Sea Surface Microlayer Research Articles

Ice-nucleating particles in Canadian Arctic sea-surface microlayer and bulk seawater

Abstract. The sea-surface microlayer and bulk seawater can contain ice-nucleating particles (INPs) and these INPs can be emitted into the atmosphere. Our current understanding of the properties, concentrations, and spatial and temporal distributions of INPs in the microlayer and bulk seawater is limited. In this study we investigate the concentrations and properties of INPs in microlayer and bulk seawater samples collected in the Canadian Arctic during the summer of 2014. INPs were ubiquitous in the microlayer and bulk seawater with freezing temperatures in the immersion mode as high as −14 °C. A strong negative correlation (R = −0. 7, p = 0. 02) was observed between salinity and freezing temperatures (after correction for freezing depression by the salts). One possible explanation is that INPs were associated with melting sea ice. Heat and filtration treatments of the samples show that the INPs were likely heat-labile biological materials with sizes between 0.02 and 0.2 µm in diameter, consistent with previous measurements off the coast of North America and near Greenland in the Arctic. The concentrations of INPs in the microlayer and bulk seawater were consistent with previous measurements at several other locations off the coast of North America. However, our average microlayer concentration was lower than previous observations made near Greenland in the Arctic. This difference could not be explained by chlorophyll a concentrations derived from satellite measurements. In addition, previous studies found significant INP enrichment in the microlayer, relative to bulk seawater, which we did not observe in this study. While further studies are needed to understand these differences, we confirm that there is a source of INP in the microlayer and bulk seawater in the Canadian Arctic that may be important for atmospheric INP concentrations.

On the imprint of surfactant‐driven stabilization of laboratory breaking wave foam with comparison to oceanic whitecaps

AbstractSurfactants are ubiquitous in the global oceans: they help form the materially‐distinct sea surface microlayer (SML) across which global ocean‐atmosphere exchanges take place, and they reside on the surfaces of bubbles and whitecap foam cells prolonging their lifetime thus altering ocean albedo. Despite their importance, the occurrence, spatial distribution, and composition of surfactants within the upper ocean and the SML remains under‐characterized during conditions of vigorous wave breaking when in‐situ sampling methods are difficult to implement. Additionally, no quantitative framework exists to evaluate the importance of surfactant activity on ocean whitecap foam coverage estimates. Here we use individual laboratory breaking waves generated in filtered seawater and seawater with added soluble surfactant to identify the imprint of surfactant activity in whitecap foam evolution. The data show a distinct surfactant imprint in the decay phase of foam evolution. The area‐time‐integral of foam evolution is used to develop a time‐varying stabilization function, and a stabilization factor, , which can be used to identify and quantify the extent of this surfactant imprint for individual breaking waves. The approach is then applied to wind‐driven oceanic whitecaps, and the laboratory and ocean distributions overlap. It is proposed that whitecap foam evolution may be used to determine the occurrence and extent of oceanic surfactant activity to complement traditional in‐situ techniques and extend measurement capabilities to more severe sea states occurring at wind speeds in excess of about 10 m/s. The analysis procedure also provides a framework to assess surfactant‐driven variability within and between whitecap coverage data sets.

Sea Surface Scanner (S3): A Catamaran for High-Resolution Measurements of Biogeochemical Properties of the Sea Surface Microlayer

AbstractThis 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.

The Ocean's Vital Skin: Toward an Integrated Understanding of the Sea Surface Microlayer

Despite the huge extent of the ocean's surface, until now relatively little attention has been paid to the sea surface microlayer (SML) as the ultimate interface where heat, momentum and mass exchange between the ocean and the atmosphere takes place. Via the SML, large-scale environmental changes in the ocean such as warming, acidification, deoxygenation, and eutrophication potentially influence cloud formation, precipitation, and the global radiation balance. Due to the deep connectivity between biological, chemical, and physical processes, studies of the SML may reveal multiple sensitivities to global and regional changes. Understanding the processes at the ocean's surface, in particular involving the SML as an important and determinant interface, could therefore provide an essential contribution to the reduction of uncertainties regarding ocean-climate feedbacks. This review identifies gaps in our current knowledge of the SML and highlights a need to develop a holistic and mechanistic understanding of the diverse biological, chemical, and physical processes occurring at the ocean-atmosphere interface. We advocate the development of strong interdisciplinary expertise and collaboration in order to bridge between ocean and atmospheric sciences. Although this will pose significant methodological challenges, such an initiative would represent a new role model for interdisciplinary research in Earth System sciences.

