Changing carbon-to-nitrogen ratios of organic-matter export under ocean acidification

Ocean acidification (OA) will affect marine biotas from the organism to the ecosystem level. Yet, the consequences for the biological carbon pump and thereby the oceanic sink for atmospheric CO 2 are still unclear. Here we show that OA considerably alters the C/N ratio of organic-matter export (C/N export ), a key factor determining efficiency of the biological pump. By synthesizing sediment-trap data from in situ mesocosm studies in different marine biomes, we find distinct but highly variable impacts of OA on C/N export , reaching up to a 20% increase/decrease under partial pressure of CO 2 ( p CO 2 ) conditions projected for 2100. These changes are driven by p CO 2 effects on a variety of plankton taxa and corresponding shifts in food-web structure. Notably, our findings suggest a pivotal role of heterotrophic processes in controlling the response of C/N export to OA, thus contradicting the paradigm of primary producers as the principal driver of biogeochemical responses to ocean change. The biological pump sequesters carbon to the deep ocean. Ocean acidification, through impacts on plankton and food-web structure, is shown to alter the carbon-to-nitrogen ratio of organic material export, with heterotrophic processes playing a key role.