Water samples were taken during the spring (29 April to 7 May 2016) expedition HE462 with RV Heincke in the North Sea and Sogne Fjord. Water samples were collected from CTD Niskin bottles at six different depths from the upper 100 m at 17 Stations. These are the same stations as sampled in Bracher and Wiegmann (2019). 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.863445.

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.