Microlayer source of oxygenated volatile organic compounds in the summertime marine Arctic boundary layer

Summertime Arctic shipboard observations of oxygenated volatile organic compounds (OVOCs) such as organic acids, key precursors of climatically active secondary organic aerosol (SOA), are consistent with a novel source of OVOCs to the marine boundary layer via chemistry at the sea surface microlayer. Although this source has been studied in a laboratory setting, organic acid emissions from the sea surface microlayer have not previously been observed in ambient marine environments. Correlations between measurements of OVOCs, including high levels of formic acid, in the atmosphere (measured by an online high-resolution time-of-flight mass spectrometer) and dissolved organic matter in the ocean point to a marine source for the measured OVOCs. That this source is photomediated is indicated by correlations between the diurnal cycles of the OVOC measurements and solar radiation. In contrast, the OVOCs do not correlate with levels of isoprene, monoterpenes, or dimethyl sulfide. Results from box model calculations are consistent with heterogeneous chemistry as the source of the measured OVOCs. As sea ice retreats and dissolved organic carbon inputs to the Arctic increase, the impact of this source on the summer Arctic atmosphere is likely to increase. Globally, this source should be assessed in other marine environments to quantify its impact on OVOC and SOA burdens in the atmosphere, and ultimately on climate.

Microbial Extracellular Polymeric Substances (EPSs) in Ocean Systems.

Microbial cells (i.e., bacteria, archaea, microeukaryotes) in oceans secrete a diverse array of large molecules, collectively called extracellular polymeric substances (EPSs) or simply exopolymers. These secretions facilitate attachment to surfaces that lead to the formation of structured ‘biofilm’ communities. In open-water environments, they also lead to formation of organic colloids, and larger aggregations of cells, called ‘marine snow.’ Secretion of EPS is now recognized as a fundamental microbial adaptation, occurring under many environmental conditions, and one that influences many ocean processes. This relatively recent realization has revolutionized our understanding of microbial impacts on ocean systems. EPS occur in a range of molecular sizes, conformations and physical/chemical properties, and polysaccharides, proteins, lipids, and even nucleic acids are actively secreted components. Interestingly, however, the physical ultrastructure of how individual EPS interact with each other is poorly understood. Together, the EPS matrix molecules form a three-dimensional architecture from which cells may localize extracellular activities and conduct cooperative/antagonistic interactions that cannot be accomplished efficiently by free-living cells. EPS alter optical signatures of sediments and seawater, and are involved in biogeomineral precipitation and the construction of microbial macrostructures, and horizontal-transfers of genetic information. In the water-column, they contribute to the formation of marine snow, transparent exopolymer particles (TEPs), sea-surface microlayer biofilm, and marine oil snow. Excessive production of EPS occurs during later-stages of phytoplankton blooms as an excess metabolic by product and releases a carbon pool that transitions among dissolved-, colloidal-, and gel-states. Some EPS are highly labile carbon forms, while other forms appear quite refractory to degradation. Emerging studies suggest that EPS contribute to efficient trophic-transfer of environmental contaminants, and may provide a protective refugia for pathogenic cells within marine systems; one that enhances their survival/persistence. Finally, these secretions are prominent in ‘extreme’ environments ranging from sea-ice communities to hypersaline systems to the high-temperatures/pressures of hydrothermal-vent systems. This overview summarizes some of the roles of exopolymer in oceans.

The Atlantic Ocean surface microlayer from 50°N to 50°S is ubiquitously enriched in surfactants at wind speeds up to 13 m s−1

AbstractWe report the first measurements of surfactant activity (SA) in the sea surface microlayer (SML) and in subsurface waters (SSW) at the ocean basin scale, for two Atlantic Meridional Transect from cruises 50°N to 50°S during 2014 and 2015. Northern Hemisphere (NH) SA was significantly higher than Southern Hemisphere (SH) SA in the SML and in the SSW. SA enrichment factors (EF = SASML/SASSW) were also higher in the NH, for wind speeds up to ~13 m s−1, questioning a prior assertion that Atlantic Ocean wind speeds >12 m s−1 poleward of 30°N and 30°S would preclude high EFs and showing the SML to be self‐sustaining with respect to SA. Our results imply that surfactants exert a control on air‐sea CO2 exchange across the whole North Atlantic CO2 sink region and that the contribution made by high wind, high latitude oceans to air‐sea gas exchange globally should be reexamined.

