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) projected to reach 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. We used 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 grani adults collected from a laboratory culture were acclimatized for 3 d at food suspensions of Heterocapsa from each pH treatment (ca. 500 cells/ml; 300 ?g C/l). 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 are in agreement with the lack of difference in the cellular stoichiometry (C : N, C : P, and N : P ratios) 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 may be largely associated with interspecific differences among prey items with regard to their sensitivity to elevated CO2 levels.

Differing responses of three Southern Ocean Emiliania huxleyi ecotypes to changing seawater carbonate chemistry

The invasion of anthropogenic carbon dioxide into the surface ocean is altering seawater carbonate speciation, a process commonly called ocean acidification. The high latitude waters of the Southern Ocean are one of the primary and most severely affected regions. Coccolithophores are an important phytoplankton group, responsible for the majority of pelagic calcium carbonate production in the world's oceans, with a distribution that ranges from tropical to polar waters. Emiliania huxleyi is numerically the most abundant coccolithophore species and appears in several different ecotypes. We tested the effects of ocean acidification on 3 carefully selected E. huxleyi ecotypes isolated from the Southern Ocean. Their responses were measured in terms of growth, photosynthesis, calcification, cellular geometry, and stoichiometry. The 3 ecotypes exhibited differing sensitivities in regards to seawater carbonate chemistry when cultured at the same temperature (14°C) and continuous light (110 µmol photons/m2/s). Under future ocean acidification scenarios, particulate inorganic to organic carbon ratios (PIC:POC) decreased by 38-44, 47-51 and 71-98% in morphotype A 'over-calcified' (A o/c), A and B/C, respectively. All ecotypes reduced their rate of calcification, but the cold-water adapted ecotype (morphotype B/C) was by far the most sensitive, and almost ceased calcification at partial pressure of carbon dioxide ( pCO2) levels above 1000 µatm. We recommend that future surveys for E. huxleyi cells in the Southern Ocean should include the capability of recognising 'naked cells' by molecular and microscopic tools. The distinct differences in the physiological responses of these 3 dominant Southern Ocean coccolithophore ecotypes are likely to have consequences for future coccolithophore community structures and thereby the Southern Ocean carbon cycle.