Enrichment of Dissolved Organic Matter in the Sea Surface Microlayer During a Phytoplankton Bloom – Preliminary Results from a Mesocosm Study

The ocean's uppermost layer, the sea surface microlayer (SML), significantly influences physical and chemical properties due to the enrichment with dissolved organic matter (DOM). Biomolecules exhibiting amphiphilic properties are referred to as surfactants and preferentially accumulate in the SML. Surfactants were previously shown to significantly damp capillary waves and reduce air-sea gas fluxes. However, their source dynamics and chemical identity remain unknown. Phytoplankton communities are the primary producers of major biomolecule classes such as carbohydrates and amino acids. We explored how phytoplankton bloom development shapes enrichment and composition processes in SML and in relation to surface activity. As part of the “BASS” (Biogeochemical processes and air-sea exchange in the sea surface microlayer) project, an experiment was conducted in the mesocosm facility “SURF” in 2023 to study changes in the SML over the course of a phytoplankton bloom for one month. During the experiment, we collected samples for dissolved amino acids (DAA) and dissolved combined carbohydrates (DCCHO) from the SML and the underlying water (ULW). Overall, concentrations of DAA and DCCHO were enriched in the SML compared to the ULW by a factor of 2.88 ± 1.16 and 2.68 ± 1.47, respectively. The highest enrichment factors for DCCHO and DAA occurred a few days after the peak of the phytoplankton bloom. Particularly high enrichment factors were calculated for the polar amino acids arginine (4.67 ± 2.64), glutamic acid (4.31 ± 2.24), and tyrosine (4.46 ± 2.92). However, nonpolar amino acids leucine and phenylalanine showed enhanced enrichment factors as well. Extremely high enrichment with factors were observed for glucose (8.79 ± 8.30), while other DCCHO only showed slight enrichment. Our results point towards a strong effect on the surface activity of polar and freshly produced, very labile DOM. Investigating variations in the biomolecular composition of the SML in relation to potential source dynamics further enhances our understanding of biogeochemical and climate-relevant processes in the SML, such as air-sea gas exchange.