For decades, the effect of surfactants in the sea-surface microlayer (SML) on gas transfer velocity ( k ) has been recognized; however, it has not been quantified under natural conditions due to missing coherent data on in situ k of carbon dioxide (CO ₂ ) and characterization of the SML. Moreover, a sea-surface phenomenon of wave-dampening, known as slicks, has been observed frequently in the ocean and potentially reduces the transfer of climate-relevant gases between the ocean and atmosphere. Therefore, this study aims to quantify the effect of natural surfactant and slicks on the in situ k of CO ₂ . A catamaran, Sea Surface Scanner (S ³ ), was deployed to sample the SML and corresponding underlying water, and a drifting buoy with a floating chamber was deployed to measure the in situ k of CO ₂ . We found a significant 23% reduction of k above surfactant concentrations of 200 µg Teq l ⁻¹ , which were common in the SML except for the Western Pacific. We conclude that an error of approximately 20% in CO ₂ fluxes for the Western Pacific is induced by applying wind-based parametrization not developed in low surfactant regimes. Furthermore, we observed an additional 62% reduction in natural slicks, reducing global CO ₂ fluxes by 19% considering known frequency of slick coverage. From our observation, we identified surfactant concentrations with two different end-members which lead to an error in global CO ₂ flux estimation if ignored.

The sea-surface microlayer (SML) at the boundary between atmosphere and hydrosphere represents a demanding habitat for bacteria. Wind speed is a crucial but poorly studied factor for its physical integrity. Increasing atmospheric burden of CO₂, as suggested for future climate scenarios, may particularly act on this habitat at the air–sea interface. We investigated the effect of increasing wind speeds and different pCO₂ levels on SML microbial communities in a wind-wave tunnel, which offered the advantage of low spatial and temporal variability. We found that enrichment of bacteria in the SML occurred solely at a U₁₀ wind speed of ≤5.6 m s⁻¹ in the tunnel and ≤4.1 m s⁻¹ in the Baltic Sea. High pCO₂ levels further intensified the bacterial enrichment in the SML during low wind speed. In addition, low wind speed and pCO₂ induced the formation of a distinctive bacterial community as revealed by 16S rRNA gene fingerprints and influenced the presence or absence of individual taxonomic units within the SML. We conclude that physical stability of the SML below a system-specific wind speed threshold induces specific bacterial communities in the SML entailing strong implications for ecosystem functioning by wind-driven impacts on habitat properties, gas exchange and matter cycling processes.

This paper describes a state-of-the-art research catamaran to investigate processes such as air–sea gas exchange, heat exchange, surface blooms, and photochemistry at the sea surface microlayer (SML) with high-resolution measurements of 0.1-Hz frequency. As the boundary layer between the ocean and the atmosphere, the SML covers 70% of Earth. The remote-controlled Sea Surface Scanner is based on a glass disk sampler to automate the sampling of the thin SML, overcoming the disadvantages of techniques such as low volume sampling and ex situ measurement of the SML. A suite of in situ sensors for seven biogeochemical parameters (temperature, pH, dissolved oxygen, salinity, chromophoric dissolved organic matter, chlorophyll-a, and photosynthetic efficiency) was implemented to characterize the SML in reference to the mixed bulk water. The Sea Surface Scanner has the capability to collect 24 discrete water samples with a volume of 1 L each for further laboratory analysis. Meteorological parameters such as wind speed influence SML properties and are continuously monitored. This paper reports the use of the Sea Surface Scanner to identify and study (i) upwelling regions and associated fronts, (ii) rain events, and (iii) the occurrence of surface blooms. The high patchiness of the SML was detected during the observed sea surface phenomena, and high-resolution mapping of the biogeochemical parameters of the oceanic boundary layer to the atmosphere are presented for the first time. The Sea Surface Scanner is a new technology to map and understand sea surface processes and, ultimately, to fill the gaps in knowledge about ocean–atmosphere interactions relevant to ocean and climate science.