Respiratory Microbiome of Endangered Southern Resident Killer Whales and Microbiota of Surrounding Sea Surface Microlayer in the Eastern North Pacific

In the Salish Sea, the endangered Southern Resident Killer Whale (SRKW) is a high trophic indicator of ecosystem health. Three major threats have been identified for this population: reduced prey availability, anthropogenic contaminants, and marine vessel disturbances. These perturbations can culminate in significant morbidity and mortality, usually associated with secondary infections that have a predilection to the respiratory system. To characterize the composition of the respiratory microbiota and identify recognized pathogens of SRKW, exhaled breath samples were collected between 2006–2009 and analyzed for bacteria, fungi and viruses using (1) culture-dependent, targeted PCR-based methodologies and (2) taxonomically broad, non-culture dependent PCR-based methodologies. Results were compared with sea surface microlayer (SML) samples to characterize the respective microbial constituents. An array of bacteria and fungi in breath and SML samples were identified, as well as microorganisms that exhibited resistance to multiple antimicrobial agents. The SML microbes and respiratory microbiota carry a pathogenic risk which we propose as an additional, fourth putative stressor (pathogens), which may adversely impact the endangered SRKW population.

High wind speeds prevent formation of a distinct bacterioneuston community in the sea-surface microlayer.

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.

Size-resolved characterization of the polysaccharidic and proteinaceous components of sea spray aerosol

Dissolved organic polymers released by phytoplankton and bacteria abiologically self-assemble in surface ocean waters into nano-to micro-sized gels containing polysaccharides, proteins, lipids and other components. These gels concentrate in the sea surface microlayer (SML), where they can potentially contribute to sea spray aerosol (SSA). Sea spray is a major source of atmospheric aerosol mass over much of the earth's surface, and knowledge of its properties (including the amount and nature of the organic content), size distributions and fluxes are fundamental for determining its role in atmospheric chemistry and climate. Using a cascade impactor, we collected size-fractionated aerosol particles from ambient air and from freshly generated Sea Sweep SSA in the western North Atlantic Ocean together with biological and chemical characterization of subsurface and SML waters. Spectrophotometric methods were applied to quantify the polysaccharide-containing transparent exopolymer (TEP) and protein-containing Coomassie stainable material (CSM) in these particles and waters. This study demonstrates that both TEP and CSM in surface ocean waters are aerosolized with sea spray with the greatest total TEP associated with particles <180 nm in diameter and >5 000 nm. The higher concentrations of TEP and CSM in particles >5 000 nm most likely reflects collection of microorganism cells and/or fragments. The greater concentration of CSM in larger size particles may also reflect greater stability of proteinaceous gels compared to polysaccharide-rich gels in surface waters and the SML. Both TEP and CSM were measured in the ambient marine air sample with concentrations of 2.1 ± 0.16 μg xanthan gum equivalents (XG eq.) m−3 and 14 ± 1.0 μg bovine serum albumin equivalents (BSA eq.) m−3. TEP in Sea Sweep SSA averaged 4.7 ± 3.1 μg XG eq. m−3 and CSM 8.6 ± 7.3 μg BSA eq. m−3. This work shows the transport of marine biogenic material across the air-sea interface through primary particle emission and the first demonstration of particle size discriminated TEP and CSM characterization of SSA and ambient aerosol under field conditions.

Distribution of transparent exopolymer particles (TEP) across an organic carbon gradient in the western North Atlantic Ocean

In this study, the abundance of transparent exopolymer particles (TEP) is examined across a surface water gradient of organic carbon from a phytoplankton bloom region in the western North Atlantic to oligotrophic waters in the Sargasso Sea, including a coastal region sampled near Cape Cod. TEP are macrogels that reach up to millimeters in size and abiotically assemble from dissolved acidic polysaccharides secreted by phytoplankton. Due to their great stickiness, TEP self aggregate and also form aggregates with non-TEP particulate organic carbon (POC). Aggregation enhances ballasting, thereby mutually facilitating export and subsequent sedimentation of both TEP and POC, increasing the efficiency of the biological pump. Here, four distinct regions with varying chlorophyll a concentrations, temperature, and salinity were sampled in the upper column, at the surface, and from the sea surface microlayer (SML). While TEP in seawater shows no correlation to chlorophyll a, nutrients, or total organic carbon (TOC) concentration, a strong correlation exists between TEP and TOC in the SML; and concentrations of both variables are inversely proportional to surface productivity as indicated by chlorophyll a concentrations. As open ocean regions show greater enrichments of TEP and TOC in the SML compared to the coastal region, we suggest that the role of the SML in organic carbon cycling is dependent on regional biogeochemistry and productivity. We hypothesize that a lower abundance of particles in oligotrophic regions compared to bloom regions limits TEP export by sinking, thus increasing the residence time of TEP in the upper water column and the SML.

