POSTER SESSION 1
15:30–17:30, Monday, September 14
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ABSTRACT 1102 | POSTER 225
DOM TRANSFORMATION IN THE BALTIC SEA’S SEA-SURFACE MICROLAYER DURING A CYANOBACTERIAL BLOOM: WHAT ARE THE DOMINATING PROCESSES?
The sea-surface microlayer (SML) is located at the ocean-atmosphere boundary, exposed to high UV-radiation, able to accumulate surface-active organic matter, including carbohydrates, lipids, and amino acids, through various enrichment processes. Understanding the organic matter transformation and enrichment is key to estimating the SML’s ability to modulate ocean-atmosphere fluxes such as gas exchange. Photodegradation and microbial processing have distinguishable effects on colored dissolved organic matter (CDOM) spectra. Photodegradation typically destroys large aromatic chromophores, increasing the spectral slope ratio (SR = S275-295/S350-400) and reducing molecular weight, while microbial processing consumes labile protein-like compounds and produces humic-like fluorescence. In the Baltic Sea, a semi-enclosed, brackish shelf sea under high anthropogenic pressure, cyanobacteria are key ecosystem organisms, fixing atmospheric nitrogen and contributing to eutrophication. At the sea surface, cyanobacteria and other plankton exude surface-active substances that fuel both enrichment and transformation processes in the SML. Based on the Baltic Sea research cruise AL597 (Kiel-Tallinn-Kiel) with RV Alkor, a multi-parameter optical and biological dataset was collected from paired SML and underlying water samples and clustered into three biogeochemical regimes. Combined optical measurements of CDOM and fluorescent DOM (FDOM) reveal higher protein-like fluorescence in the SML, indicating increased fresh biological production which provides labile substrates. A lower slope ratio, and lower humification indices, relative to the underlying water, further support that microbial processing likely dominates dissolved organic matter transformation in the SML. This pattern persists across three hydrographically and biologically distinct regimes, ranging from cool, saline, cyanobacteria influenced waters to warm, low-salinity, terrestrially influenced bloom conditions.
Claudia Thölen*, Carl von Ossietzky Universität Oldenburg, [email protected], https://orcid.org/0000-0002-7600-5313
Michael G. Novak, Helmholtz-Center hereon, [email protected], https://orcid.org/0000-0001-7687-2938
Rüdiger Röttgers, Helmholtz-Center hereon, [email protected], https://orcid.org/0009-0000-6820-1237
Jochen Wollschläger, Carl von Ossietzky Universität Oldenburg, Institute for Chemistry and Biology of the Marine Environment, [email protected], https://orcid.org/0000-0001-5399-4414
Oliver Zielinski, Universität Rostock, Faculty for Agriculture, Civil and Environmental Engineering, [email protected], https://orcid.org/0000-0002-6018-5030
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