Through the production and export of their calcite coccoliths, coccolithophores form a key component of the global carbon cycle. Despite this key role, very little is known about the biogeochemical role of different coccolithophore species in terms of calcite production, and how these species will respond to future climate change and ocean acidification. Here we present the first study to estimate species-specific calcite production, from samples collected in the Arctic Ocean and subarctic Iceland Basin in June 2012. We show that although the coccolithophorid Coccolithus pelagicus comprised only a small fraction of the total community in terms of abundance (2%), our estimates indicate that it was the major calcite producer in the Arctic Ocean and Iceland Basin (57% of total calcite production). In contrast, Emiliania huxleyi formed 27% of the total abundance and was responsible for only 20% of the calcite production. That C. pelagicus was able to dominate calcite production was due to its relatively high cellular calcite content compared with the other species present. Our results demonstrate for the first time the importance of investigating the complete coccolithophore community when considering pelagic calcite production, as relatively rare but heavily calcified species such as C. pelagicus can be the key calcite producers in mixed communities. The response of C. pelagicus to ocean acidification and climate change has the potential therefore to have a major impact on carbon cycling within the North Atlantic and Arctic Ocean

We provide a large data set on salinity anomalies in the ocean's skin layer together with temperature anomalies and meteorological forcing. We observed an average salinity anomaly of 0.40 ± 0.41 practical salinity unity (n = 23,743), and in 83% of the observations the salinity anomaly was positive; that is, the skin layer was more saline. Temperature anomalies determined by an infrared camera were −0.23 ± 0.28 °C (upper 20‐μm layer in reference to nominal 1‐mm depth) and slightly warmer with −0.19 ± 0.25 °C in an upper 80‐μm layer in reference to 1‐m depth. In 75% of the observations, our data confirmed the presence of a cooler skin layer. Light rain rates (<4 mm/hr) induced an immediate freshening by 0.25 practical salinity unit in the skin layer without any effect in the mixed layer at 1‐m depth. Vertical mixing by strong winds (12 m/s) masked freshening during a heavy rain fall (47 mm/hr) by the intrusion of saltier deeper waters, but a freshening was observed after the wind and rain calmed down. We computed density anomalies, which suggest that denser skin layers can remain afloat up to a density anomaly of 1.3 g/L, likely due to the interfacial tension between the skin layer and underlying bulk water. It implies that salinization by evaporation regulates buoyancy fluxes, a key process for the exchange of climate‐relevant gases and heat between the ocean and atmosphere.

Wind, chemical enhancement, phytoplankton activity, and surfactants are potential factors driving the air-sea gas exchange of carbon dioxide (CO₂). We investigated their effects on the gas transfer velocity of CO₂ in a large annular wind-wave tank filled with natural seawater from the North Atlantic Ocean. Experiments were run under 11 different wind speed conditions (ranging from 1.5 ms⁻¹ to 22.8 ms⁻¹), and we increased the water pCO₂ concentration twice by more than 950 μatm for two of the seven experimental days. We develop a conceptual box model that incorporated the thermodynamics of the marine CO₂ system. Surfactant concentrations in the sea surface microlayer (SML) ranged from 301 to 1015 μgL⁻¹ (as Triton X-100 equivalents) with enrichments ranged from 1.0 to 5.7 in comparison to the samples from the underlying bulk water. With wind speeds up to 8.5 ms⁻¹, surfactants in the SML can reduce the gas transfer velocity by 54%. Wind-wave tank experiments in combination with modeling are useful tools for obtaining a better understanding of the gas transfer velocities of CO₂ across the air-sea boundary. The tank allowed for measuring the gas exchange velocity under extreme low and high wind speeds; in contrast, most previous parametrizations have fallen short because measurements of gas exchange velocities in the field are challenging, especially at low wind conditions. High variability in the CO₂ transfer velocities suggests that gas exchange is a complex process not solely controlled by wind forces, especially in low wind conditions.