The oceans are becoming more acidic. What does this mean for the global climate? To find out, researchers have simulated the ocean of the future off the coast of Sweden. A team from Oldenburg took part - and were in for a surprise.
The rise in CO2 in the atmosphere is making the oceans increasingly acidic. This jeopardises the lives of many sea creatures: acidic water damages corals, for example, and smaller organisms with shells or skeletons made of calcium carbonate also suffer from the changes. But the oceans are not only the habitat of plants and animals. They also play an important role in the global climate, as they store a lot of carbon dioxide (CO2), including in the form of dissolved organic molecules.
Until now, scientists assumed that the acidification of the oceans was fuelling this natural process. A long-term experiment by the Institute of Chemistry and Biology of the Marine Environment (ICBM) at the University of Oldenburg and the GEOMAR Helmholtz Centre for Ocean Research Kiel now shows otherwise: apparently the amount and composition of the dissolved organic material does not change. The research team led by Oldenburg scientist Maren Zark from the ICBM's bridge group with the Max Planck Institute for Marine Microbiology has published its surprising results in the journal "Science Advances".
The ocean contributes to reducing the greenhouse effect by binding large amounts of CO2. Among other things, it stores carbon in dissolved organic matter, abbreviated as DOM. The amount of carbon bound in DOM is comparable to the amount of CO2 in the atmosphere. A large proportion of the DOM survives in seawater for many thousands of years.
In their study, the Oldenburg researchers wanted to find out what happens to the DOM when the ocean gradually acidifies. It can already be observed today that the pH value of the water is falling because the ocean is absorbing more and more CO2 from the atmosphere. In order to investigate the effects of acidification on natural processes in the water, the researchers simulated the ocean of the future as part of the multi-year "BIOACID" (Biological Impacts of Ocean ACIDification) project.
In 2013, the first long-term study brought together a total of 60 scientists working on various issues relating to ocean acidification off the coast of Sweden. They anchored so-called mesocosms in the Gullmarsfjord. These structures, which float vertically in the water with plastic tubes around 17 metres long, hold around 55,000 litres of water and represent a separate living environment shielded from the rest of the fjord water. The scientists added CO2 to five of these mesocosms to simulate the ocean of the year 2100, while five other mesocosms received no additional CO2 and served as controls. They assumed that the CO2 content would rise in line with current forecasts.
The water samples were then analysed in detail at the University of Oldenburg. Surprisingly, the DOM had developed in exactly the same way, regardless of which mesocosm it came from. The additional CO2 in the seawater apparently has no influence on the composition and concentration of the DOM. "However, this does not necessarily mean that the DOM remains just as good a long-term store of carbon in other regions of the world's oceans," explains Zark. It could also be that the DOM will behave completely differently in other ocean regions in the future, according to Zark.
Further studies are currently being conducted under the umbrella of BIOACID to clarify questions such as these. The results of a similar mesocosm experiment in the waters off Gran Canaria will be available soon. The scientists hope that these will provide new insights, such as whether the DOM behaves differently in the open sea than near the coast.
About BIOACID:
Under the umbrella of BIOACID (Biological Impacts of Ocean Acidification), ten Institutes are investigating how marine biocoenoses react to ocean acidification and what consequences this has for the food web, material and energy turnover in the sea and ultimately also for the economy and society. The project is supported by the Federal Ministry of Education and Research (BMBF). It is coordinated by the GEOMAR Helmholtz Centre for Ocean Research Kiel. A list of member institutions and further information can be found at www.bioacid.de.