Water samples were taken during North Sea to Fram Strait expedition PS121 with RV Polarstern from 11 Aug to 10 Sep 2019. Water samples were collected from the ship's seawater supply pumped through teflon tubing from about 11m depth during underway as described in Liu et al (2018). The same water samples were measured as in Bracher et al. (2021). Water samples for CDOM absorption analysis are filtered through 0.2 µm filters and analysed onboard with a 2.5-m path length liquid waveguide capillary cell system (LWCC, WPI) following Levering et al. (2017). Details on method adaptation to our instrumentation set-up are provided in Alvarez et al. (2022). Salinity data were extracted from https://doi.pangaea.de/10.1594/PANGAEA.930022.

Further specifications on the measurements:a) Instrument set-up:light source: UV-VIS lamp (Mikropack DH-2000 Deuterium-Tungsten Halogen Light Source, Artisan Technology Group, USA)cuvette: Liquid Waveguide Capillary Cell system (LWCC, World Precision Instruments Inc, USA) with 197.8 cm pathlengthdetector: photodiode array spectrometer (Sensline AVASPEC-2048-RS, Avantes, Netherlands)pump: Ismatec Reglo Analogb) configurations:wavelength range of scan and resolution: 200.0-760.0 nm, 0.3 nm.slit band width: 1 nm.Dark current was recorded before each sample measurement and samples were measured against reference of MQ three times.c) further specifications about data analysis:interpolated wavelength range and resolution: 350-730 nm, 1nm. Salinity correction applied based on CTD and thermosalinograph data.