Orals: Thu, 1 May | Room 1.34
The sea-surface microlayer (SML) represents the thin (< 1000 µm) uppermost layer of the ocean. Due to its unique position between ocean and atmosphere, the SML plays a central role in marine biogeochemical cycles. Changes in the phytoplankton biomass and community composition are linked to profound changes in the physical, chemical, and biological properties of the SML. And this influences air-sea interaction such as heat and gas exchange, organic matter composition, and surface-active substances in the SML and underlying water (ULW). Dynamic interactions between the SML and the ULW and the connectivity of the biogeochemical processes in the SML remain unclear. To fill this knowledge gap, we conducted a multidisciplinary mesocosm study. Here we report the general setup in a 17 m3 mesocosm facility, the progression of an induced phytoplankton bloom, and the general description and coupling of the changes in biogeochemical properties of the SML and the ULW.
SML and ULW samples were collected daily to analyze inorganic nutrients (NO3-, NO2-, PO43-, SiO32-), turbidity, solar radiation, phytopigments, surfactants, dissolved and particulate organic carbon (DOC, POC), total dissolved and particulate nitrogen (TDN, PN), phytoplankton and bacterial abundance, and their utilized substrates.
A self-organizing map (SOM) configuration revealed a clear temporal segregation of nutrient samples in SML and ULW. Based on nutrient levels, phytoplankton bloom progression over the time of the mesocosm experiment could be clearly classified into pre-bloom, bloom, and post-bloom phases. During this time, Chla concentrations varied from 1.0 to 11.4 μg L-1 with an average of 7.3 µg L-1. POC and PN exhibited a strongly positive relationship (r = 0.95) and followed the trend of Chla. Turbidity demonstrated a peak during bloom phase, which was associated with a high biological activity. Phytopigment composition data showed that haptophytes were the dominant phytoplankton group, followed by diatoms which could be confirmed by optical methods.
The daily average solar irradiance aligned with the local weather variability. Surfactants were enriched in the SML compared to the ULW. A discrepancy between the onset of increases in phytoplankton biomass and surfactant concentrations was observed with a lag of five days. This mismatch suggests a physiological acclimation of phytoplankton towards less favorable growth conditions, for example, nutrient limitations after the bloom phase. The high surfactant concentrations were also mirrored as DOC and TDN enrichment in the SML compared to ULW. A distinct slick formation with high turbidity was observed, indicating a biofilm-like SML habitat during the bloom and post-bloom phases. This biofilm was characterized by higher bacterial cell counts in SML. Bacterial metabolic profiles assessed by Biolog EcoPlates showed that the bacterial community utilized amino acids as key substrates in both water layers.
The main findings of our study emphasize that changes in biological parameters were linked to changes in chemical and physical parameters in SML. Our study provides deeper insights into the biogeochemical controls of the SML at a mechanistic level. Further spatio-temporal studies are needed to investigate the coupling of biogeochemical processes between the SML and ULW at both regional and global scales.
How to cite: Bibi, R., Ribas-Ribas, M., Lehners, C., Jaeger, L., Gassen, L., Mintah Ayim, S., H. Badewien, T., Wollschläger, J., Thölen, C., H. Brinkhoff, T., Athale, I., Waska, H., Zöbelein, J., Röttgers, R., Novak, M., Engel, A., Karnatz, J., and Wurl, O.: Biogeochemical Links in the Sea-Surface Microlayer: A Multidisciplinary Mesocosm Study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8823, https://doi.org/10.5194/egusphere-egu25-8823, 2025.
The sea surface microlayer (SML) is of global importance as all exchange processes of heat and gases between the ocean and the atmosphere have to pass through it and are regulated by the features of the SML. These exchanges occur not only permanently between the SML and the atmosphere, but also between the SML and the underlying water (ULW). The properties of the SML are strongly influenced by surface-active substances known as surfactants, which are mostly of biological origin. Events such as algal blooms can produce large amounts of surfactants, thus changing the properties of the SML and the ULW. Obtaining in situ data of the SML proved very difficult in the past, due to its small thickness. Using microsensors gives the opportunity to close this gap by obtaining in situ data of the SML and to directly show the influence an algal bloom has on the SML.
