Enhanced biological carbon consumption in a high CO2 ocean

Ocean carbon dioxide Nearly half of the fossil-fuel carbon dioxide produced since pre-industrial times has been absorbed by the oceans, causing measurable acidification and carbonate saturation. A series of recent reports has raised the spectre of severe seawater acidification in the future if anthropogenic CO 2 production continues unchecked. Previous work has dealt largely with the effects on individual marine species. Now in an experimental study at Raune Fjord in Norway, using environment-scale mesocosm enclosures, the impact of CO 2 absorption has been estimated in a natural community ecosystem. The results show that CO 2 consumption by marine phytoplankton increases markedly as partial pressures of the gas increase, yet nutrient uptake is unchanged. If applicable to the oceans as a whole, this feedback might be an important constraint on atmospheric CO 2 concentrations. It is estimated that global increasing CO 2 emissions will lead to significant ocean acidification. Although the effects on individual marine species have been explored previously, this presents first empirical results to estimate the impact on a natural community ecosystem. The oceans have absorbed nearly half of the fossil-fuel carbon dioxide (CO 2 ) emitted into the atmosphere since pre-industrial times 1 , causing a measurable reduction in seawater pH and carbonate saturation 2 . If CO 2 emissions continue to rise at current rates, upper-ocean pH will decrease to levels lower than have existed for tens of millions of years and, critically, at a rate of change 100 times greater than at any time over this period 3 . Recent studies have shown effects of ocean acidification on a variety of marine life forms, in particular calcifying organisms 4 , 5 , 6 . Consequences at the community to ecosystem level, in contrast, are largely unknown. Here we show that dissolved inorganic carbon consumption of a natural plankton community maintained in mesocosm enclosures at initial CO 2 partial pressures of 350, 700 and 1,050 μatm increases with rising CO 2 . The community consumed up to 39% more dissolved inorganic carbon at increased CO 2 partial pressures compared to present levels, whereas nutrient uptake remained the same. The stoichiometry of carbon to nitrogen drawdown increased from 6.0 at low CO 2 to 8.0 at high CO 2 , thus exceeding the Redfield carbon:nitrogen ratio of 6.6 in today’s ocean 7 . This excess carbon consumption was associated with higher loss of organic carbon from the upper