The global ocean plays a central role in Earth’s climate system and has considerably slowed down climate change by taking up about one third of the greenhouse gas carbon dioxide (CO 2 ) emitted through human activities.

To what extent this will continue in the future depends on a variety of physical and chemical processes - and, as marine scientists from the Leibniz Institute of Marine Sciences (IFM-GEOMAR) together with colleagues from the Alfred Wegener Institute for Polar and Marine Research (Bremerhaven, Germany) and the Leibniz Institute for Baltic Sea Research (Warnemünde, Germany) have now shown in an experiment with natural plankton communities, also depends on biological factors.

Atmospheric CO 2 enters the ocean at the sea surface. To allow for continued uptake, it has to be transported to deeper layers. “One of the mechanisms responsible for this transport to depth is the biological carbon pump”, explains Julia Wohlers from IFM-GEOMAR, the first author of the study, which has just been published in PNAS. In spring, when temperatures rise and light conditions improve, planktonic algae start to build up biomass using CO 2 and nutrients. After the algae have died, a fraction of this biomass sinks to greater depth – taking with it the photosynthetically bound carbon.

To investigate how this system will change in response to surface-ocean warming, the scientists from Kiel have enclosed natural plankton communities in eight tanks of 1,400 litres each. The communities were exposed to different temperatures, which corresponded to the warming scenarios projected by the International Panel of Climate Change (IPCC) until the year 2100. In these “miniature ecosystems” the build-up and decline of the spring plankton bloom was monitored over a period of one month. “As expected, the metabolic rates of all components of the plankton community were accelerated with increasing temperature. What came as a surprise to us was that the plankton consumed up to one third less CO 2 at elevated temperatures. Ultimately, this may cause a weakening of the biological carbon pump”, says Prof. Ulf Riebesell from IFM-GEOMAR, the principal investigator of the study.

The reason for this weakening: While the photosynthetic build-up of biomass by planktonic algae shows only a minor response to warming, its consumption by bacteria strongly increases with rising temperature. This causes a greater portion of algal biomass to be degraded before it can sink to deeper waters. Thus, more CO 2 is retained in the surface layer, which, in turn, will take up less CO 2 from the atmosphere.

“This study underscores the importance to improve the incorporation of biological processes and feedbacks in Earth system models”, Julia Wohlers points out. For a global estimate of the magnitude of the observed feedback, however, it is still too early at this stage. “Data in this research area are still sparse and further research efforts are needed to fill this gap of knowledge”, says Wohlers.