High-resolution variability of the enrichment of fluorescence dissolved organic matter in the sea surface microlayer of an upwelling region

Enrichment of fluorescence dissolved organic matter (FDOM) in the sea surface microlayer (SML) provides insights into biogeochemical processes occurring at the sea surface, including cycling of organic matter, photochemistry, and air-sea gas exchange. We present data concerning the variability of FDOM enrichment in the SML during upwelling events in the Baltic Sea (Cruise M117). Our results show that FDOM is frequently enriched (75% of all samples) and that enrichment factors are significantly higher in SMLs located in regions with upwelling (pooled median = 1.4) compared to a non-upwelling region (median = 1.1). The enrichment factor of FDOM showed short time-scale variability, changing by 6% within ten-minute intervals. Larger variabilities (standard deviation up to ±0.14 µg L–1 compared to background of ±0.01 µg L–1) occurred when fronts were present and when the SML was mixed with underlying bulk water. Small-scale patchiness, indicated by changes in the variability of FDOM enrichment in SML, was a common feature of the sea surface. Wind speed played a potential role in controlling the enrichment of FDOM in the SML, but the effects of solar radiation on photochemical processes, mixing and upwelling of water masses, and biological processes as a source of FDOM also influence enrichment at this critical interface between ocean and atmosphere.

Sea surface microlayer in a changing ocean – A perspective

The sea surface microlayer (SML) is the boundary interface between the atmosphere and ocean, covering about 70% of the Earth’s surface. With an operationally defined thickness between 1 and 1000 µm, the SML has physicochemical and biological properties that are measurably distinct from underlying waters. Recent studies now indicate that the SML covers the ocean to a significant extent, and evidence shows that it is an aggregate-enriched biofilm environment with distinct microbial communities. Because of its unique position at the air-sea interface, the SML is central to a range of global biogeochemical and climate-related processes. The redeveloped SML paradigm pushes the SML into a new and wider context that is relevant to many ocean and climate sciences.

Trace elements in the sea surface microlayer: rapid responses to changes in aerosol deposition

Natural and anthropogenic aerosols are a significant source of trace elements to oligotrophic ocean surface waters, where they provide episodic pulses of limiting micronutrients for the microbial community. However, little is known about the fate of trace elements at the air-sea interface, i.e. the sea surface microlayer. In this study, samples of aerosols, sea surface microlayer, and underlying water column were collected in the Florida Keys during a dusty season (July 2014) and non-dusty season (May 2015) and analyzed for the dissolved and particulate elements Al, Fe, Ni, Cu, Zn, and Pb. Microlayer samples were collected using a cylinder of ultra-pure SiO2 (quartz glass), a novel adaptation of the glass plate technique. A significant dust deposition event occurred during the 2014 sampling period which resulted in elevated concentrations of trace elements in the microlayer. Residence times in the microlayer from this event ranged from 12 to 94 minutes for dissolved trace elements and from 1.3 to 3.4 minutes for particulate trace elements. These residence times are potentially long enough for the atmospherically derived trace elements to undergo chemical and biological alterations within the microlayer. Characterizing the trace element distributions within the three regimes is an important step towards our overall goals of understanding the rates and mechanisms of the solubilization of trace elements following aeolian dust deposition and how this might affect microorganisms in surface waters.