A mesocosm experiment was conducted to obtain a more mechanistic understanding of the effect of an algal bloom on the physicochemical properties of the SML. An algal bloom was artificially induced in a seawater basin and physiochemical changes in the SML and ULW were investigated over time by applying multiple techniques. To directly study changes in temperature and oxygen, very precise microsensors (UNISENSE) were used for continuous in situ profiling, measuring from the air, through the SML, and into the ULW on a scale of tens of micrometers. We conducted the experiment over a continuous 30-day period during the algal bloom, allowing us to gain insights into the boundary layer, including the formation of oxygen and temperature gradients and the thickness of the SML.
The microsensor data showed, that the oxygen gradient in the SML is strongly correlated to the chlorophyll a concentration (r = 0.76, p < 0.01) and thus the algal bloom, while the thickness of the oxygen diffusion boundary layer, however, only shows a weak correlation to the surfactant concentration (r = 0.47, p = 0.01). The oxygen measurements deliver the in situ data to verify previous assumptions on oxygen gradients (-10 – 50 µmol L-1) and the thickness of the oxygen diffusion boundary layer (500 – 1500 µm) at the sea surface. The temperature gradient in the SML and the thickness of the thermal boundary layer were not influenced by the algal bloom, but the in situ measurements also confirm previous assumptions on temperature gradients (0.05 – 0.2 °C) and the thermal boundary layer thickness (750 – 2000 µm).
Obtaining gradients of gases or temperature in the SML and calculating the SML thickness was in the past only possible via indirect methods like measuring gas concentration differences between air and ULW or with computing surface temperatures from the emitted longwave irradiance. The in situ microsensor measurements now enable us to directly investigate processes inside the SML without relying on indirect measurements. Overall, we investigated the effect of an algal bloom on the SML and demonstrated a new in situ approach using microsensors to investigate physicochemical changes in and across the SML.
How to cite: Rauch, C., Cortés, E., Jaeger, L., and Wurl, O.: The Effects of Algal Blooms on Oxygen Concentration and Temperature in the Sea Surface Microlayer – a Mesocosm Study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6365, https://doi.org/10.5194/egusphere-egu25-6365, 2025.
The sea surface microlayer (SML) is a microscopic boundary that covers the ocean’s surface, influences CO2 exchange with the atmosphere, and is often exposed to high levels of UV irradiation. The SML is a unique biome and shelters diverse microbial communities. Bioaggregates, containing carbohydrates, lipids and proteinaceous material accumulate in the SML, affecting gas exchange. Despite its role in the global carbon cycle, the biogeochemical processes controlling the production and turnover of organic matter in the SML are poorly understood. This study is part of the collaborative research unit ’Biogeochemical processes and Air-sea exchange in the Sea-Surface microlayer’ (BASS). Our goal is to decipher the underlaying forces behind the accumulation of dissolved organic matter (DOM) in the SML and its spatial and temporal dynamics. Furthermore, we aim to link the molecular properties of DOM in the SML to the microbial communities living in the SML, to air-sea gas exchange, and to carbonate chemistry. To address these objectives, we conducted a large-scale mesocosm study with coastal seawater from Jade Bay (North Sea, Germany). Following nutrient addition, a bloom of the coccolithophore Emiliania huxleyi occurred. The SML was sampled with a glass plate, and the underlying water (ULW) was sampled with a tube at a depth of 60 cm. Dissolved organic carbon (DOC) was quantified in filtered samples, which were then desalinated and concentrated for molecular analysis of DOM with ultra-high resolution mass spectrometry. In both the SML and ULW, DOC concentrations almost doubled from pre-bloom to post-bloom conditions. Overall, DOC was higher in the SML than in the ULW, and this discrepancy increased after the algal bloom. Furthermore, the ratio of DOC to DON was significantly higher in the SML than in the ULW after the bloom. Molecular indicators of DOM lability increased concurrently with DOC concentrations, reflecting freshly produced DOM in both SML and ULW during the late algal bloom stages. At the same time, the contributions of aromatic fractions in DOM and a photodegradation index decreased, possibly related to UV exposure of the mesocosm. Overall, our results suggest that primary production is likely to drive organic matter accumulation in the SML.
