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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

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 days 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 88.11 and the explained percentage was 99.55%. 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).

Grain size distribution of soil samples from the White Main catchment, Germany, November 2024

This dataset contains grain size classifications and class proportions of dried soil samples from the White Main catchment in northern Bavaria, Germany. It includes sample dry mass, grain size classes, minimum and maximum grain size ranges, and the relative proportion of each grain size class.

Fluorescence spectra of stream water samples from the White Main headwater stream, Germany, 2023–2024

This dataset contains fluorescence spectra of stream water samples collected from the White Main headwater stream in northern Bavaria, Germany, from August 2023 to April 2024. Fluorescence intensities from 240 to 400 nm were measured to characterize dissolved organic matter properties.

Fluorescence spectra of acid- and base-treated soil extracts from the White Main catchment, Germany, November 2024

This dataset contains fluorescence spectra of acid- and base-treated soil extracts from soil samples collected at three locations within the White Main catchment in northern Bavaria, Germany. Fluorescence intensities from 240 to 400 nm were measured to characterize soil-derived organic matter properties.

Carbon concentrations, stable carbon isotopes, calculated pCO2 and CO2 fluxes from the White Main headwater stream, Germany, 2023–2024

This dataset contains laboratory-measured alkalinity, dissolved inorganic and organic carbon concentrations, δ13C values of DIC and DOC, and calculated pCO2, CO2 fluxes, and k600 from stream water samples collected along the White Main headwater stream in northern Bavaria, Germany, from March 2023 to April 2024.

UV-Vis absorbance spectra of stream water samples from the White Main headwater stream, Germany, 2023–2024

This dataset contains UV-Vis absorbance spectra of stream water samples collected from the White Main headwater stream in northern Bavaria, Germany, from August 2023 to April 2024. Absorption coefficients between 230 and 700 nm were measured using a microplate spectrophotometer to characterize dissolved organic matter properties.

Zeta potential, particle concentration, and electrical conductivity of soil-derived water extracts from the White Main catchment, Germany, November 2024

This dataset contains zeta potential, particle concentration, electrical conductivity, and pH measurements of soil-derived samples from three locations within the White Main catchment in northern Bavaria, Germany, treated with either Milli-Q water or White Main stream water. The data also include sample treatment, dilution factor, and replicate information.

UV-Vis absorbance spectra of soil sample acid-base extracts from the White Main catchment, Germany, November 2024

This dataset contains UV-Vis absorbance spectra of acid and base extracts from soil samples collected at three locations within the White Main catchment in northern Bavaria, Germany. Absorption coefficients were measured between 230 and 698 nm using a microplate spectrophotometer to characterize soil-derived dissolved organic matter properties.

Hydrological and hydrochemical field parameters from the White Main headwater stream, Germany, 2023–2024

This dataset contains field parameters measured along the first 1.3 km downstream of the White Main spring in northern Bavaria, Germany, from March 2023 to April 2024. It includes sampling location and site information, stream geometry, discharge, and in-situ physicochemical parameters such as water temperature, pH, electrical conductivity, and redox potential.

Major ions and trace elements from the White Main headwater stream, Germany, 2023–2024

This dataset contains major anion, major cation, and trace element concentrations measured in stream water samples collected along the White Main headwater stream in northern Bavaria, Germany, from March 2023 to April 2024. Analyses include fluoride, chloride, sulfate, nitrate, sodium, magnesium, potassium, silicon, calcium, aluminium, manganese, and iron.

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