Paper Reviewed
Cooper, H.L., Potts, D.C. and Paytan, A. 2017. Effects of elevated pCO2 on the survival, growth, and moulting of the Pacific krill species, Euphausia pacifica. ICES Journal of Marine Science 74: 1005-1012.

Euphausia pacifica is a dominant krill species inhabiting the northeast Pacific off the coast of North America. Its range extends from Alaska in the north to Southern California in the south. In the late spring and early summer, this region experiences large increases in E. pacifica populations following upwelling of deep waters off the coast. On any given day, this species will travel hundreds of meters in the vertical -- from the ocean depths below during the day to the surface heights above where they feed at night. Such migration results in natural exposure to seawater pH that fluctuates from around 8.1 at the surface to 7.6 at depths maintained during the day. Thus, E. pacifica routinely experiences in its habitat pH fluctuations that are larger than the decline predicted to occur by the end of this century due to ocean acidification, which exposure may well predispose them to adapt and tolerate such projected changes.

Exploring this hypothesis was the objective of Cooper et al. (2017), who recently assessed the survival, growth and moulting frequency of E. pacifica exposed to high pCO2/low pH seawater conditions during a two-month incubation experiment. And what did that experiment reveal?

In the words of the authors, although high pCO2 appeared to slow growth of the krill species (more particularly in the 10th and 90th quantiles of the size distribution), "a pCO2 level of 1200 µatm [(pH 7.6)] had no effect on either survival or moulting frequency." In fact, krill survival remained unaffected by ocean acidification until seawater pH values were lowered to 6.96, corresponding to a pCO2 of over 6,000 µatm.

Commenting on their findings, Cooper et al. say they suggest that "E. pacifica survival is unlikely to decline as a direct response to future OA levels," adding that "since E. pacifica regularly experiences a range of pCO2 values during its diel migration, it probably has well-developed acid-base regulation systems, and these may enhance its survival when exposed to elevated pCO2 for long periods, and also during extreme though short-term pH declines."