How to cite: Zöbelein, J., Dittmar, T., and Waska, H.: Spatial and temporal dynamics of dissolved organic matter in the sea surface microlayer during a bloom of coccolithophores, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19728, https://doi.org/10.5194/egusphere-egu25-19728, 2025.
The ocean's uppermost layer, the sea surface microlayer (SML), significantly influences physical and chemical properties due to the enrichment with dissolved organic matter (DOM). Biomolecules exhibiting amphiphilic properties are referred to as surfactants and preferentially accumulate in the SML. Surfactants were previously shown to significantly damp capillary waves and reduce air-sea gas fluxes. However, their source dynamics and chemical identity remain unknown. Phytoplankton communities are the primary producers of major biomolecule classes such as carbohydrates and amino acids. We explored how phytoplankton bloom development shapes enrichment and composition processes in SML and in relation to surface activity. As part of the “BASS” (Biogeochemical processes and air-sea exchange in the sea surface microlayer) project, an experiment was conducted in the mesocosm facility “SURF” in 2023 to study changes in the SML over the course of a phytoplankton bloom for one month. During the experiment, we collected samples for dissolved amino acids (DAA) and dissolved combined carbohydrates (DCCHO) from the SML and the underlying water (ULW). Overall, concentrations of DAA and DCCHO were enriched in the SML compared to the ULW by a factor of 2.88 ± 1.16 and 2.68 ± 1.47, respectively. The highest enrichment factors for DCCHO and DAA occurred a few days after the peak of the phytoplankton bloom. Particularly high enrichment factors were calculated for the polar amino acids arginine (4.67 ± 2.64), glutamic acid (4.31 ± 2.24), and tyrosine (4.46 ± 2.92). However, nonpolar amino acids leucine and phenylalanine showed enhanced enrichment factors as well. Extremely high enrichment with factors were observed for glucose (8.79 ± 8.30), while other DCCHO only showed slight enrichment. Our results point towards a strong effect on the surface activity of polar and freshly produced, very labile DOM. Investigating variations in the biomolecular composition of the SML in relation to potential source dynamics further enhances our understanding of biogeochemical and climate-relevant processes in the SML, such as air-sea gas exchange.
How to cite: Karnatz, J., Barthelmess, T., Ribas-Ribas, M., Lehners, C., Wurl, O., and Engel, A.: Enrichment of Dissolved Organic Matter in the Sea Surface Microlayer During a Phytoplankton Bloom – Preliminary Results from a Mesocosm Study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20162, https://doi.org/10.5194/egusphere-egu25-20162, 2025.
The sea surface microlayer (SML) acts as a biogeochemical and (photo)chemical reactor. It is enriched with surfactants that modulate the physico-chemical properties of the interface. As such, the SML reduces the formation of capillary waves and thus turbulent air-sea gas exchange.
In recent years, the surface sensitive methods of Vibrational Sum Frequency Generation (VSFG) and Langmuir through compression isotherms (LT) have been used to characterize the state of the SML on the nanoscale. Here, we give a brief overview of the results obtained during the last decade, reporting on a variety of experiments ranging from (i) artificial laboratory experiments with model wet and dry surfactants (Triton X-100 and DPPC), (ii) semi-natural large-scale mesocosm experiments (SURF facility in Wilhelmshaven, Germany, 2023), and (iii) the analysis of natural samples. These include samples from a study targeting slick versus non-slick conditions (near Helgoland island, Germany, 2024), year-long time-series measurements at Boknis Eck Time series Station as well as during the Baltic GasEX campaign (Eckernförde Bay, Germany, 2009-2019). In this context, we have derived a surface coverage index as a proxy parameter to reduce the spectral VSFG information to a single parameter in order to enable correlation with other biogeochemical and physical variables, including surfactant activity based on AC voltammetry and wave damping data from previous studies.
