The effects of a phytoplankton bloom and photobleaching on colored dissolved organic matter (CDOM) in the sea-surface microlayer (SML) and the underlying water (ULW) were studied in a 33-day mesocosm study at the Institute for Chemistry and Biology of the Marine Environment in Wilhelmshaven, Germany, in 2023. The mesocosm study was conducted by the DFG research group BASS (Biogeochemical processes and Air–sea exchange in the Sea-Surface microlayer, Bibi et al., 2025). Daily SML and ULW samples were collected for spectral photometric analysis, alternately in the morning and afternoon. SML and ULW samples were filtered through pre-flushed 0.7 µm Whatman GF/F and 0.2 nucleopore filters into brown bottles and were stored dark and at 4 °C until measurement within weeks of the study. The brown bottles were previously combusted at 500 °C. CDOM was measured with three liquid waveguide capillary cells (LWCC, WPI, USA) of different pathlengths (10 cm, 50 cm, 250 cm) to increase the measurement sensitivity following the protocols of Röttgers et al. (2024) using a spectral detector (Avantes, Netherlands) for a total spectral range from 230 to 750 nm. A sodium chloride (NaCl) solution was used for the salinity correction. The blank-corrected absorbance spectra were then converted into Napierian absorption coefficients (Bricaud et al., 1981).