Biochemical Characteristics of the Sea Surface Microlayer in the Central Baltic Sea and Potential Signatures of Cyanobacterial Blooms

Abstract. The sea surface microlayer (SML) forms the <1mm thin ocean’s boundary with the atmosphere and plays a critical role in mediating air–sea gas exchange and biogeochemical cycling. However, the biological processes shaping its molecular composition remain insufficiently understood. During a research cruise in the central Baltic Sea (Eastern Gotland Basin), we investigated how phytoplankton, including cyanobacteria, influence the biomolecular composition of the SML. Although no major bloom was detected, distinct shifts in phytoplankton composition were observed, leading to pronounced differences in biomolecular characteristics between the SML and underlying water (ULW), and between conditions characterized by high and low cyanobacteria abundance. While SML enrichment patterns and carbohydrate concentrations were comparable to those previously reported for the Western Baltic Sea, concentrations of total amino acids (TAA) and surfactants were substantially higher, suggesting enhanced production by cyanobacteria. Distinct molecular signatures were associated with different phytoplankton size classes. During periods of high abundance of small phytoplankton (HPA<20 µm; Synechococcus-dominated), the SML was characterized by elevated surfactant and total combined carbohydrate (TCCHO) concentrations. Furthermore, Synechococcus sp. co-varied with the non-protein amino acid g-aminobutyric acid (GABA), particularly under HPA<20 µm conditions. This suggests that the production of surface-active organic matter may be linked to Synechococcus sp. In contrast, under high abundance of large phytoplankton (HPA>20µm; filamentous and colonial cyanobacteria), total amino acids (TAA), particulate amino acids (PAA>20µm), and particulate combined carbohydrates (PCCHO>20µm) were enhanced in the ULW, mirroring POC>20µm and cyanobacterial biomass patterns. The significant correlation between phytoplankton >20µm biomass and POC>20µm suggests that the particulate organic carbon pool was largely cyanobacteria-derived, even in the absence of a distinct bloom. Together, our results imply that phytoplankton size structure and taxonomy exert distinct biomolecular imprints on SML chemistry in the Central Baltic Sea. The contrasting roles of filamentous/colonial cyanobacteria (proteinaceous signatures) and Synechococcus (carbohydrate/surface-activity imprint) imply community-dependent modulation of surface activity and, indicate that changes in biodiversity may potentially impact air–sea gas exchange in the ocean.