The influence of environmental drivers on the enrichment of organic carbon in the sea surface microlayer and in submicron aerosol particles – measurements from the Atlantic Ocean

The export of organic matter from ocean to atmosphere represents a substantial carbon flux in the Earth system, yet the impact of environmental drivers on this transfer is not fully understood. This work presents dissolved and particulate organic carbon (DOC, POC) concentrations, their enrichment factors in the sea surface microlayer (SML), and equivalent measurements in marine aerosol particles across the Atlantic Ocean. DOC concentrations averaged 161 ± 139 µmol L–1 (n = 78) in bulk seawater and 225 ± 175 µmol L–1 (n = 79) in the SML; POC concentrations averaged 13 ± 11 µmol L–1 (n = 80) and 17 ± 10 µmol L–1 (n = 80), respectively. High DOC and POC enrichment factors were observed when samples had low concentrations, and lower enrichments when concentrations were high. The impacts of wind speed and chlorophyll-a levels on concentrations and enrichment of DOC and POC in seawater were insignificant. In ambient submicron marine aerosol particles the concentration of water-soluble organic carbon was approximately 0.2 µg m–3. Water-insoluble organic carbon concentrations varied between 0.01 and 0.9 µg m–3, with highest concentrations observed when chlorophyll-a concentrations were high. Concerted measurements of bulk seawater, the SML and aerosol particles enabled calculation of enrichment factors of organic carbon in submicron marine ambient aerosols, which ranged from 103 to 104 during periods of low chlorophyll-a concentrations and up to 105 when chlorophyll-a levels were high. The results suggest that elevated local biological activity enhances the enrichment of marine-sourced organic carbon on aerosol particles. However, implementation of the results in source functions based on wind speed and chlorophyll-a concentrations underestimated the organic fraction at low biological activity by about 30%. There may be additional atmospheric and oceanic parameters to consider for accurately predicting organic fractions on aerosol particles.

Seasonal dynamics of atmospheric and river inputs of black carbon, and impacts on biogeochemical cycles in Halong Bay, Vietnam

Emissions of black carbon (BC), a product of incomplete combustion of fossil fuels, biofuels and biomass, are high in the Asia-Pacific region, yet input pathways and rates to the ocean are not well constrained. Atmospheric and riverine inputs of BC in Halong Bay (Vietnam), a hotspot of atmospheric BC, were studied at monthly intervals during one year. Climate in Halong Bay is governed by the monsoon regime, characterized by a northeast winter monsoon (dry season) and southeast summer monsoon (wet season). During the dry season, atmospheric BC concentrations averaged twice those observed during the wet season. In the sea surface microlayer (SML) and underlying water (ULW), concentrations of particulate BC (PBC) averaged 539 and 11 µmol C L–1, respectively. Dissolved BC (DBC) concentrations averaged 2.6 µmol C L–1 in both the SML and ULW. Seasonal variations indicated that PBC concentration in the SML was controlled by atmospheric deposition during the dry season, while riverine inputs controlled both PBC and DBC concentrations in ULW during the wet season. Spatiotemporal variations of PBC and DBC during the wet season suggest that river runoff was efficient in transporting PBC that had accumulated on land during the dry season, and in mobilizing and transporting DBC to the ocean. The annual river flux of PBC was about 3.8 times higher than that of DBC. The monsoon regime controls BC input to Halong Bay by favoring dry deposition of BC originating from the north during the dry season, and wet deposition and river runoff during the wet season. High PBC concentrations seem to enhance the transfer of organic carbon from dissolved to particulate phase by adsorbing dissolved organic carbon and stimulating aggregation. Such processes may impact the availability and biogeochemical cycling of other dissolved substances, including nutrients, for the coastal marine ecosystem.

Impact of anions on the surface organisation of lipid monolayers at the air–water interface

Environmental contextLipids released from lysis of phytoplankton cells are enriched in the sea surface microlayer. Such surface-active organics can be transferred through bursting bubbles to sea-spray aerosols where they can influence atmospheric chemistry. The results presented here suggest that phospholipids combine more readily with SO42− than with Br−, leading to enrichment of organic-coated sulfate salts in marine aerosols. AbstractInorganic salts and organic matter are known to be present at higher levels in the sea surface microlayer and marine aerosols; however, the impact of common anions on their surface properties is not well understood. Here, a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer was enriched with the sodium and ammonium salts of different anions (Br−, Cl−, NO3−, SO42−, CH3COO−, and HCO3−), and the effects on the surface properties of the monolayer were investigated. The monolayer phase behaviour and the structure of the lipid phases were studied by surface pressure–area (π–A) isotherms and infrared reflection-absorption spectroscopy (IRRAS). The presence of salts in the subphase was found to increase the surface pressure of the DPPC monolayer at a fixed area per molecule. The effect of the anions follows the order of the Hofmeister series. The higher concentration of salt solution caused the π–A isotherm to shift to larger area. The IRRAS spectra demonstrate that the ordering of the DPPC molecules in the liquid condensed phase remains essentially unaffected, even at higher electrolyte concentrations. DPPC molecules combined with SO42− could be transferred from the ocean to sea spray aerosol. The present study finds that the anions have significant influence on the surface organisation and, consequently, the interfacial properties, of the surface-active species at the air–water interface, a finding that has further implications for atmospheric aerosol nucleation.

