Ocean acidification alters marine food chains

Chair: Ana Queiros

Ivan Nagelkerken(1), Sean D. Connell(1)

1 School of Biological Sciences, The University of Adelaide, Adelaide SA 5005, Australia

Background
Ocean acidification has detrimental effects on a wide range of species, but we know relatively little about how it affects species interactions, particularly trophic interactions.

Methods
We performed a meta-analysis, a large mesocosm study, and a study on natural CO 2 vents to evaluate how ocean acidification – and its combination with ocean warming – affects species at different trophic levels.

Findings
Our meta-analysis shows that ocean warming causes a shift towards smaller pico-/nanoplankton species to the detriment of microplankton, which are less suitable as a food source for zooplankton. Total primary production by temperate non-calcifying plankton increases with elevated temperature and CO 2, whereas tropical plankton decreases productivity due to acidification. Temperature increases consumption by and metabolic rates of herbivores, but this response does not translate to greater secondary production, which instead decreases with acidification. This effect creates a mismatch with carnivores whose metabolic and foraging costs increase with temperature. Our mesocosm study supports this analysis by showing that elevated temperature and CO₂ increased energetic demands of sharks but also reduced their ability to locate food through olfaction, leading to a considerable reduction in shark growth rate. On natural CO2 vents, we found that predators such as groupers showed a steep reduction in their abundances which was associated with a strong increase in some prey species.

Conclusions
Our results suggest that species at higher trophic levels are particularly vulnerable to the combination of ocean warming and acidification due to elevated energetic demands, which are contrasted by reduced food intake resulting from a diminished ability to effectively hunt. This can lead to reduced top-down control over food webs, favouring some prey species at lower trophic levels. We find that ocean warming and acidification increase the potential for an overall simplification of ecosystem structure and function with reduced energy flow among trophic levels.