We hypothesize that gas exchange reduction can be constrained by a surfactant coverage threshold. Working out solid correlation of biogeochemical parameters with surfactant coverage would help to better model the influence of the SML on large-scale air-sea gas exchange based on their climatologies.
How to cite: Schäfer, F., Lange, F.-D., Laß, K., and Friedrichs, G.: VSFG Based Surfactant Coverage Index - A Feasible Approach to Assess the Effect of the SML on Air-Sea Gas Exchange?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20597, https://doi.org/10.5194/egusphere-egu25-20597, 2025.
The air and water flow boundary layers are strongly coupled with the wave field and the physical phenomena involved are essentially based on small submillimeter/millimeter scale features and dynamic processes within the first millimeters above and below the SML (Sea surface Micro Layer). The scale at which these complex feed-back mechanisms operate make their study particularly challenging.
Surfactants at the air-sea interface strongly dampen both the dominant gravity-capillary waves and micro-breaking waves and hence dramatically influence the dynamics and associated air-sea fluxes. Even though the general effect of these monolayers on the waves is well known by the scientific community, their influence on the surface dynamics and air-sea fluxes associated still need to be carefully studied.
A series of experiments were conducted at the 26-m long, 1.5-m high, 1-m wide wind-wave tank of the University of Hamburg (Germany), where a measurement system was developed and installed at a fetch of 15.5m. The system offers the possibility to perform high resolution (33 µm/pixel) PIV (Particle Image Velocimetry) to capture the motion in the air-water flows in the SML’s vicinity, and LIF (Laser Induced Fluorescence) to accurately detect the wavy interface, with a resolution of 55 µm/pixel. Experiments were carried out at a reference wind speed of 4.5 m/s, without and with an insoluble surfactant (oleyl alcohol).
In slick free conditions, high vorticity regions are observed under the wave crests. On the air-side, the viscous sublayer detaches from the crest of most of the observed waves, and is being regenerated on the windward side of the directly following wave. However, thanks to the wide 50-cm field of view, some evidence was found that, under specific conditions, the sheltered region past the airflow separation can overcome a wave, hence strongly affecting its growth. After deployment of oleyl alcohol at the water surface, the dominant gravity-capillary waves are strongly dampened, and the capillaries mostly disappeared. Waves are still sheltering the airflow on their leeward side, but no clear airflow separation is being seen, and the enhanced turbulent regions, which were observed below the crest in slick-free conditions, are thinner, more elongated, and less intense. In the waterside, it has also been noticed that with surfactants, some streaks are being ejected away from the wavy interface.
How to cite: Tondu, C., Buckley, M., and Gade, M.: Influence of a Surfactant on Physical Processes Above and Below Wind-Generated Waves in a Wind-Wave Tank , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13644, https://doi.org/10.5194/egusphere-egu25-13644, 2025.
The sea-surface microlayer (SML), the thin boundary interface between the ocean and the atmosphere, is of global relevance as oceans are largely assumed to carry an SML. Characterized by its enrichment in organic material and exposure to strong solar radiation, the SML is expected to be a photochemically active zone that plays a critical role in the cycling of organic compounds and that influences air-sea exchange processes. Carbonyl compounds are particularly important as known products of photochemical reactions at the ocean's surface, making their behavior potentially relevant for understanding abiotic reactions and exchanges with the atmosphere. This study investigates the photochemical production and degradation of aldehydes and ketones in both ambient SML and bulk seawater samples. Samples were collected during a mesocosm field campaign at the Sea-surface Facility (SURF), located at the Institute for Chemistry and Biology of the Marine Environment (ICBM) in Wilhelmshaven. To simulate natural conditions, the samples were irradiated for 5 hours using a temperature-controlled aqueous-phase photoreactor equipped with a light source that mimics actinic radiation. The formation and degradation of target carbonyl compounds were analyzed using a derivatization technique with o-(2,3,4,5,6-Pentafluorobenzyl)hydroxylamine (PFBHA), followed by solvent extraction and GC-MS analysis. The findings provide a quantitative evaluation of the formation and degradation dynamics of carbonyl compounds to understand differences between the SML and the underlying bulk seawater. First results suggest the photochemical formation of acetaldehyde and methyl vinyl ketone, and the photochemical degradation of trans-2-hexenal. For other target compounds, including acetophenone, acrolein, butyraldehyde, crotonaldehyde, glyoxal, hexanal, heptanal, hydroxyacetone, methacrolein and propionaldehyde, no consistent trend of formation or degradation was observed. The concentrations of these carbonyl compounds varied significantly depending on the sample, ranging from a few ng L-1 to a few mg L-1. This study contributes to a deeper understanding of the role of the SML as a reactive environment and its implications for biogeochemical cycles and air-sea interactions.