Enriching particles on a bubble through drainage: Measuring and modeling the concentration of microbial particles in a bubble film at rupture

The concentration of microbes and other particulates is frequently enriched in the droplets produced by bursting bubbles. As a bubble rises to the ocean surface, particulates in the bulk liquid can be transported to the sea surface microlayer by attaching to the bubble’s interface. When the bubble eventually ruptures, a fraction of these particulates is often ejected into the surroundings in film droplets with a particulate concentration that is higher than in the liquid from which they formed. The precise mechanisms responsible for this enrichment are unclear, yet such enrichment at the ocean surface influences important exchange processes with the atmosphere. Here we provide evidence that drainage, coupled with scavenging, is responsible for the enrichment. By simultaneously recording the drainage and rupture effects with high-speed and standard photography, we directly measured the particulate concentrations in the thin film of a bubble cap at the moment before it ruptures. We observed that the enrichment factor strongly depends on the film thickness at rupture, and developed a physical model, based on scavenging and drainage, that is consistent with our observations. We have also demonstrated that this model is quantitatively consistent with prior observations of film drop enrichment, indicating its potential for a broader range of applications in the study of the sea surface microlayer and related phenomena.

The Impact of Dry Atmospheric Deposition on the Sea-Surface Microlayer in the SE Mediterranean Sea: An Experimental Approach

The oligotrophic southeastern Mediterranean Sea (SEMS) is frequently exposed to desert-dust deposition which supplies nutrients, trace metals and a wide array of viable airborne microorganisms. In this study, we experimentally examined the impact of aerosol addition, collected during an intense dust storm event in early September 2015, on the biomass and activity of pico-phytoplankton and heterotrophic bacterial populations at the sea-surface micro layer (SML) relative to the sub surface layer (SSL). Aerosol (1.5 mg L-1) was added to SML and SSL water samples in microcosms (4.5 L) and the water was frequently sampled over a period of 48 hours. While the aerosol amendment triggered a moderate 1.5-2 fold increase in primary production in both the SML and the SSL, bacterial production increased by ~3 and ~7 folds in the SSL and SML, respectively. Concurrently, the abundance and flow-cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells showed a significant increase in the %HNA, in both SML and SSL samples following aerosol amendment. This shift in nucleic acid content took place at a much faster rate in the SML, suggesting a more active heterotrophic community. These changes were likely a result of higher rates of carbon utilizations in the SML following the dust addition, as assessed by a selected hydrocarbons and saccharides analysis. Additionally, a high absorption rate of hydrocarbons by the aerosol particles was measured following the additions, leaving less than 10% of these molecules available for potential heterotrophic microbial utilization. Our results suggest that the heterotrophic microbial community inhabiting the SML is more efficient in utilizing aerosol associated constituents than the community in the SSL.

Enrichment of Saccharides and Divalent Cations in Sea Spray Aerosol During Two Phytoplankton Blooms.

Sea spray aerosol (SSA) is a globally important source of particulate matter. A mesocosm study was performed to determine the relative enrichment of saccharides and inorganic ions in nascent fine (PM2.5) and coarse (PM10-2.5) SSA and the sea surface microlayer (SSML) relative to bulk seawater. Saccharides comprise a significant fraction of organic matter in fine and coarse SSA (11 and 27%, respectively). Relative to sodium, individual saccharides were enriched 14-1314-fold in fine SSA, 3-138-fold in coarse SSA, but only up to 1.0-16.2-fold in SSML. Enrichments in SSML were attributed to rising bubbles that scavenge surface-active species from seawater, while further enrichment in fine SSA likely derives from bubble films. Mean enrichment factors for major ions demonstrated significant enrichment in fine SSA for potassium (1.3), magnesium (1.4), and calcium (1.7), likely because of their interactions with organic matter. Consequently, fine SSA develops a salt profile significantly different from that of seawater. Maximal enrichments of saccharides and ions coincided with the second of two phytoplankton blooms, signifying the influence of ocean biology on selective mass transfer across the ocean-air interface.