As the climate changes and oceans’ acidity increases, tiny plankton seem set to succeed. An international team of marine scientists has found that the smallest plankton groups thrive under elevated carbon dioxide (CO₂) levels. This could cause an imbalance in the food web as well as decrease ocean CO₂ uptake, an important regulator of global climate. The results of the study, conducted off the coast of Svalbard, Norway, in 2010, are now compiled in a special issue published in Biogeosciences, a journal of the European Geosciences Union.

“If the tiny plankton blooms, it consumes the nutrients that are normally also available to larger plankton species,” explains Ulf Riebesell, a professor of biological oceanography at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany and head of the experimental team. This could mean the larger plankton run short of food.

Large plankton play an important role in carbon export to the deep ocean, but in a system dominated by the so-called pico- and nanoplankton, less carbon is transported out of surface waters. “This may cause the oceans to absorb less CO₂ in the future,” says Riebesell.

The potential imbalance in the plankton food web may have an even bigger climate impact. Large plankton are also important producers of a climate-cooling gas called dimethyl sulphide, which stimulates cloud-formation over the oceans. Less dimethyl sulphide means more sunlight reaches the Earth’s surface, adding to the greenhouse effect. “These important services of the ocean may thus be significantly affected by acidification.”

Ecosystems in the Arctic are some of the most vulnerable to acidification because the cold temperatures here mean that the ocean absorbs more carbon dioxide. “Acidification is faster there than in temperate or tropical regions,” explains the coordinator of the European Project on Ocean Acidification (EPOCA), Jean-Pierre Gattuso of the Laboratory of Oceanography of Villefranche-sur-Mer of the French National Centre for Scientific Research (CNRS).

The increasing acidity is known to affect some calcifying organisms in the Arctic, including certain sea snails, mussels and other molluscs. But scientists did not know until now how ocean acidification alters both the base of the marine food web and carbon transport in the ocean.

The five-week long field study conducted in the Kongsfjord off the Arctic archipelago of Svalbard, under the EPOCA framework, intended to close this knowledge gap. For the experiment, the scientists deployed nine large ‘mesocosms’, eight-metre long floatation frames carrying plastic bags with a capacity of 50 cubic metres. These water enclosures, developed at GEOMAR, allow researchers to study plankton communities in their natural environment under controlled conditions, rather than in a beaker in the lab. Few studies have looked at whole communities before.

The scientists gradually added CO₂ to the mesocom water so that it reached acidity levels expected in 20, 40, 60, 80 and 100 years, with two bags left as controls. They also added nutrients to simulate a natural plankton bloom, as reported in the Biogeosciences special issue.

The team found that, where CO₂ was elevated, pico- and, to a lesser extent, nanoplankton grew, drawing down nutrients so there were less available to larger plankton. “The different responses we observed made it clear that the communities’ sensitivity to acidification depends strongly on whether or not nutrients are available,” Riebesell summarises.

“Time and [time] again the tiniest plankton benefits from the surplus CO₂, they produce more biomass and more organic carbon, and dimethyl sulphide production and carbon export are decreasing,” he concludes.

With 35 participants from 13 European institutions, the GEOMAR-coordinated mesocosm experiment was the largest project of EPOCA, a large EU-funded research initiative on ocean acidification that ran from 2008 to 2012. It was supported by the French-German Arctic Research Base.

The research results obtained in the mesocosm study are presented in the special issue ‘Arctic ocean acidification: pelagic ecosystem and biogeochemical responses during a mesocosm study’ of the EGU open access journal Biogeosciences. The papers on the special issue were published between March and August 2013. Please mention the publication if reporting on this story and, if reporting online, include a link to the special issue or to the journal website.

The scientific articles that make up the special issue are available online, free of charge, at http://www.biogeosciences.net/special_issue120.html.

The issue was edited by U. Riebesell (GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany), J.-P. Gattuso (CNRS Laboratory of Oceanography of Villefranche sur Mer and Université Pierre et Marie Curie-Paris 6, France), T. F. Thingstad (Department of Biology, University of Bergen, Norway) and J. J. Middelburg (Faculty of Geosciences, Utrecht University, the Netherlands).

For additional high-resolution images and video footage of the experiment, please email Maike Nicolai (GEOMAR Communication & Media) at mnicolai@geomar.de.

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