It is wafer-thin, but nevertheless important - the boundary layer between the ocean and the atmosphere. It influences the exchange of gases such as carbon dioxide and oxygen between water and air - and thus has an impact on the climate. An international team of researchers is now investigating this layer in the Jade Bay.
The oceans cover more than 70 per cent of the Earth's surface and play a major role in the Earth's climate. The transition between water and air is particularly important - many processes take place at this boundary layer. But how exactly do these processes affect the climate? And what happens at the boundary layer at night?
To answer these questions, an international team of researchers has come together at the Institute of Chemistry and Biology of the Marine Environment (ICBM) at the University of Oldenburg. For a fortnight, the scientists in the MILAN project on the Jade Bay are investigating the wafer-thin surface layer of the sea - even at night.
Most of the oceans are covered by thin natural skins, known as surface films. Organic molecules of biological origin collect in this boundary layer as there is hardly any turbulence. This layer also slows down the exchange of gases between the ocean and the atmosphere. It often provides ideal living conditions for microorganisms, which also actively influence this exchange.
However, scientists have not yet understood in detail what exactly happens in this boundary layer. This is because it is difficult to take measurements and samples from this wafer-thin film - especially at night. "This is the first time that we have investigated surface films at night, and it is also an international and interdisciplinary endeavour. It will be a challenge, because working at sea in the dark is not easy," says Dr Mariana Ribas Ribas, oceanographer in the Marine Surfaces working group at the ICBM. She initiated the project together with her colleague Dr Christian Stolle, who also conducts research at the Institute for Baltic Sea Research Warnemünde.
In order to obtain a comprehensive picture of the quantities of gases and other substances that are exchanged through the surface film, the researchers have to take measurements around the clock. This is because the activities of the microorganisms differ between day and night. During the day, for example, solar radiation causes microalgae to produce oxygen - after all, the oceans contribute half of the oxygen in the air on Earth. In addition, the oceans absorb around a third of the greenhouse gas carbon dioxide produced by humans. At night, however, when organisms are unable to photosynthesise, respiring microorganisms in the boundary layer come to the fore and produce carbon dioxide.
Over the next two weeks at the Jade Bay, the main questions will be: Do the composition and metabolic performance of the microbial communities fluctuate significantly during the course of the day? And to what extent do they influence the exchange of oxygen and carbon dioxide through the surface film? The scientists are also interested in the extent to which biological, physical and (photo)chemical processes characterise the quantity and composition of the finest suspended particles, aerosols, above the sea surface - these also have an effect on cloud formation.
The idea for MILAN (sea-surface microlayer functioning during the night) came about in an unconventional way on the fringes of a specialist conference. "Normally you raise money and then carry out a project. Here it went the other way round," explains Ribas Ribas. Together, they now want to develop MILAN into a European project.
The team includes researchers from Costa Rica, Denmark, Great Britain, Italy, Croatia, Poland, Sweden and Spain. In addition to the Oldenburg team, researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel, the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen and the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig are involved on the German side.
The project team uses the ICBM research boat "Otzum" and the research cutter "Senckenberg" from the Institute Senckenberg am Meer in Wilhelmshaven. In addition to a special sensor-equipped drift buoy, a remote-controlled research catamaran from the Marine Surfaces working group is also used on board the ship: it collects large quantities of the surface film for laboratory investigations.