Feedbacks of CO₂ dependent dissolved organic carbon production on atmospheric CO₂ in an ocean biogeochemical model

Abstract. We use an ocean biogeochemical model to test whether a potential CO₂ dependence of dissolved organic carbon (DOC) production may have an influence on marine carbon sequestration via particle aggregation and so may represent a potentially larger ocean sink for anthropogenic CO₂. The hypothesis is based on mesocosm experiments that have shown enhanced carbon uptake by phytoplankton when more CO₂ is available, and where the extra carbon was probably directly routed into DOC instead of the particulate phase. Sensitivity experiments are carried out where phytoplankton exudation of DOC is systematically increased with and without a parallel rise in atmospheric CO₂. We find that under a fourfold increase of the DOC formation rate, there is a slight reduction of global particle export, which results in a shallower turnover of nutrients and carbon. As a consequence, a positive feedback loop develops, such that the ocean becomes a weak source of CO₂ to the atmosphere. The effect is amplified under high CO₂ conditions due to the decreased CO₂ buffer capacity of the ocean, however, it is rather low compared to the total anthropogenic perturbation. The positive feedback is in contrast to a very similar study, where a slight stimulation of particle export was found. Therefore, we conclude that the sign of the feedback depends on the actual pathway the extra carbon is taking and on the overall background conditions of marine primary production and ocean circulation.