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 the ship's seawater supply pumped through teflon tubing from about 5 m depth during underway. These are the same samples 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.873226.

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.