Thölen, Claudia; Wollschläger, Jochen; Novak, Michael; Röttgers, Rüdiger; Zielinski, Oliver: PARAFAC components and fluorescent dissolved organic matter (FDOM) indices on organic matter transformation processes in the sea-surface microlayer and the underlying water during a mesocosm phytoplankton bloom in 2023 [dataset]. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.988058 (dataset in review)

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 month-long mesocosm study, in May and June of 2023, at the Institute for Chemistry and Biology of the Marine Environment (ICBM) in Wilhelmshaven, Germany. 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) in the Sea Surface Facility (SURF) of the ICBM. The facility contains an 8 m × 1.5 m × 0.8 m large outdoor basin with a retractable roof, which was closed at night and during rain events. The basin was filled with North Sea water from the adjacent Jade Bay. Homogeneity of the ULW in the basin was achieved by constant mixing of the water column. The daily SML and ULW samples were collected alternating in the morning, about 1 h after sunrise, and in the afternoon, about 10 h after sunrise. The alternation of sampling times intended to capture a potential effect of sun-exposure duration on DOM transformations and elucidated the day and night variability of the layers. The SML was collected via glass plate sampling (Cunliffe and Wurl, 2014). The ULW was sampled via a submerged tube and a connected syringe suction system in 0.4 m depth. The removed sample volume was refilled with Jade Bay water every day. SML and ULW samples were filtered through pre-flushed 0.7 µm Whatman GF/F and 0.2 nucleopore filters into clear 40 ml SUPELCO bottles. These bottles were acid-washed twice and combusted at 500 °C for 5 h. The samples were stored dark and at 4 °C and measured within a few months of the study. FDOM was measured using a Aqualog fluorescence spectrometer (Horiba Scientific, Japan) with 10 seconds integration time and high gain of the CCD (charge-coupled device) sensor within an excitation range from 240 to 500 nm, and an emission range from 209.15 to 618.53 nm. The Aqualog measures fluorescence as well as absorption. The resulting data includes an excitation-emission-matrix (EEM) of the blank (MilliQ Starna cuvette), an EEM of the sample, and the absorption values of the sample. The raw exported Aqualog data was corrected for errors and lamp shifts. The corrected EEM data is then decomposed by PARAFAC (Murphy et al., 2013) for its underlying fluorophore components. Before running the PARAFAC routine, the corrected data needed to undergo a correction process by subtracting the blank from the sample EEM and canceling the influences of the inner-filter effect (IFE, Parker & Rees, 1962; Kothawala et al., 2013). The fluorescence intensity of the IFE-corrected EEM is calibrated by using the Raman scatter peak of water (Lawaetz & Stedmon, 2009). For PARAFAC the corrected data was processed using the drEEM and NWAY toolbox (version 0.6.5; Murphy et al., 2013) in MATLAB (R2020b). A 4-component model was validated with the validation style S4C6T3 for the split half analysis with nonnegativity constraints and 1-8e as the convergence criteria with 50 random starts and a maximum number of 2500 iterations. The resulting final model had a core consistency of 82.04 and the explained percentage was 99.54%. Furthermore, four fluorescence indices were calculated from the corrected EEM data (HIX – Humification index, Zsolnay et al., 1999; BIX – Biological index, Huguet et al., 2009; REPIX – Recently produced index, Parlanti et al., 2000, Drozdowska et al., 2015; ARIX, Murphy, 2025).

The dataset consists of the sample ID, datetime of the sampling event (DD.MM.YYYY hh:mm:ss), the source (SML: sea-surface microlayer; ULW: underlying water), the PARAFAC component data (Fmax1-Fmax4) and the calculated indices (HIX: Humification index, Zsolnay et al., 1999; BIX: Biological index, Huguet et al., 2009; REPIX: Recently produced index, Parlanti et al., 2000, Drozdowska et al., 2015; ARIX, Murphy, 2025). The REPIX index was calculated using the PARAFAC components: (Fmax1 + Fmax3) / Fmax2, as they correspond to the components Drozdowska et al. (2015) use for their calculation of REPIX, (M + T) / (C + A). The nomenclature of the components Drozdowska et al. (2015) used is based on Coble (1996). Fmax1 has an excitation maximum at <240/312 nm, and an emission maximum at 405 nm, which corresponds to marine humic-like components (M) according to Coble, 1996. Likewise, Fmax2 has an excitation maximum at 264/368 nm, and an emission maximum at 464 nm, which corresponds to humic-like components (A&C). Fmax3 has an excitation maximum at 296 nm, and an emission maximum at 340 nm, which corresponds to tryptophan-like components (T). Lastly, Fmax4 has an excitation maximum at 276 nm, and an emission maximum at 307 nm, which corresponds to tyrosine-like components (B). The HIX, BIX and ARIX indices are calculated based on the excitation and emission pairs given in the respective literature.