How to cite: Jibaja Valderrama, O., Schaefer, T., van Pinxteren, M., and Herrmann, H.: Photochemical dynamics of carbonyl compounds in the sea-surface microlayer (SML) based on a mesocosm study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8931, https://doi.org/10.5194/egusphere-egu25-8931, 2025.
The near-surface ocean is central to exchanging energy, gases, and particles between the atmosphere and the upper ocean. In particular, the interaction processes between the sea surface microlayer and the underlying water are crucial for biogeochemical processes and climate science. An innovative approach using free-floating, minimal-invasive Lagrangian sensor drifters is employed to investigate hydrographic and dynamical processes in the near-surface layer. Each drifter is equipped with a sensor chain containing temperature and salinity sensors, enabling high-resolution vertical measurements down to a depth of 1.8 m.
The Lagrangian measurement method enables the dynamics of a water mass to be recorded in its natural inertial system without external influences such as ship-induced disturbances. During a field campaign in the North Sea near Helgoland in July 2024, temperature and salinity data were collected during slick events associated with algal blooms. Processes inside and outside the slicks, as well as their formation, dispersion and decay processes, were studied to understand the underlying mechanisms. This allows the analysis of horizontal and vertical gradients, as well as the investigation of the spatial and temporal dynamics of slicks, understanding their impact on the exchange processes and quantifying the importance of the sea surface microlayer and the underlying water.
Initial results reveal significant differences in temperature and salinity gradients between slick and non-slick areas. Slicks act as hydrodynamic microhabitats and critical boundaries, influencing vertical convection patterns and current shear in the near-surface layer. These results are confirmed by ADCP backscatter data collected from an autonomous catamaran, providing additional insights into current structures and particle distributions. Horizontal comparisons between multiple sensor-equipped drifters illustrate the variability of processes at small spatial scales.
The presented results demonstrate the potential of Lagrangian drifters as a minimally invasive, innovative and highly accurate method for studying slicks and climate-relevant processes in the near-surface layer. These approaches can significantly improve our understanding of air-sea interaction mechanisms and their role in global biogeochemical cycles.
How to cite: Deyle, L., Albinus, M., Meyerjürgens, J., and Badewien, T. H.: Hydrodynamic Processes and Temperature-Salinity Gradients in Slicks: Insights from Lagrangian Observations in the Near-Surface Layer, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15118, https://doi.org/10.5194/egusphere-egu25-15118, 2025.
The sea surface microlayer (SML) is the boundary layer at the surface of the ocean, distinct from the water below and highly variable in space and time. SML is influenced by organisms that aslicks. Slicks are the result of surfactants dampening capillary waves, which can be seen in synthetic aperture radar (SAR) imagery because the smooth surface reflecting backscatter away from the receiver. This experiment investigated the presence and abundance of surfactant-associated bacteria in the SML above a coral reef and in slicks in a coastal seagrass ecosystem. During the experiment in the Florida Keys, 220 SML and subsurface water (SSW) samples were collected above a coral reef area and in slicks above a coastal seagrass ecosystem. During our previous experiments in the Gulf of Mexico samples were collected in the daylight only; while, in the Straits of Florida, in both daylight and nighttime (due to the study of the coral sponging, which however did not happen at the time of the experiment). All SML and SSW samples were sequenced on the Illumina MiSeq, 12 surfactant associated bacteria genera were found. Increasing wind speed had a negative effect on the abundance of these genera, with lower wind speeds showing a more habitable environment. The ratio of abundance of surfactant-associated bacteria between the SML and SSW was found different and affected by the ultraviolet component of solar radiation. Thus, the concentration of bio-surfactants in the SML may be different during the daylight and nighttime with corresponding consequences for the SAR imagery and air-sea interactions.
