fatty acids is provided. The dataset is linked to a manuscript that compares pattern seen in sea ice- and water column POM in the CAO with previously published data from Arctic shelf regions. This manuscript focusses mainly on two key long-chain omega-3 FA (eicosapentaenoic acid and docosahexaenoic acid) that are considered essential for the nutrition of higher trophic levels, including humans, and their production to decline with global temperature rise. Contributions by KS were funded by the UK's Natural Environment Research Council MOSAiC Thematic project SYM-PEL: "Quantifying the contribution of sympagic versus pelagic diatoms to Arctic food webs and biogeochemical fluxes: application of source-specific highly branched isoprenoid biomarkers" (NE/S002502/1).
description: The MOSAiC cruise: The MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate; tag MOSAiC20192020 and the Project ID: AWI_PS122_00) expedition represents the first year-round interdisciplinary study of the atmosphere, the sea ice, the ocean, the ecosystem, and biogeochemical processes during the transpolar drift across the CAO with a unique opportunity for intensive field sampling (Shupe et al. 2022; Rabe et al. 2022; Nicolaus et al. 2022). The observational year was divided into 5 legs: Leg 1 started on October 4, 2019 with the setup of the first Central Observatory (CO1) and installations on the research icebreaker RV Polarstern north of the Laptev Sea (Krumpen et al. 2020) (Fig. 2b). The winter Leg 2 and spring Leg 3 continued the work on CO1, before RV Polarstern had to leave the floe, for logistical reasons, on May 16, 2020. The vessel returned to the original ice floe on June 19, 2020, but at a different location some hundred meters away. Leg 4 continued the drift with the new CO2 over the summer until the disintegration of the floe in the Fram Strait on July 31, 2020. During Leg 5, RV Polarstern travelled back into the ice and started the setup of CO3 on August 21, 2020, near the North Pole. The third drift ended on September 20, 2020, when the vessel started the return voyage. The water column sampling: Particulate organic matter samples for lipid biomarker analysis were collected from 12 L Niskin bottles attached to the shipboard 24-bottle CTD-rosette (PS-CTD). During legs 1-3, additional water column sampling was conducted via a 5 L Niskin bottle-CTD-rosette from Ocean City (OC), an in-ice hole 300 meters from RV Polarstern (OC-CTD). In the period between mid-March and mid-May, all the water column samples were collected at OC due to the loss of the ice hole alongside the ship (see Rabe et al. 2022). Pelagic particulate organic matter (POM) for FA analysis was collected at 2 m and/or the chl a max (based on CTD fluorescence sensor profiles). Volumes of 7 to 10 L of seawater were filtered via a vacuum pump (-20 kPA) onto pre-combusted (3h, 550°C) 47 mm Whatman GF/F filters and stored in aluminium foil at -80 °C until further processing. Additional under-ice sampling took place during MOSAiC Leg 5 from August 26 to September 15, 2020. An in-situ automatic pump phytoplankton sampler (PPS; serial number 12697-01, McLane; USA) was deployed at a depth of 11 m to collect suspended particles. In 2-3 day intervals, 4-6 L seawater were filtered through combusted GF/F filters under different pre-set sample parameters of the PPS. Sea ice sampling: During Leg 1, areas of first year ice (FYI) and second year ice (SYI) were identified that were safely accessible, relatively homogeneous, and large enough to accommodate repeat visits, potentially for the entire drift. The sites were located away from RV Polarstern to minimize the human impacts (e.g. artificial lights, traffic, fumes, noise). Cores for biological properties were collected using a 9-cm diameter KOVACS Mark II ice corer. Cores were usually sectioned and parsed into sterile Whirlpak bags directly inside the ice coring tent under low and/or red-light conditions to minimize artefacts. Small-scale horizontal variability was reduced by pooling 3-4 ice cores, creating a more homogeneous master sample from which related properties were derived. FA profiles of sea ice POM were derived from the two bottom 5 cm intervals (0-5 cm, 5-10 cm) of the pooled ice cores (ECO Pool 1), alongside with samples for pigment analysis (Chl a, HPLC), particulate organic matter (POC/PON) and flow cytometry. All pooled samples were melted after the addition of filtered surface seawater (typically 50 ml per 1 cm of core section) to reduce the impact of osmotic stress and cell loss (Garrison and Buck 1986, Campbell et al. 2019). Ice core sections in bags were melted in the dark at room temperature (18-22 degrees Celsius) and checked every 4-6 hours. Upon completed melt, which took from 12 to 40 hours, bags were transferred into dark, temperature-controlled lab containers, and parsed for sub-sampling of biological properties under red light to minimize artificial light stimulation of biological activities. The subsamples were filtered via a vacuum pump (-20 kPA) onto pre-combusted 47 mm Whatman GF/F filters and stored in aluminium foil at -80 degrees Celsius until further processing.
description: In situ-sample collection: 12 L Niskin bottles attached to the shipboard 24-bottle CTD-rosette; automatic pump phytoplankton sampler (PPS; serial number 12697-01, McLane; USA); 9-cm diameter KOVACS Mark II ice corer. Fatty acid analysis: FAME were quantified using an Agilent 6890N gas chromatograph (Agilent Technologies, USA) with a DB-FFAP capillary column (60 m, 0.25 mm I.D., 0.25 micrometer film thickness, Agilent Technologies, USA) supplied with a splitless injector and a flame ionization detector using temperature programming. Chromatogram data evaluation: Clarity chromatography software system (version 8.8.0, DataApex, Czech Republic)
description: Due to the low concentrations of the POM, the water volume that needed to be filtered was often &gt;10000 mL and filtration of replicate samples was not feasible. A total of 9, out of 153 samples, were lost for FA analysis due to the low amount of material filtered. The fatty acid profiles were compared to several commercial- and self-produced standards (e.g. Arctic algae standard, Bacteria standard, Calanus spp. standard), and fatty acid peaks were identified accordingly. In a few cases, samples were also analysed with the mass spectrometer and peaks were identified via (1) the mass of the compound, (2) the retention time of the compound and (3) the equivalent chain length method.