Oral Session 5

Shining light on FDOM dynamics in the sea-surface microlayer: A mesocosm study

The thin boundary layer between ocean and atmosphere, the sea-surface microlayer (SML), is of high relevance for ocean biogeochemistry and climate-related exchange processes. Literature shows, that an SML, enriched in organic matter, is known to hinder light, gas, momentum, and heat exchanges between ocean and atmosphere. However, underlying processes of organic matter enrichment in the SML are insufficiently understood. Heterogeneity and dynamics in the open sea make it difficult to differentiate between transport processes, environmental drivers, and biogeochemical processes. Therefore, a late-spring phytoplankton bloom mesocosm experiment was conducted for a month in the framework of the project BASS (Biogeochemical Processes and Air-Sea Exchange in the Sea Surface Microlayer). Daily SML and underlying water (ULW) samples were collected, alternately in the morning and afternoon. Supplementary parameters like irradiance, surfactants, and chlorophyll-a were recorded within the mesocosm basin, generating a unique dataset on biological SML formation and degradation. Since SML glass plate sampling produces small sample volumes, spectral fluorometric methods, which exhibit high sensitivity and structural specificity with respect to organic matter, are used for fluorescent dissolved organic matter (FDOM) analysis. Results support organic matter enrichment in the SML. During higher biological activity in the peak of the phytoplankton bloom larger FDOM intensity differences in SML and ULW were observed. Humic-like and protein-like components, depend negatively and positively, respectively, on the incident light, as the morning and afternoon samples had different lengths of sun exposition. This study implies that FDOM component analysis shines light on organic matter transformation processes in the SML.

*Claudia Thölen, Carl von Ossietzky Universität Oldenburg, Germany, https://orcid.org/0000-0002-7600-5313

Jochen Wollschläger, Carl von Ossietzky Universität Oldenburg, Germany, https://orcid.org/0000-0001-5399-4414

Oliver Zielinski, Leibniz-Institute for Baltic Sea Research Warnemünde, Germany, https://orcid.org/0000-0002-6018-5030

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