Glacier meltwater supplies a significant amount of silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these essential nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Si and Fe within fjord systems, given the rapid deposition of reactive particulate fractions at fjord heads. The dataset is used to examine the benthic cycling of the two nutrients at Patagonian fjord heads through geochemical analyses of sediment pore waters and reaction-transport modeling for Si. The dataset contains: (i) pore water redox-sensitive nitrate (NO3-) and dissolved manganese (DMn) concentration data, nutrient dissolved silicon (DSi) and iron (DFe) concentration and isotope data (delta30 Si, delta56 Fe); (ii) mild alkaline leachable (Si-Alk) and acid leachable (Si-HCl) sediment silica content and isotope data; and (iii) reaction transport model output for the benthic cycling of Si. The pore water and sediment samples were collected from four sites: SJ (48.228o S, 73.502o W, 106 m depth), SH (47.679 S, 73.715 W, 203 m depth), SP (48.179 S, 73.347 W, 248 m depth) and SB (47.787 S, 73.610 W, 151 m depth) in the Baker-Martinez Fjord Complex on the research vessel Sur-Austral in February 2017. Funded by NERC-CONICYT grant NE/P003133/1-PII20150106.

Measurements of benthic invertebrate particle reworking, bioirrigation, and associated nutrient concentrations used in the analyses by Solan et al. 2020 (see referenced publication) for shipboard incubations of replicate macrofaunal communities from the Western Barents Sea during summer 2017 and summer 2018. Samples were collected on cruises JR16006 and JR17007. Funding was provided by ''The Changing Arctic Ocean Seafloor (ChAOS) - how changing sea ice conditions impact biological communities, biogeochemical processes and ecosystems'' project (NE/N015894/1 and NE/P006426/1, 2017-2021), part of the NERC funded Changing Arctic Ocean programme.

A seasonal cycle of the FA composition of particulate organic matter from surface waters, Chlorophyll-a maximum layer and bottom sea ice, sampled during the MOSAiC expedition in the Central Arctic Ocean (2019-2020), suggests the importance of phylogenetic and environmental drivers. To improve our understanding of these different drivers, we conducted culture experiments with 32 cold-water algal strains where temperature, light intensity, and nutrient supply were manipulated individually or in combination. The culture experiments were carried out at the Culture Collection of Algae and Protozoa (CCAP; Oban, Scotland), the Roscoff Culture Collection (RCC; Roscoff, France) and the Alfred-Wegener-Institute-Helmholtz-Centre for Polar and Marine Research (AWI; Bremerhaven, Germany). The strains were part of the culture collections, had been isolated in the Arctic (25 strains), Southern Ocean (2 strains) or North Atlantic (5 strains), and included diatoms, chlorophytes, haptophytes, cryptophytes, chrysophytes, dinoflagellates and cyanobacteria. Some of the species are Arctic sea ice diatoms (e.g. Nitzschia frigida, Attheya spp.) or pelagic diatoms (e.g. Thalassiosira gravida), while others are non-diatom species that are becoming increasingly prominent in the Arctic, e.g. the coccolithophore Emiliania huxleyi (synonym Gephyrocapsa huxleyi), the prymnesiophyte Phaeocystis pouchetii, the chlorophyte Micromonas spp. and the cyanobacterium Synechococcus spp.. The experiments can be divided into three groups: First, those that tested a low light-low temperature setting, second, those that tested a low light-low temperature and a higher light-higher temperature setting and, third, those that tested the effect of nutrient (nitrate, phosphate and silicate) shortage in combination with low and high light intensity. The first set of experiments was conducted with all 32 strains, the second set with all strains grown at CCAP and AWI, and the third set focuses on the keystone under-ice diatom Melosira arctica. The experiments were run for 4-7 weeks to accumulate sufficient biomass for biomarker extractions (FA and sterols), C:N analysis and light-microscopy of cell size and cell concentration. At the end of the experiments, the algae were filtered onto GF/F filters and deep frozen until analysis. After addition of internal standards for FA and sterols, the filters were saponified with KOH. Thereafter, non-saponifiable lipids (sterols) were extracted with hexane and purified by open column chromatography on silica gel. FA were obtained by adding concentrated HCl to the saponified solution and re-extracted with hexane. Samples were converted into fatty acid methyl esters (FAME) and analysed using an Agilent 6890N gas chromatograph with FID detector. 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 FA datasets of cultured algae are presented in a manuscript together with the FA pattern seen in sea ice- and water column POM in the CAO during the MOSAiC expedition and in previously published data from Arctic shelf regions. The manuscript focusses mainly on two important 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). CRM was funded by the NERC National Capability Services and Facilities Programme (NE/R017050/1).