Biogeochemical dynamics of the sea-surface microlayer in a multidisciplinary mesocosm study

Abstract. The sea-surface microlayer (SML) represents the thin uppermost layer of the ocean, typically less than 1000 µm in thickness. As an interface between the ocean and the atmosphere, the SML plays a key role in marine biogeochemical cycles. Its physical and chemical properties are intrinsically linked to the dynamics of the surface ocean's biological communities, especially those of phytoplankton and phytoneuston. These properties, in turn, influence air–sea interactions, such as heat and gas exchange, which are modulated by the interaction between organic matter composition and surfactants in the SML and the underlying water (ULW). However, the dynamic coupling of biogeochemical processes between the SML and the ULW remains poorly understood. To contribute to filling this knowledge gap, we conducted a multidisciplinary mesocosm study at the Center for Marine Sensor Technology (ZfMarS), Institute of Chemistry and Biology of the Marine Environment (ICBM), Wilhelmshaven, Germany. In this study, we induced a phytoplankton bloom and observed the subsequent community shift to investigate the effects on the SML biogeochemistry. Samples were collected daily to analyse inorganic nutrients, phytopigments, surfactants, dissolved and particulate organic carbon (DOC, POC), total dissolved and particulate nitrogen (TDN, PN), phytoplankton and bacterial abundances, and bacterial utilisation of organic matter. A clear temporal segregation of nutrient samples in the SML and ULW was observed through a self-organising map (SOM) analysis. Phytoplankton bloom progression throughout the mesocosm experiment was classified into three phases: pre-bloom, bloom, and post-bloom based on Chlorophyll a (Chl a) concentration. Chl a concentration varied from 1.0 to 11.4 µg L−1. POC and PN followed the Chl a trend. Haptophytes, specifically Emiliania huxleyi, dominated the phytoplankton community, followed by diatoms, primarily Cylindrotheca closterium. An enrichment of surfactants and DOC was observed after the bloom. During the bloom, a distinct surface slick with complete surfactant coverage of the air–sea interface created a biofilm-like habitat in the SML, leading to increased bacterial cell abundance. The bacterial community utilised amino acids as the preferred carbon source, followed by carbohydrates in both water layers. Our findings highlight that the SML is a biogeochemical reactor, playing a crucial role in the production, transformation, and microbial activity of autochthonous organic matter, thus exhibiting the potential to strongly affect air–sea exchange. Incorporating SML dynamics into Earth system models will enhance climate predictions and improve ocean-atmosphere interaction studies on both regional and global scales.