Water samples were taken during North Sea to Fram Strait expedition PS99.1 with RV Polarstern from 14 to 23 Jun 2016. Water samples were collected from CTD Niskin bottles at five to six different depths from the upper 100 m. The same water samples were measured as in Liu et al. (2021). Water samples for CDOM absorption analysis were 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 taken using SeaBird 911-plus CTD (Conductivity-Temperature-Depth) mounted on the SBE 32 Carousel Water Sampler (Rosette) equipped with 24 Niskin Bottles (12-L capacity). CTD data were also quality checked using salinity measured from water samples and analysed in laboratory with Autosal Salinometer Guildine 8400B. Extrapolated data were used from CTD downcasts based on data at depths were also water samples were taken.

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.