Description: The current understanding of Arctic ecosystems is deeply rooted in the classical view of a bottom-up controlled system with strong physical forcing and seasonality in primary-production regimes. Consequently, the Arctic polar night is commonly disregarded as a time of year when biological activities are reduced to a minimum due to a reduced food supply. Here, based upon a multidisciplinary ecosystem-scale study from the polar night at 79°N, we present an entirely different view. Instead of an ecosystem that has entered a resting state, we document a system with high activity levels and biological interactions across most trophic levels. In some habitats, biological diversity and presence of juvenile stages were elevated in winter months compared to the more productive and sunlit periods. Ultimately, our results suggest a different perspective regarding ecosystem function that will be of importance for future environmental management and decision making, especially at a time when Arctic regions are experiencing accelerated environmental change.

Description: We present an accurate, fast, simple and non-destructive photographic method to estimate wax ester and lipid content in single individuals of the calanoid copepod genus Calanus and test this method against gas-chromatographic lipid measurements.

Description: Temperature and heating-induced temperature differences were measured along a chain of thermistors. SIMBA 2020T61 (a.k.a. UiT0206) is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during MOSAiC (Leg 2) in February 2020. The thermistor chain was 5 m long with 250 evenly distributed temperature sensors. The resulting time series describes the evolution of temperature and temperature differences after two heating cycles of 30 and 120 s as a function of location (GPS), depth and time between February 5th, 2020 and July 27th 2020 at 6 hour intervals for temperature and 24 hours for temperature differences. The buoy was installed on the transect across “Alli's Ridge” on the first year ice (FYI) flank of the ridge, away of the second year ice (SYI) floe of the Central Observatory. In addition to temperature, geographic position (GPS), barometric pressure, air temperature measured 1m over the ice level, tilt and compass were measured. This instrument was deployed as part of the project "Ridges - Safe HAVens for ice-associated Flora and Fauna in a Seasonally ice-covered Arctic OCean" (HAVOC), funded by the Research Council of Norway, project number: 280292, "Coordinated Arctic Acoustic Thermometry Experiment" (CAATEX), funded by Norwegian Research Council, project number: 280531 and "Arctic Ocean ecosystems - Applied technology, Biological interactions and Consequences in an era of abrupt climate change" (Arctic ABC Development, Research Council of Norway, project number: 245923).

Description: Bioacoustic data from an Acoustic Zooplankton Fish Profiler (AZFP) deployed in the MOSAiC Central Observatory the four AZFP frequencies (38, 125, 200 and 455kHz) are published. This echosounder was deployed on February 16th, 2020 and retrieved on August the 2nd 2020, after 168 days of operation. The conditions during deployment are described as second year ice in the MOSAiC Central Observatory. The AZFP was connected to the surface buoy with a 5 m long umbilical cable and acoustic beam of the echosounder was pointed downward. Due to its dimensions, weight and color this buoy type was sometimes referred to as the “white monster”, or “AZFP-2”, “POPEDOWN” or “ITO306” in the different MOSAIC-related reports. Data from the Iridium messages used to control the surface buoy are also included. These include geographical position together with date and time stamp of the AZFP. An independent GPS tracker of the type XEOS Rover was attached to the AZFP and its GPS track is also part of this data set. For the AZFP data, the initial and final deployment (.DPL) and configuration (.XML) files are included. these describe the instrument configuration and sampling plan. The bioacoustics data sets (.01A,.01B, ..., .02A,...) are consecutive, divided hourly. the first two digits describe the phase of the sampling program being used when the file was created while the letter B and C are for files generated within a same hour when the process is interrupted, for example, when backing up the data from the echosounder. During MOSAiC two AZFPs were deployed. The other one had an AZFP pointing upwards, see doi:10.1594/PANGAEA.953621.

Description: Bioacoustic data from an upward looking echosounder, of the model Acoustic Zooplankton Fish Profiler (AZFP) are published. this instrument was deployed in the MOSAiC Central Observatory. Four AZFP frequencies (38, 125, 200 and 455kHz) are included. This echosounder was deployed on January 29th 2020 and retrieved on August the 3nd 2020, after 187 days of operation. The buoy was placed on a 20-m wide area of level ice surrounded by SYI old ridges of the main floe. Access via the road to the Dark site with a junction to the buoy area right before the crossing of Allis ridge was made. Data from the Iridium messages used to control the surface buoy are also included. These include geographical position together with date and time stamp of the AZFP. An independent GPS tracker of the type XEOS Rover was attached to the AZFP and its GPS track is also part of this data set. For the AZFP data, the initial and final deployment (.DPL) and configuration (.XML) files are included. These describe the instrument configuration and sampling plan. The bioacoustics data sets (.01A,.01B, ..., .02A,...) are consecutive, divided hourly. the first two digits describe the phase of the sampling program being used when the file was created while the letter B and C are for files generated within a same hour when the process is interrupted, for example, when backing up the data from the echosounder. During MOSAiC two AZFPs were deployed. The other one had an AZFP pointing downwards, see doi:10.1594/PANGAEA.949811.

Description: Three ice-tethered / floating systems measuring sub - sea-ice Photosynthetic Active Radiation (PAR) were deployed within the MOSAiC ice camp from February 2020, and recorded hourly data through to later summer 2020. These Optical Chain And Logger (OptiCAL) systems capture PAR-like data at 12 logarithmically-spaced levels down to 50 m depth, and, along with positional data, transmit these data hourly via Iridium. The PAR sensors have a wide dynamic range, able to detect PAR from 5x10⁻⁵ µMol s⁻¹ m⁻² to 100 µMol s⁻¹ m⁻² and thereby graph a wide range of light behaviour from winter to summer and from surface to depth. Sampling at the 12 levels is accomplished by resin-containment 'nodes' located along a power and communication cable that hangs vertically from the underside of the surface spar-buoy. The nodes are at nominal depths of 1, 2, 3, 4, 5, 6, 7, 9, 14, 21, 32, and 50m. Each node has six 'PAR' sensors, three detecting downwelling light and three upwelling light. Two of each set of three sensor sets have simple optical filters: one transmitting 'orange' the other 'blue'. The third sensor, being unfiltered, is 'white'. PAR data are archived in CSV files as 'counts' from the sensors, and are in units of millihertz (mHz). The detail of the CSV format is provided in this directory (see OptiCAL 'hh' buoy: Detail of the CSV format under Documentation), but to convert from mHz to the expected SI unit of µMol s⁻¹ m⁻² requires a conversion function. Details of this conversion are provided in this directory (see OptiCAL 'hh' buoy: Data description under Documentation). The 'hh' unit was deployed in the Dark site Second Year Ice, next to ITBOB and Salt/Light Harps. Its drift started at N 88.0651900, E 79.20462533 on 16th Feb 2020 and last data transmission was on 9th August at 13:00 UTC due to battery power being exhausted. Intended recovery was not possible before positional data were lost. This instrument was deployed as part of the project Ridges - Safe HAVens for ice-associated Flora and Fauna in a Seasonally ice-covered Arctic OCean (HAVOC), funded by the Research Council of Norway, project number: 280292 and Arctic Ocean ecosystems - Applied technology, Biological interactions and Consequences in an era of abrupt climate change (Arctic ABC Development, Research Council of Norway, project number: 245923). Ship-time associated to the participation of the DEARice team (D. Divine on leg 2) on the MOSAiC Expedition has been supported by the European Union's Horizon 2020 research and innovation programme under grant agreement No 730965 (ARICE).