How biogenic polymers control surfactant dynamics in the surface microlayer: Insights from a coastal Baltic Sea study

Surfactants can hamper gas exchange by up to 50 % in coastal seas, however, their small-scale temporal and spatial dynamics are poorly constrained. This study investigated possible biogenic sources of surfactants in the sea surface microlayer (SML) and the underlying water at a coastal Baltic Sea site. To relate surfactant dynamics to biogenic production, we conducted two field studies (June and September 2018) and focused on amino acids and carbohydrates as the main components of organic matter derived from phytoplankton. The composition of the biochemicals provided furthermore insights into microbial degradation dynamics and was complemented by flow-cytometry-based community analysis. In total, 76 samples were collected within an area of approx. 50 km2 allowing for high spatial resolution. Moreover, morning and afternoon sampling enabled us to also investigate diel cycles. Our results reveal that surfactant concentrations were tightly coupled to the abundance of nano-phytoplankton and generally higher in September than in June, when cell abundance was three-times higher. Surfactant concentration in June was best explained by the combined effect of the particulate fraction of the non-essential amino acid serine, the concentration of particulate combined carbohydrates (PCHO), and dissolved organic carbon (DOC). Surfactant and PCHO concentrations were significantly enriched in the SML and followed a pronounced diel cycle, possibly linked to microbial- and/or photo-processing. In contrast to June, the surfactant pool in September correlated to a diverse mixture of semi-labile organic matter components, represented best by dissolved glucose and the essential amino acid isoleucine. We conclude that the surfactant pool in surface seawater is mainly composed of organic matter components that resist rapid microbial degradation. Elevated surfactant concentrations are triggered by the release of fresh organic matter. While the effect of the resistant but less surface-active stock is potentially longer-lasting, the additive effect of labile, highly surface-active agents on gas exchange may diminish on short timescales