Study of the Sea-Surface Microlayer and Air-Sea Boundary

Mark Schrope

Everything in nature serves a purpose, even if we do not understand it or fail to identify it. Some natural processes are so subtle it can be easy to think that they simply do not exist. Look at the sky and then at the ocean: one over the other, so distinguishable, and never seeming to touch. But they do. There is a thin layer of ocean surface in contact with the air, and in it, many chemical and biological processes take place. But our understanding of the sea’s surface microlayer is still very limited.

Gas and climate exchange.
Gas and climate exchange.NIWA

The surface microlayer is a boundary that serves as a mediator of air-sea gas exchange. The microlayer is mainly formed by chemicals (carbohydrates, proteins, and fatty acids) released by microorganisms, either in the underlying bulk water or directly in the microlayer. It is as thin as one millimeter, but still it controls all exchange between the atmosphere and the ocean. It is the link between interfacial processes at the sea surface and marine biogeochemical cycles, air-sea interactions and climate. So for something so subtle and thin, the surface microlayer sure serves a big purpose.

Its composition is also unique. The diversity of microorganisms in the surface microlayer can be significantly different from that in the underlying water, even a few centimeters below it. The living organisms and suspended organic compounds inside the layer have an impact on chemical, physical biological and climatological processes. For instance, certain bacteria can affect the gas exchange between ocean and air. It is this overlooked but very determinant piece of our ocean’s puzzle that research vessel Falkor embarked to study, in the Indian and Pacific oceans.

Holistic approach

Little is known about the surface microlayer, so focusing on just one science field would prove inadequate in order to begin deciphering how it works. This is why the first cruise on Falkor dedicated to the surface microlayer, took a holistic approach linking multidisciplinary approaches including, atmospheric deposition, aerosol chemistry, gas and heat exchange, sea surfaces as microbiological habitat, and new technologies for in situ sensing.

State-of-the-art technology was deployed in a series of experiments in order to characterize sea surfaces. Experts identified the exchange of elements and organic compounds between the atmosphere and the sea surface, creating a set of new parameters to characterize air-sea exchange of climate-relevant gases and heat, and study the sea surface as a habitat for complex microbial communities.

Thinking of every angle – Four working groups

DSCN2139In order to take advantage of the combined expertise and knowledge onboard, a series of working packages were designed. In each of them, every scientific field was able to shine while making sure that all of the other teams benefited from the data and findings. There was a total of four working packages.

  • Biological aspects

The first one focused on biological aspects of the surface microlayer. The scientists in this group quantified and characterized the abundance and diversity of different organisms on the surface microlayer, compared different communities of organisms between them, and linked changes in microbial abundance and diversity. Organic compounds create a sort of film that greatly impacts the exchange between sea and air, this group provided new insights into the workings of such film through the study of the surface microlayer’s biology.

  • Climate change – Carbon

The second working package was more concerned with carbon, and therefore was relevant for climate change considerations. The processes controlling carbon transport and transformation in the ocean remain uncertain, in order to assess them, studies require accurate estimates of the air-water carbon dioxide flux. The scientists on Falkor measured these processes and sought to resolve the relative importance of wind, waves, buoyancy, stratification, and surface material play, in order to ultimately determine the role of the surface microlayer on the air-water carbon dioxide flux versus other carbon budget terms in oceans.

  • Climate change – Trace elements

The experiments of the third working group also yielded relevant information regarding oceans and climate change. There are many elements in our oceans that are not related to the water’s salinity, we refer to them as trace elements. Trace elements and their isotopes act as nutrients and tracers of oceanic cycling processes both today and in the past. The experts participating on this working package quantified the elements and their mechanisms, an essential part of understanding ocean ecosystems and the global carbon cycle (and thus climate change).

  • Organic components

Finally, the fourth working group was concerned with the organic components of the surface microlayer. These compounds behave like a film, and so the microlayer affects gas exchange by reducing gas transfer velocity through diffusion, but also affects the carbon dioxide difference between air and sea through biogeochemical processes. The scientists working in this group measured gas exchange rates and acquired high resolution scans of biochemical properties of the microlayer to investigate links between its properties, gas transfer velocities, and aerosol chemistry.

Concentrated power

The thin microlayer no more than one millimeter, is strong enough that all exchange between air and sea depend on it, yet fragile enough that its composition depends on climate conditions, precipitation, and wind speed. It could be easy to overlook it. But it’s fascinating being able to understand how such a subtle presence can have a global impact that we are just beginning to grasp. Learn how the cruise and the team deployed top of the line technology to explore the concentrated power of the ocean’s surface microlayer.


Cruise Log

Wrapping Up

Keeping At It

Data & Publications

The resulting shipboard dataset is being archived at the Rolling Deck to Repository and is now available.

  • Wurl, O., Ribas Ribas, M., Landing, W., and Zappa, C. (2017).  Surface Skin Salinity – First in situ Measurements in the Tropical Ocean. Oral Presentation at the Global Ocean Salinity Workshop, Woods Hole, MA, USA.

  • Wurl, O., Ekau, W., Landing, W., and Zappa, C. (2017). Sea Surface Microlayer in a Changing Ocean – A perspective. Elem Sci Anth. 2017; 5:31. DOI: [This article is published as OPEN ACCESS].

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