How to cite: Soloviev, A., Parks, G., and Tartar, A.: Effect of Solar Radiation on Presence and Abundance of Surfactant Associated Bacteria in the Sea Surface Microlayer , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18260, https://doi.org/10.5194/egusphere-egu25-18260, 2025.
The sea surface microlayer (SML) refers to the uppermost millimeter of the ocean surface that is in direct contact with the atmosphere. It has physicochemical and biological properties that are distinct from the underlying water and its properties determine air-sea exchange of momentum, mass and energy. Gas transfer velocity is mostly determined by wind forcing, where gas transfer is enhanced at low to moderate wind speeds. However, biological and pollutant enrichment of the SML with surfactants reduces gas transfer by suppressing turbulence and damping waves. Local impacts from surfactants can be significant, reducing air-sea gas transfer by single to double-digit percentages at moderate wind speeds.
I calculate a timeseries of contemporary global air-sea CO2 fluxes using FluxEngine, adjusting the calculation for the presence of biological surfactants. Surfactant suppression of air-sea gas transfer is estimated as a function of total organic carbon concentration, which is in turn estimated using global satellite products of particulate and dissolved organic carbon. Results will be compared to previous regional estimates of surfactant regulation of CO2 fluxes. This approach will produce a novel global estimate of biological surfactants’ regulation of CO2 fluxes across the air-sea interface, supporting further work isolating pollutants’ role in regulation of gas transfer.
How to cite: Kvale, K.: Surfactants’ global regulation of CO2 fluxes across the air-sea interface, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7269, https://doi.org/10.5194/egusphere-egu25-7269, 2025.
The sea surface microlayer (SML), the critical interface between the ocean and atmosphere (≤ 1000 μm thick), plays a vital role in regulating the exchange of climate-relevant gases, such as CO2. This study provides the first evaluation of the SML in a tropical estuarine system, covering over 80 km of the Gulf of Urabá in Caribbean Colombia. It investigates the distribution and influence of surfactants, focusing on the effect of fluvial inputs during the rainy and dry seasons. Samples were collected from fluvial and marine zones, revealing no significant differences in surfactant concentrations or enrichment factors. However, surfactant concentrations were significantly higher during the rainy season (1011.63 ± 745.21 μg Teq L⁻¹, August 2021) than the dry season (428.34 ± 189.44 μg Teq L⁻¹, April 2022). Notably, all sampling stations exhibited surfactant concentrations exceeding 200 μg Teq L⁻¹, a threshold associated with reductions of up to 23% in the rate of ocean-atmosphere CO2 transfer. Approximately 55% of the recorded concentrations represented a high surfactant regime, while 28% corresponded to slick zones. These values and enrichment factors were higher than those reported in other coastal and oceanic studies. Our findings underscore the significant role of surfactants in tropical biogeochemical cycles and provide valuable new insights into the SML in tropical regions where data is scarce. This research highlights the potential impact of surfactants on CO2 exchange in coastal tropical environments, enhancing our understanding of the ocean-atmosphere interface in such regions.
How to cite: Ribas-Ribas, M., Moreno-Polo, K., Tobón-Monsalve, D., Lehners, C., Wurl, O., Pacheco, W., and Florez-Leiva, L.: Surfactant distribution can impact air-sea exchange in a Tropical Estuarine System in the Caribbean., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8521, https://doi.org/10.5194/egusphere-egu25-8521, 2025.
