Description: Between Greenland and Spitsbergen, Fram Strait is a region where cold ice-covered Polar Water exits the Arctic Ocean with the East Greenland Current (EGC) and warm Atlantic Water enters the Arctic Ocean with the West Spitsbergen Current (WSC). In this compilation, we present two different data sets from plankton ecological observations in Fram Strait: (1) long-term measurements of satellite-derived (1998-2012) and in situ chlorophyll a (chl a) measurements (mainly summer cruises, 1991-2012) plus protist compositions (a station in WSC, eight summer cruises, 1998-2011); and (2) short-term measurements of a multidisciplinary approach that includes traditional plankton investigations, remote sensing, zooplankton, microbiological and molecular studies, and biogeochemical analyses carried out during two expeditions in June/July in the years 2010 and 2011. Both summer satellite-derived and in situ chl a concentrations showed slight trends towards higher values in the WSC since 1998 and 1991, respectively. In contrast, no trends were visible in the EGC. The protist composition in the WSC showed differences for the summer months: a dominance of diatoms was replaced by a dominance of Phaeocystis pouchetii and other small pico- and nanoplankton species. The observed differences in eastern Fram Strait were partially due to a warm anomaly in the WSC. Although changes associated with warmer water temperatures were observed, further long-term investigations are needed to distinguish between natural variability and climate change in Fram Strait. Results of two summer studies in 2010 and 2011 revealed the variability in plankton ecology in Fram Strait.

Description: At each study station, surface water temperature (±0.2°C) and salinity (±0.06) were measured in situ by triplicate readings with a multiparameter probe (Horiba U-10, Japan). Using Niskin bottles, surface water samples were collected for quantification of plankton and estimations of chlorophyll a. For the former, 250 ml were fixed with Lugol's solution (1% final concentration) and kept in the dark until analysis under the microscope, while for chlorophyll a, a volume of 200–250 mL was filtrated onboard through filter GF/C and kept at -20°C.

Description: Water sampling was carried out weekly between April and October 2016. Water samples were taken from two shallow sites (less than 5 km apart) in the Orkneys. Sampling was done using a bucket at both sampling locations. Water for filtration was transported to the Orkney Lobster Hatchery where 500–1000 mL of water was filtered over 0.4 µm polycarbonate (PC) filter (Whatman, 47mm). Filters were stored at -20 °C until DNA isolation in the laboratory. The samples were processed at the AWI to produce a metabarcoding data set with the goal of analysing the general diversity at the two sites but also to investigate the dynamics of harmful algal bloom species (HABs). As the data also contained a wealth of information on protistan parasites they were used to produce an additional manuscript on oomycete infections in the planktonic foodweb at the two sites. The bioinformatic processing of the raw sequence files was conducted as follows. The low-quality 3'-ends of the reads were trimmed by Trimmomatic (version 0.38) and the paired-ends were merged by VSEARCH (version 2.3.0). Cutadapt (version 1.19, was used to adjust the sequence orientation and to remove the forward and reverse primer matching sequence segments. Sequences were only kept in the sequence pool if both primer matching segments could be detected. To ensure a high sequence quality the remaining sequences were filtered by VSEARCH and only kept if the expected base error (sum of all base error probabilities) of a sequence was above 0.25. Chimeric sequences were predicted sample-wise by VSEARCH in de novo mode with default settings and removed from the sequence pool as well. Only samples which consisted of at least 10000 sequences after filtering were considered for further analyses. The remaining sequences were clustered into OTUs by the tool swarm (version 2.2.2) with default settings. For each OTU the most abundant amplicon was selected as representative and taxonomically annotated with the default classifier implemented in mothur (version 1.38.1). As reference the Protist Ribosomal Reference database (PR2), version 4.10 was chosen and the minimum confidence cut-off for annotation was set to a value of 80. Further details of the pipeline are provided in (Sprong, et al. 2020; doi:10.1016/j.seares.2020.101914).

Description: Water samples were collected from the long-term ecological research (LTER) site at Helgoland and the eukaryotic microbial community was assessed. 18S V4 region was amplified using the primer set 528iF /964iR and amplicon sequencing was performed on an Illumina MiSeq™ sequencer in a 2 × 300 bp paired-end run. Sequence data have been deposited in the European Nucleotide Archive (ENA) at the European Bioinformatics Institute (EMBL-EBI) under accession number PRJEB37135 using the data brokerage service of the German Federation for Biological Data (GFBio). 21 million sequences remained after bioinformatic processing and were clustered into Operational Taxonomic Units (OTUs). The Protist Ribosomal Reference database (PR2), version 4.11.1 was used as reference database.

Description: Raw data acquired by two thermosalinographs (SBE21, SeaBird GmbH) on board RV Polarstern were processed to yield a calibrated and validated data set of temperature, conductivity and salinity during expedition PS121. Both sensors were equipped with a more accurate external temperature sensor (SBE38, Sea-Bird GmbH). Data were downloaded from the DAVIS SHIP data base (https://dship.awi.de) with a resolution of 1 sec. The raw hex data were converted to temperature and conductivity while a sensor drift correction was applied using calibration coefficients from before and after the expedition. Salinity was calculated according to the instructions from the Practical Salinity Scale PSS-78, using the obtained (internal) temperature and conductivity data and a pressure of 11 dbar which represents the water depth of the inlet of the TSG system on Polarstern. A speed filter of 0.5 knots minimum speed was applied. Processed data are provided as 10min means of seawater salinity, conductivity and temperature, aligned with position data taken from the master track of PS121 (doi:10.1594/PANGAEA.908157). Further details and evaluation of the data is outlined in the data processing report found at the EPIC repository under URL (https://hdl.handle.net/10013/epic.7fffb528-06bd-48ae-8489-cea0444c4eab).

Description: Time series data of physical oceanography (seawater conductivity, temperature, pressure, salinity), ocean current velocities and hydroacoustics were obtained from mooring F4-18 in the Fram Strait in July 2018 - August 2019 as part of the Helmholtz infrastructure program Frontiers in Arctic Marine Monitoring (FRAM) and the long-term monitoring program at AWI HAUSGARTEN. The mooring was deployed during Polarstern expedition PS114, and recovered during PS121. The attached archive contains raw data files of 6 Seabird SBE37 microcats (nominal depths: 63m, 150m, 250m, 500m, 731m, 1202m; sampling interval 1h), one RDI Workhorse Longranger ADCP (depth: 380m; sampling interval: 1h) and two AADI RCM11 current meters (depths: 730m, 1208m; sampling interval: 2h). The mooring also included a Develogic Sonovault (nominal depth: 800m; data archived elsewhere). Auxiliary information such as sensor calibration sheets, mooring diagrams and schedule files are also provided, if applicable.