Lack of evidence for elevated CO₂-induced bottom-up effects on marine copepods: A dinoflagellate-calanoid prey-predator pair

Rising levels of atmospheric CO₂ are responsible for a change in the carbonate chemistry of seawater with associated pH drops (acidification) pro-jectedtoreach 0.4 units from 1950 to 2100. We investigated possible indirect effects of seawater acidification on the feeding, fecundity, and hatching success of the calanoid copepod Acartia grani, mediated by potential CO₂-induced changes in the nutritional characteristics of their prey. Weused as prey the autotrophic dinoflagellate Heterocapsa sp., cultured at three distinct pH levels (control: 8.17, medium: 7.96, and low: 7.75) by bubbling pure CO₂ via a computer automated system. Acartia graniadults collected fromalaboratory culture were acclimatized for 3d at food suspensions of Heterocapsa from each pH treatment (ca. 500 cells ml^-1; 300 μg C l^-1). Feeding and egg production rates of the preconditioned females did not differ significantly among the three Heterocapsa diets. Egg hatching success, monitored once per day for the 72 h, did not reveal significant difference among treatments. These results areinagreement with the lack of difference in the cellular stoichiometry(C: N,C: P, and N: Pratios) and fatty acid concentration and composition encountered between the three tested Heterocapsa treatments. Our findings disagree with those of other studies using distinct types of prey, suggesting that this kind of indirect influence of acidification on copepods maybe largely associated with interspecific differences among prey items with regard to their sensitivity to elevated CO₂ levels.