Short- and long-lived trace gases impact atmospheric chemistry and climate, via processes like hydroxyl radical chemistry, aerosol formation, cloud condensation nuclei formation, or the greenhouse effect. As the oceans serve as sources and sinks for atmospheric trace gases, understanding the drivers of trace gas cycling in surface waters and their release to the atmosphere is crucial for climate predictions. Furthermore, there is a serious lack of information related to trace gas cycling in the uppermost ocean, the Sea surface microlayer (SML). Production and consumption of trace gases was investigated in a five-week mesocosm study with North Sea water at the SURF facility (Wilhelmshaven, Germany), during which an extreme slick formed under a combined diatom and coccolith bloom. In addition to bulk sampling, the glass plate method was used successfully to sample trace gases in the SML. Findings are supported by an extensive set of parameters from other BASS subprojects.
How to cite: Lange, L., Booge, D., Stoltenberg, I., Feil, H., Bange, H. W., and Marandino, C. A.: BASS Mesocosm Study: trace gas processes during a phytoplankton bloom with extreme slick formation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-346, https://doi.org/10.5194/egusphere-egu25-346, 2025.
The air-sea CO₂ exchange is a critical process in regulating Earth's carbon cycle. At the ocean's surface, the sea-surface microlayer (SML) - a thin, organic-rich layer - serves as the critical interface between the air and sea and acts as a microreactor where unique chemical transformations occur, driven by sunlight, biological activity and surface-active materials. However, its role in air-sea CO₂ exchange is not well explored. In this study, we present the first direct measurements of organic alkalinity (OA) in the SML during a mesocosm experiment simulating a coccolithophore bloom of Emiliania huxleyi, aiming to better understand the contribution of organic matter to the air-sea CO₂ exchange.
Our every third day-resolution data on dissolved inorganic carbon (DIC), total alkalinity (TA), pH, and OA, quantified using back-titration, reveal significant differences between the SML and the underlying water (ULW). OA concentrations in the SML were consistently higher, contributing 8.07% ± 2.60 of TA, 2.58 times higher than the 3.12% ± 1.24 contribution observed in the ULW. This enrichment suggests that the SML serve as a significant reservoir for OA, influencing the overall acid-base balance.
During the coccolithophore bloom phase, we observed that photosynthesis and calcification—the dominant biogeochemical processes—resulted in decreases in both TA and DIC in the SML. Normally, changes in DIC would lead to a decrease in pH (increased acidity), while changes in TA might buffer this effect. However, the observed pH variability could not be explained by DIC and TA alone. Only by considering OA concentrations we can explain the observed pH variability. A strong negative correlation between the OA contribution to TA and pH (r = -0.82, p < 0.05) highlighted OA's role in modulating pH only in the SML. While calcification produces CO₂ and lowers pH through the dissociation of carbonic acid, coccolithophores also release organic acids, including humic-like fluorescent dissolved organic matter (fDOM). These acids may contribute to TA, but their primary effect is to release H⁺ ions, further acidifying the surface layer.
The increased OA in the SML contributes to its buffering capacity, but it does not fully counteract the acidification induced by calcification. These findings underscore the importance of incorporating OA dynamics in studies of the SML, particularly in the context of intense biological activity, such as coccolithophore blooms. Our results suggest that pH changes in the SML cannot be fully explained by TA alone, highlighting the need to consider OA in the analysis of the marine carbon system and the air-sea CO₂ exchange. While the specific organic acids contributing to OA remain unidentified for this work, future research into these compounds will be essential for improving our understanding of OA’s role in modulating the Earth's carbon cycle.
How to cite: Cortés, E., Wüst, A. R. I., Lopez Puertas, A., Wurl, O., Hernández Ayón, J. M., Waska, H., and Ribas Ribas, M.: Organic Alkalinity in the Sea-Surface Microlayer: Implications for Ocean Acid-Base Chemistry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11560, https://doi.org/10.5194/egusphere-egu25-11560, 2025.
