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.

The air-sea heat exchange has a substantial impact on the heat budget of the surface ocean. Due to the heat loss across the air-sea interface, the uppermost millimeter of the ocean is generally cooler than the underlying water. This cool-skin layer can be naturally disrupted by processes such as wave breaking or wind stress, as well as by anthropogenic factors like ships or offshore wind turbines. Understanding recovery times after complete disturbance is hindered by observational challenges. Previous studies, which used stationary observations with thermal imagery, have focused on natural disruptions. This study presents in situ high-resolution temperature measurements of the skin and near-surface layer during the complete disturbance of the ocean’s surface layer using the autonomous surface vehicle Halobates. The vehicle drifted within artificially disturbed water masses, enabling observation and analysis of thermal recovery. The average temperature difference between skin and near-surface layer was –0.240 ± 0.037°C. The thickness of the skin layer was computed as 1.06 ± 0.16 mm approaching 0.2 mm during disturbances. Recovery times of the cool-skin layer ranged from 62 to 157 seconds, with rates between –0.047 and –0.140°C min–1. The main findings demonstrate that the complete recovery of the cool-skin layer can take up to three minutes following intense surface disruption, such as that caused by vessel-induced turbulence. These results have important implications for modeling air-sea interactions in areas with frequent human activity. Combined with prior research on rapid recovery from natural disturbances, these results enhance our understanding of surface layer dynamics.

The biochemical composition of seston along a salinity gradient were examined in the low-turbidity, temperate, estuarine embayment, Gwangyang Bay in Korea. Seasonal variations in sestonic protein (PRT), carbohydrate (CHO), and lipid (LIP) concentrations were analyzed to assess the effects of physiological status and taxonomic composition of phytoplankton. The concentrations of biochemical compounds displayed a close relationship with chlorophyll a (Chla). PRT:CHO ratios were high (>1.0) in the estuarine channel in warmer months and in whole bay in February, indicating a N-replete condition for phytoplankton growth. High CHO:LIP ratios (>2.5) in the saline deep-bay area during the warmer months (>2.0) emphasized the importance of temperature and photoperiod over nutritional conditions. The low POC:Chla (<200), molar C:N (~7) ratios, and biopolymeric carbon concentrations coupled with high primary productivity indicated a low detrital contribution to the particulate organic matter pool. Diatom dominance throughout the year contributed to consistently high carbohydrate concentrations. Furthermore, generalized additive models highlighted that phytoplankton community (i.e., size) structure may serve as an important descriptor of sestonic biochemical composition. Collectively, our results suggest that physiological and taxonomic features of phytoplankton play prominent roles in determining the biochemical composition of seston, supporting the fact that the ecosystem processes in Gwangyang Bay are largely based on phytoplankton dynamics.

The sea-surface microlayer (SML) represents the thin uppermost layer of the ocean, typically less than 1,000 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. 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 (Chla) concentration. Chla concentration varied from 1.0 to 11.4 μg L⁻¹. POC and PN followed the Chla 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 hotspot, 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.