The effects of a phytoplankton bloom and photobleaching on the colored and fluorescent dissolved organic matter (CDOM and FDOM, respectively) in the sea-surface microlayer (SML) and the underlying water (ULW) were studied in a 33-day mesocosm experiment at the Institute for Chemistry and Biology of the Marine Environment in Wilhelmshaven, Germany. The SML is the thin (< 100 µm) boundary layer between the ocean and the atmosphere and highly relevant to ocean biogeochemistry and climate-related exchange processes. Previous work has shown that when the SML is enriched in organic matter it can hinder gas, light, momentum, and heat exchanges between ocean and atmosphere. However, the underlying processes of organic matter enrichment in the SML are insufficiently understood. Heterogeneity and dynamics in the open sea make it difficult to differentiate between transport processes, environmental drivers, and biogeochemical processes. Hence, the mesocosm study was conducted to gain a deeper understanding of organic matter formation and degradation processes in the SML and ULW. To gain an understanding of different sources and sinks, the hypotheses tested were (1.) phytoplankton blooms result in different FDOM component compositions in the SML and ULW and (2.) photodegradation affects the component composition of the SML and the ULW differently.
Daily SML and ULW samples were collected for spectral fluorometric and photometric analysis, alternately in the morning and afternoon. Supplementary parameters like irradiance, temperature, and chlorophyll-a were also recorded within the mesocosm basin with high temporal resolution (approx. 1 min). Spectral photometric and fluorometric methods, which exhibit high sensitivity and structural specificity with respect to organic matter are used for CDOM and FDOM analysis.
The mesocosm experiment was divided into three phases (bloom onset, peak, and decay). Degradation of larger, complex molecules or production of new organic matter was assessed via the “humification index” and is dependent on the water layer (SML or ULW), the phase of the bloom, and the sampling time. As samples were taken alternatively in the morning and in the afternoon, the exposure time to UV-light and therefore photodegradation as a sink varies differently for SML and ULW. CDOM slope results showed a high variability and generally higher molecular weights and higher molecule aromaticity in the SML compared to the ULW. Protein-like component concentrations increased in both SML and ULW which indicates higher microbial activity towards the peak and decay phase of the experiment. These results suggest that photodegradation and possibly microbial activity have different effects on SML and ULW, verifying hypothesis 2. The affect of higher biological activity during the phytoplankton bloom led to the most pronounced differences between the concentration and composition of organic matter in the SML and ULW, especially in the protein-like components. This finding supports the premises of hypothesis 1.
How to cite: Thölen, C., Wollschläger, J., Novak, M., Röttgers, R., and Zielinski, O.: Effects of a Phytoplankton Bloom and Photobleaching on Colored and Fluorescent Dissolved Organic Matter in the Sea-Surface Microlayer, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11844, https://doi.org/10.5194/egusphere-egu25-11844, 2025.
The Sea Surface Microlayer (SML) in aquatic environments is a thin layer (1–100 μm) at the air-water boundary, characterized by unique biogeochemical properties distinct from the underlying water. The production of organic biofilms and surfactants within the SML stabilizes the layer, often leading to a "slick-like" environment. The organic matrix within the SML can trap phytoplankton, subjecting them to intense light and ultraviolet (UV) radiation. Mycosporine-like amino acids (MAAs) are pigments produced by certain types of phytoplankton, exhibiting photoprotective absorption bands in UV and visible wavelengths. While numerous studies have documented MAAs in surface waters, particularly in equatorial regions, there is limited documentation of MAAs produced specifically within the SML. Here, we present data collected from the Baltic Sea near the summer solstice under both slick and non-slick conditions. Using High-Performance Liquid Chromatography (HPLC), we detected MAAs in SML samples. Absorption spectra measured from these samples revealed distinct UV absorption peaks characteristic of MAAs. Interestingly, many corresponding subsurface water samples contained either no detectable MAAs or only trace amounts. These findings highlight the unique environment of the SML and the biological acclimations that the neuston undergo to survive under these conditions.
How to cite: Novak, M., Roettgers, R., Thoelen, C., and Wollschlaeger, J.: Detection and Characterization of Mycosporine-like Amino Acids in the Sea Surface Microlayer of the Baltic Sea during Summer, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16057, https://doi.org/10.5194/egusphere-egu25-16057, 2025.
